Manual for Judging Oregon Soils

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Manual 6 • Reprinted September 2007$12.50

Archival copy. For current version, see: https://catalog.extension.oregonstate.edu/manual6

This publication was prepared by J. Herbert Huddleston, Extension soil science specialist, and Gerald F. Kling, associate professor of soil science, Oregon State University. To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products that are not mentioned.

Plates 5, 8, 9, and 12 are reproduced from The Marbut Memorial Slides, slide nos. 5‑10, 6‑2, 6‑16, and 9‑1, by permission of the Soil Science Society of America.

Plates 2, 3, 4, 7, 10, and 11 are repro duced from the William M. John son slide set illustrating the United States Soil Classification System, by permission of M.A. Fosberg, Department of Plant and Soil Sciences, University of Idaho.

Plates 1, 6, 13, 14, 15, and 16 are from the authors’ collections.Figures 1, 5, 7, 8, and 25 were drawn by Cody Bustamante, OSU Communi‑

cations Media Center.Figures 2 and 3 are reproduced from Soil Microbiology and Biochemistry, slide

nos. 120 and 48, by permission of the Soil Science Society of America.Figures 4, 6, 17, 18, 19, 20, 21, 22, 23, and 24 are from the authors’ collec tions.Figures 9, 10, 11, and 12 are by courtesy of Tom Gentle, OSU Extension and

Experiment Station Communications.Figure 13 is reproduced from The Marbut Memorial Slides, slide no. 10‑4, by

permission of the Soil Science Soci ety of America.Figure 14 is by courtesy of Herb Futter, Natural Resources Conservation

Service, Ontario, Oregon.Figures 15, 29, and 33 are by courtesy of Don Baldwin, Natural Resources

Conservation Service, Enterprise, Oregon.Figure 16 is by courtesy of the Oregon Agricul tural Experiment Station.Figure 26 is by courtesy of Billie Forrest, Natural Resources Conservation

Service, Tangent, Oregon.Figure 27 is by courtesy of OSU Extension and Experiment Station Communica‑

tions.Figure 28 is by courtesy of Wilbur Bluhm, OSU Extension Service, Salem,

Oregon.Figures 30 and 31 are by courtesy of Dr. Steve Sharrow, OSU Department of

Rangeland Resources.Figure 32 is by courtesy of Sandy Macnab, OSU Extension Service, The Dalles,

Oregon.Figure 34 is by courtesy of Dr. Robert Paeth, Department of Environmen tal

Quality, Portland, Oregon.

Authors

Photo and Figure Credits

© 1996 Oregon State University

This publication was produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon counties. Oregon State University Extension Service offers educational programs, activities, and materials without discrimination based on age, color, disability, gender identity or expression, marital status, national origin, race, religion, sex, sexual orientation, or veteran’s status. Oregon State University Extension Service is an Equal Opportunity Employer.

Reprinted September 2007


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............................................................................................... 1

Factors of Soil Formation ................................................... 4Processes of Soil Formation ............................................... 6From Rock to Soil ................................................................ 7

Master Horizons .................................................................. 9Special Kinds of A, B, and C Horizons ............................ 10Transition Horizons .......................................................... 12Subdivisions of Thick Horizons ....................................... 13More Than One Kind of Parent Material ......................... 13Typical Horizon Sequences .............................................. 14Locating Boundaries Between Horizons ........................ 14

Color of the Soil Matrix ..................................................... 15Mottling .............................................................................. 17Texture ................................................................................ 18Coarse Fragments .............................................................. 23Soil Structure ..................................................................... 25Soil Consistence ................................................................ 28Horizon Boundaries .......................................................... 29Special Features of Soil Horizons .................................... 30

Effective Depth of Rooting ............................................... 33Available Water-Holding Capacity ................................... 34Permeability ....................................................................... 36Water Erosion Hazard ....................................................... 39Wind Erosion Hazard ........................................................ 41Internal Soil Drainage........................................................ 42Color Plates ........................................................................ 47

Landform ............................................................................ 51Parent Materials ................................................................ 54Stoniness and Rockiness .................................................. 57Slope.................................................................................... 58Aspect ................................................................................. 59

Feasibility of Artificial Drainage ...................................... 61Suitability for Irrigation .................................................... 63Most Intensive Crop .......................................................... 64Erosion Control Practice .................................................. 70Reaction Correction .......................................................... 73Limitation for Septic Tank Drainfields ............................ 74

A. How to Use Interpretation Guides .............................. 79B. How to Set Up and Run a Soil Judging Contest ......... 81C. Horizon Names Used Before 1983 ............................... 85Glossary .............................................................................. 87Scorecard............................................................................ 95Interpretation Guide ......................................................... 97

ContentsChapter lWhy Study Soil?

Chapter 2The Soil We Study

Chapter 3Kinds of Soil Horizons

Chapter 4Properties of Soil Horizons

Chapter 5Properties of the Whole Soil

Chapter 6Site Characteristics

Chapter 7Management Interpretations

Appendixes


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Soil judging has been part of thevocational agriculture curriculum inOregon high schools for more than

40 years. Since 1956, the OSU ExtensionService and the Oregon Association ofConservation Districts have workedtogether to organize and operate the StateSoil Judging Contest.

The Extension Service also has pro-vided educational materials for the stu-dents and conducted training sessions forthe instructors. For many years, Exten-sion Bulletin 769, Soil Judging from theGround Up, was used as the primary textfor the program.

As we learned more about Oregon soils,however, and as soil management alterna-tives became more complicated, werealized that just revising EB 769 wouldnot do the job. Instead a completely newmanual was needed.

We began writing a new soil judgingmanual, and developing a new format forjudging soils, in 1978, when we introducedseveral new ideas to a group of about 20Vo-Ag instructors. They liked the newformat, and their comments were veryhelpful as we wrote a draft of a newmanual. We used this draft for 4 years whilewe tested new ideas and revised the text.

Publication of this manual would nothave been possible without numeroussuggestions from many soil scientists andVo-Ag instructors who have been veryclosely associated with the soil judgingprogram for a number of years. To each ofthem, we express our sincere apprecia-tion.

There are several major changes in thisnew Manual.1. We encourage students to study the

whole soil profile and evaluate each ofthe natural soil horizons present.

2. We stress gathering primary data onjust a few important properties thatstudents can determine in the field—color, texture, and structure.

3. We show students how to use thesebasic properties to discover some

Forewordimportant aspects of soil behavior,such as effective depth, water-holdingcapacity, susceptibility to erosion, andinternal drainage class.

4. We explain why soil properties and soilbehavior are important to soil manage-ment.

5. We develop keys for students to use torelate soil properties to importantdecisions about irrigation suitability,crop choices, erosion control, andother questions.We don’t expect students to memorize

all the factors and interactions that enterinto a management decision. We do hope,however, that they will learn how to usethe keys to translate their evaluations ofprofile data into solutions to real prob-lems facing modern agriculturalists andenvironmental scientists.

Above all, we view soil judging prima-rily as an educational program. Contestscertainly add a dimension of challengeand fun, but they are not the primaryobjective of the program. Instead, themain objective is to encourage studentsto investigate this fascinating resource wecall soil, to discover how soils are orga-nized into horizons, to learn both how todescribe a few key properties of soilhorizons and to interpret them in terms ofmanagement practices—and to develop asense of stewardship for the soils thatsupport them.

Nothing would make us happier thanfor a student to challenge some of thegeneralized management interpretationswe describe in this manual in terms oftheir relevance to his or her own soilconditions—and for the instructor toseize that opportunity to do some realteaching about the kinds of interactionsone must consider when deciding howbest to use the soil resources available ona given farm. When this happens, theManual for Judging Oregon Soils will haveaccomplished its real purpose.


1

Soil is an essential natural resource.Without a doubt, that is the mostimportant reason for studying the

soil. Nathaniel Shaler put it this way: “Soil,ever slipping away in streams to the sea,is a kind of placenta that enables livingthings to feed upon the earth.”

The sketch in Figure 1 illustrates ourdependence on the soil. Unlike plant life,we human beings can’t manufacture ourown food from the four primary resourcesof soil, air, water, and sunlight. Instead, wedepend completely on green plants, whichtake nutrients and water from the soil andcombine them with air and sunlight toprovide our food supply.

Some of those plants, such as wheatand corn, we eat directly. Others, such as

Why Study Soil?alfalfa hay and range grasses, we processthrough livestock first. Even the fish weeat depend on plants that grow in the seausing nutrients that have been washed outof the soil and carried to the sea in riversand streams.

We study the soil, then, to increase ourunderstanding of this resource that sup-ports us.

Another important reason for studyingsoil is that soils are different. Oregon alonehas nearly 2,000 different kinds of soils,ranging from deep to shallow, clayey tosandy, wet to dry, nearly level to steeplysloping. These differences are important,because different soils require differentkinds of management practices.

Wet soils, for example, are essentialcomponents of wetland ecosystems.Proper management of wet soils dependson recognizing the imprint of saturated

Figure 1.—All of our food resources can be traced directly back to the soil.

Chapter 1

1Shaler, N.S., Man and the Earth (Chautauqua,NY: The Chautauqua Press, 1915).


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conditions on soils and maintaining watertables at desired levels.

Many soils require irrigation for maxi-mum productivity. Both the amount ofirrigation water needed and the propermethod of applying it depend on a soil’spermeability rate and water-holdingcapacity.

Still other soils have a very seriouserosion hazard. The proper choice ofconservation practices depends in part onthe texture of the surface horizon and thesteepness of the slope.

Only by studying soils can we learn howto tailor management practices to thespecific needs of each of the many differ-ent kinds of soils we depend on.

Some of us study soil because we’re justplain curious about this unique andfascinating natural resource. When youdig a hole or scrape off a road cut, youdiscover right away that there’s a wholelot more to the soil than just the top 8 or10 inches.

Soil scientists, in fact, study the soil to adepth of about 5 feet. They see severaldifferent layers, or horizons, in the soil.Together, these horizons make a soilprofile.

We describe the horizons in a soilprofile in terms of their properties, or

morphology. Some properties, such ascolor and root abundance, can be deter-mined by eye. Others, such as texture andstructure, require a keen sense of touch.

You, too, can learn how to determinethe important properties of soil horizons.Then you will be able to make a number ofimportant decisions about drainage,irrigation, crop selection, erosion con-trol—and much, much more.

At the beginning of this chapter, wetalked about how all human life dependson our soil resources. Knowing that, wemust study the soil so that we can learnhow to protect it for others to use. There’splenty of good soil on this planet—as longas we take care of it properly. But if we letit erode, or compact it, or mine it, it willfail to support us.

Farmers are the primary stewards of thesoil, for they are the tillers of the land. Allof us, however, share the responsibility toprotect this valuable resource. If wemanage our soil properly, it will continueto nourish us for generations to come. Ifwe don’t, our very civilization is threat-ened.

So study the soil, learn about its proper-ties and behavior, manage it wisely, anddo your part as a steward of the land.

Chapter 1 • Why Study Soil?


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Soil has been defined by lots of differ-ent people in lots of different ways.Here’s a very basic definition:

SOIL—the natural medium inwhich plants grow.

This definition, however, may be a littletoo simple. Here’s a better one:

SOIL—a natural body that devel-ops in profile form from a mix-ture of minerals and organicmatter. It covers the earth in avery thin layer and suppliesplants with air, water, nutrients,and mechanical support.

Our definition is, of course, the one weprefer:

SOIL—a living, dynamic system atthe interface between air androck. Soil forms in response toforces of climate and organismsthat act on parent materialin a specific landscapeover a long period of time.

We like this definitionbecause each key word sayssomething important about thesoil. Why living? Because thesoil is full of living organisms:roots large and small, animalsand insects, millions of micro-scopic fungi and bacteria.

Equally important are thedecaying remains of plants andanimals after they die. Theyform soil organic matter, orhumus, which is vitally impor-tant for good soil tilth andproductivity.

Dynamic says that the soilchanges all the time. Oregonsoils change from very wet in

the winter to very dry in the summer.Even under irrigation, the amount ofwater in the soil can vary widely.

Soil organic matter increases when cropresidues are worked in, and decreases asfresh plant materials decay. Soil nutrientsincrease as soil minerals break down.They decrease as water moving throughthe soil carries them away. Even soilacidity, or pH, changes seasonally.

The word system says that all parts ofthe soil work together to make up thedynamic whole. A change in one part maycause changes in many other parts.

Suppose, for example, we add wateruntil the soil is very wet. That reduces theamount of air available to plant roots. Itmakes the soil colder, and the activity ofroots and soil microbes (very small plantsand animals) slows down. The wet soil isstickier and cannot bear as much weight.

The Soil We Study

Chapter 2

Figure 2.—Living, dynamic soil. The microscopic worm shown is called anematode. The lower part of the nematode is being attacked and invadedby a fungus.


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Now let’s remove the excess water. Thewhole system changes to a warmer anddrier soil that is better for plants to growin and easier for farmers to manage.

The word interface stresses the ideathat soil is indeed a very thin rind at theearth’s surface. When air meets rock,especially if the air is warm and the rockis moist, the rock begins to change. Somechanges are physical. They break rocksdown into smaller pieces. Other changesare chemical. They destroy some of theoriginal minerals and create new ones.

These physical and chemical changesare called weathering. Weathering occursonly within the first few feet of the earth’ssurface. Plate 5 illustrates a stronglyweathered soil. The light-colored parts ofthe BC horizon are highly weatheredbedrock remnants not yet fully changedinto soil.

Now consider the size of the earth. Thedistance from the surface to the center ofthe earth is about 4,000 miles. Thus,10 feet of weathered rock out of4,000 miles is something less than.00005 percent. Soil does indeed occur atthe point of contact between earth andatmosphere!

Factors of Soil FormationThe rest of the key words in our defini-

tion of soil tell us something about howsoil forms. We think in terms of fivesoil-forming factors. Two of them, climateand organisms, are called active factors.They provide the forces that cause soil toform. The other three, parent material,

topography, and time, are called passivefactors. They respond to the forcesexerted by climate and organisms.

Together, the interactions between theforce factors and the response factorsresult in a new product, a unique naturalresource, which we call soil.

ClimateClimate affects soil most directly

through temperature and rainfall. Inwarm, moist climates, rocks and mineralsweather very quickly. The soil that formsoften has a reddish color. Most of the redsoils in western Oregon are red becausethey formed when the climate waswarmer than it is now.

High rainfall also causes leaching—theremoval of soil materials by water flowingthrough the soil. Free lime is completelyleached from western Oregon soils. Thesesoils are acid. Free lime is still present inmany eastern Oregon soils because thereisn’t enough rain to leach the soil com-pletely.

Warm, moist climates encourage lots ofplant growth, and that means lots of soilorganic matter. The opposite is true inhot, dry climates. Soils in the WillametteValley are dark-colored because they haveplenty of organic matter. Soils in theOntario area are light-colored becausethey have very little organic matter.

OrganismsOrganisms are of three general types:

large plants, tiny plants (microbes), andanimals. Roots of large plants help breakrocks apart and mix soil particles. Rootchannels provide pathways for water andair movement through the soil. Above-ground plant parts die and decay, therebybuilding up the organic matter in the soil.

Microscopic organisms, or microbes,are an extremely important part of thesoil. They are the primary decomposers.They change raw plant material into acomplex black substance called humus. Atthe same time they release soil nitrogen,

Five Soil-forming FactorsClimate

OrganismsParent Material

TopographyTime

Chapter 2 • The Soil We Study


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an essential nutrient that plants need inlarge quantities. Thus, rich, fertile topsoilsare rich and fertile because they are wellsupplied with humus (see Plates 3, 4, and8). Even the earthy smell of moist, rich,topsoil is caused by a microorganism.

Microbes and the humus they producealso act as a kind of glue to hold soilparticles together in aggregates. Well-aggregated soil is ideal for providing theright combination of air and water toplant roots.

Without microbes, therefore, soil wouldbe a virtually inert (lifeless) body. Withthem, soil is truly a living, dynamic sys-tem.

Soil animals include large burrowinganimals, small earthworms and insects,and microscopic worms called nema-todes. All are important because they helpmix soil. Animal mixing carries raw plantdebris that lies on the soil surface downinto the topsoil. Only then can themicrobes do their job of changing plantmaterial to humus.

Parent materialParent material is the original geologic

material that has been changed into thesoil we see today. Parent material ispassive because it simply responds to thechanges brought about by weathering andbiological activity.

Many parent materials are some kind ofbedrock, like sandstone or basalt. Othersare deposits of sediments carried bywater, wind, or ice. Volcanic ash, lake-laidsilts, dune sand, and glacial gravels all areexamples of transported parent materials.

TimeTime is the great equalizer. Young soils

inherit the properties of their parentmaterials. They tend to have the samecolor, texture, and chemical compositionas their parent materials. Later on, the

influence of parent material is not asevident.

As soils age, many original minerals aredestroyed. Many new ones are formed.Soils become more leached, more acid,and more clayey. In short, the soilbecomes more strongly developed withthe passage of time.

TopographyTopography, or landscape position,

causes localized changes in moisture,temperature, and parent material. Whenrain falls on a hillslope, for example, waterruns away from the top of the hill. Excesswater collects at the bottom of the hill.Soils at the top of the hill are relativelydry and often show the effects of erosionon the soil profile. Soils at the bottom ofthe hill not only are wetter but often areformed in materials transported downslope and deposited in lower landscapepositions.

Another effect of topography is due tothe direction that a slope faces. Soils onnorth-facing slopes, for example, tend tobe cooler and wetter than soils on south-facing slopes.

Figure 3.—Humus formation. Strands of fungus surround afreshly clipped blade of grass. Decomposition is beginningand will soon convert the clipping to soil organic matter.

Factors of Soil Formation


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Processes of Soil FormationOur definition of soil has identified five

factors of soil formation. We also can thinkin terms of four major processes thatchange parent material into life-sustainingsoil. They are additions, losses, transloca-tions, and transformations.

AdditionsThe most obvious addition is organic

matter. As soon as plant life begins togrow in fresh parent material, organicmatter begins to accumulate. Organicmatter gives a black or dark brown colorto surface soils. This is why even veryyoung soils usually have a dark-coloredsurface layer.

Other additions come with rainfall. Onthe average, rainfall adds about fivepounds of nitrogen each year to everyacre of soil. Rainfall also can be acid,especially downwind from industrialareas. Acid rain may change the rate ofsome soil processes. Rainfall, by causingrivers to flood, is indirectly responsiblefor the addition of new sediments to thesoil on a river’s floodplain.

LossesMost losses occur by leaching. Water

moving through the soil dissolves certainminerals and carries them out of the soil.Some minerals, especially salt and lime,are readily soluble. They are removedvery early in the history of a soil’s forma-tion. That’s why soils in humid regionsdon’t contain free lime.

Many fertilizers, especially nitrogenfertilizers, also are quite soluble. They,too, are readily lost by leaching, either bynatural rainfall or by irrigation water.

Other minerals, such as iron oxides andsand grains, dissolve very slowly. Theyremain in the soil until it is very old andhighly weathered.

Losses also occur as gases or solids.Oxygen and water vapor are lost from soilas fresh organic matter decays. And whensoils are very wet, nitrogen can bechanged to a gas and lost to the atmo-sphere. Solids are lost by erosion, whichremoves both mineral and organic soilparticles. Such losses are very serious, forthe soil lost by erosion usually is the mostproductive part of the soil profile.

TranslocationsTranslocation means movement from

one place to another. Usually we think ofmovement out of a horizon near the soilsurface and into another horizon that isdeeper in the soil.

In low rainfall areas, leaching often isincomplete. Water starts moving downthrough the soil, dissolving soluble miner-als as it goes. But there isn’t enough waterto move all the way through the soil.When the water stops moving, thenevaporates, the salts are left behind.That’s how subsoil accumulations of freelime are formed (Plate 9). Many hardpansin soils of dry areas form this way, too(Plate 10).

Upward translocation also is possible.Even in the dry areas of eastern Oregon,there are some wet soils that have highwater tables. Evaporation at the surfacecauses water to move upward continu-ously. Salts are dissolved on the way, andleft behind as the water evaporates(Figure 4). Salty soils are difficult tomanage, and they are not very productive.

Another kind of translocation involvesvery thin clay particles. Water movingthrough the soil can carry these particlesfrom one horizon to another, or fromplace to place within a horizon. When the

Processes of Soil FormationAdditions

LossesTranslocations

Transformations

Chapter 2 • The Soil We Study


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water stops moving, clay particles aredeposited on the surface of soil aggre-gates. We call these coatings clay skins,and they have a dark, waxy appearance(Plate 15).

TransformationsThese are changes that take place in the

soil. Microorganisms that live in the soilfeed on fresh organic matter and change itinto humus. Chemical weathering changesthe original minerals of parent materials.Some minerals are destroyed completely.Others are changed into new minerals.Many of the clay particles in soils areactually new minerals that form duringsoil development.

Still other transformations change theform of certain elements. Iron oxideusually gives soils a yellowish-brown orreddish-brown color. In waterlogged soils,however, iron oxide changes to a differentform that we call reduced. Reduced ironoxide is lost quite easily from the soil byleaching. After the iron is gone, the soilhas a gray or white color.

Repeated cycles of wetting and dryingcreate mottled soil (Plates 7 and 16). Part

of the soil is gray because of loss of iron,and part is yellow-brown where the ironoxides have accumulated in localizedareas.

From Rock to SoilHow do all these processes work

together to form soil? Let’s start with afresh parent material. Climate starts actingon it immediately. Weathering begins tochange minerals. Leaching removes salts,then free lime.

As soon as plants begin growing, theyadd organic matter to the soil. Biologicalactivity increases, and humus forms. Soona dark-colored surface horizon is present.

Weathering and leaching continue tochange soil minerals and remove solublecomponents. More horizons developbeneath the surface. The soil becomesmore acid. Clay minerals begin to form.Clay is translocated and clay skins becomevisible.

As the amount of clay in subsoil hori-zons increases, the rate of water move-ment through the soil decreases. Weather-ing continues, but leaching isn’t as rapid.After a while, further change is very slow,

and the whole soil-plant-landscape system is in a kindof steady state.

How do we know that all thishas happened? First, we dug ahole to reveal a soil profile.Next, we studied the soilcarefully, located the horizons,and determined their proper-ties. Then we interpreted theinformation from the profile interms of the factors andprocesses of soil formation.

You, too, can learn how todescribe and interpret soilprofiles. That’s what the rest ofthis Manual is all about.

Figure 4.—Upward translocation. The white areas, calledslick spots, are accumulations of salts left on the soilsurface after water moving up through the soil evaporatedat the surface.

From Rock to Soil


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9

Asoil horizon is a layer of soil paral-lel to the earth’s surface. It has aunique set of physical, chemical,

and biological properties. The propertiesof soil horizons are the results of soil-forming processes, and they distinguisheach horizon from other horizons aboveand below.

Soil horizons are named using combina-tions of letters and numbers. Six generalkinds of horizons may occur in soil pro-files (Figure 5), and they are named withcapital letters: O, A, E, B, C, R. These arecalled master horizons.

Gradual changes from one master hori-zon to another give rise to transitionhorizons. These are named with twoletters, for example, AB, BA, BC. Specialkinds of master horizons are named byadding lower case letters—for example Ap,Bt, Cr. Thick horizons may be subdividedusing Arabic numbers, as in Al, A2, or Bw1,Bw2, Bw3.

A single soil profile never has all thehorizons that are possible. Most Oregonsoils have A, B, C, and one or two transi-tion horizons. Other Oregon soils mayhave an A horizon resting directly on aC horizon, or an A-E-B-C horizon sequence,or even an O-E-B-C sequence.

Because all six master horizons occursomewhere in Oregon, we need to knowwhat each one is and how it differs fromthe others.

Master HorizonsEach master horizon has a distinct set of

properties.

O horizonThe O stands for organic. O horizons

don’t have to be 100 percent organicmaterial, but most are nearly so. Forestsoils usually have thin organic horizons atthe surface. They consist of leaves andtwigs in various stages of decay (Plate 1).

Wet soils in bogs or drained swampsoften have O horizons of peat or muck.Soils in Lake Labish, Waldo Lake, andLower Klamath Lake all have O horizons ofthis kind. Other than these, very fewagricultural or rangeland soils in Oregonhave O horizons.

Kinds of Soil Horizons

Chapter 3

Figure 5.—Generalized soil profile. Eachmaster horizon is shown in the relativeposition in which it occurs in a soil profile. Allsix master horizons are shown, even thoughany one soil usually has only three or four.

Litter layer

Mineral surface horizon, dark-colored, granular structure

Strongly leached horizon,light-colored, platy structure

Subsoil horizon of maximumdevelopment, “brown,” blockystructure

Weathered “parent material,”“brown,” massive structure

Hard bedrock


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A horizonThe A horizon is the surface horizon of

mineral soil. Its unique characteristic is adark color formed by addition of humus(Plates 3, 4, 5, 6, 8, and 12). Granular orfine blocky structure (aggregate shape)and friable consistence (ease of crushing)also are typical.

The thickness of A horizons rangesfrom a few inches in dry, rangeland soilsto over 20 inches (50 cm) in someWillamette Valley soils. Every cultivatedagricultural soil has an A horizon.

A horizons are extremely important inmaintaining soil fertility and providing afavorable environment for root growth.They should be protected from damage byerosion or compaction.

E horizonThis horizon has a light gray or white

color. It’s not present in all Oregon soils,but when it is, it usually occurs immedi-ately beneath an O or an A horizon(Plates 1, 5, 6, and 11).

E horizons are light-colored becausenearly all the iron and organic matter hasbeen removed. You can think of the E asmeaning exit or leaching.

E horizons occur in several of the sandysoils along the Oregon coast. They alsooccur in some wet, silty soils that havedense, clayey subsoils. In the wet soils,the E horizon also has noticeably less claythan the B horizon beneath it.

B horizonThe B horizon is the subsoil layer that

changes the most because of soil-formingprocesses. Several kinds of changes arepossible.

In some soils, the B horizon has thebrightest yellowish-brown or reddish-brown color (Plates 1, 2, 4, 5, 9, and 12). Inothers, it has the most evident blocky orprismatic structure (Plates 2 and 3). ManyB horizons have more clay than any other

horizon, and you may be able to see clayskins (Plates 3, 5, 6, and 9). Each of thesemajor kinds of B horizons is discussedmore fully in the next section, “SpecialKinds of A, B, and C Horizons.”

C horizonThe C horizon is weathered geologic

material below the A or B horizon. Any-thing that you can dig with a spade butwhich has not been changed very muchby soil-forming processes is consideredC horizon (Plates 1, 2, 3, 4, 9, and 13).

R horizonR stands for rock. It refers to hard

bedrock that you can’t easily dig with aspade. Depending on the depth to bed-rock, the R horizon may occur directlybeneath any of the other master horizons(Plate 12).

To judge an R horizon, mark its color,and check none for mottles and NA fortexture. Bedrock essentially is 100 percentcoarse fragments, so check more than60 percent. The structure type is massiveand the grade is structureless.

Special Kindsof A, B, and C Horizons

Many horizons are the result of uniqueprocesses that leave a distinct mark onthe horizon. We identify these horizonswith a lower-case letter immediatelyfollowing the master horizon symbol.Over 25 letters and combinations ofletters are possible. We’ll discuss onlynine that you are most likely to encounterin Oregon soils.

Ap horizonThe surface horizon of any soil that has

been plowed or cultivated is called theplow layer (Plates 2, 6, 9, and 12). That’swhat the p stands for. Cultivation thor-oughly mixes the upper 8 to 12 inches

Chapter 3 • Kinds of Soil Horizons


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(20 to 30 cm) of the soil and destroys anynatural horizons that may have beenpresent.

If the original A was very thick, plowingconverts the upper part into an Ap, andthe lower part remains simply as a secondA horizon. If the original A was very thin,then the Ap could rest on a B, a C, or atransition horizon.

Even when a soil has been severelyeroded, such that all the original A is gone,plowing an exposed B or C horizon wouldautomatically make the surface horizon an Ap.

Bt horizonThe t stands for texture. Textural B

horizons have distinctly more clay inthem than the horizons above or below.You can feel the difference.

Some of the clay comes from the soilabove the Bt. Water moving down throughthe soil carries very fine clay particleswith it. When the downward movementstops, the clays are deposited, building upthe waxy coatings we call clay skins. Someof the clay also comes from the weather-ing of original minerals in the Bt.

Bt horizons are quite common inOregon soils. They usually have well-developed blocky or prismatic structure(Plates 3, 5, 6, and 9).

Bg horizonThis horizon is gleyed. That means it’s

very wet for long periods of time. Iron inthe soil is chemically reduced, and muchof it has been leached out of the soil. As aresult, gleyed horizons usually are darkgray in color (Plates 7 and 8). They alsomay be mottled (see page 17), but notnecessarily so.

Gleyed horizons almost always tell usthat the soil is poorly or very poorlydrained. Gleying is not restricted to theBg; other gleyed horizons include Ag, BAg,BCg, Cg.

Bs horizonWe call this horizon a spodic horizon.

It’s common only in some of the sandysoils on marine terraces along the Oregoncoast. A few soils at very high elevationsin the Cascades and the Blue Mountainsalso have spodic horizons.

The color of a spodic horizon is quitedistinctive. It’s usually bright yellowish-brown or reddish-brown, and it fades withdepth (Plate 1). Often there’s a thin blacklayer at the top. The spodic horizon formswhen iron, aluminum, and organic matterall are leached out of surface horizons,carried downward, and deposited in thesubsoil.

Bw horizonThink of the w as meaning weathered.

Bw horizons have been changed by weath-ering, but not enough to form a Bt, Bg, orBs. In Oregon soils, the Bw differs from theC by having weak or moderate blockystructure (see page 25). The Bw also mayhave a little brighter color (Plates 2, 4, 11,and 12), and it may be more leached thanthe C.

Bw horizons are common in soils of theCascade and Coast Range Mountains, inyoung soils on river floodplains and lowterraces, and in many soils of easternOregon.

Bx horizonThis refers to a special feature called a

fragipan. It is a massive, dense, but notcemented, soil horizon. The fragipan isoften mottled and has streaks of gray siltscattered throughout (Plate 11). Thefragipan is so dense that neither plantroots nor water can penetrate, except inthe gray silt streaks. In Oregon, fragipansoccur only in some of the upland soils ofColumbia, Washington, Multnomah, andClackamas counties.

Special Kinds of A, B, and C Horizons


12

Bk horizonThis horizon has an accumulation of

calcium carbonate, or free lime. Carbon-ates leached from upper horizons havebeen redeposited in the Bk (Plate 9). Youshould be able to see white streaks ornodules of lime. These will bubble vio-lently when a drop of hydrochloric acid(HCl) is placed on them.

Be careful though. Some soils in easternOregon have had free lime in them rightfrom the beginning. They will react to theacid, too. We use the k only to indicate ahorizon enriched in carbonates by trans-location. A Bk horizon may very well havean ordinary C horizon beneath it thatcontains only its original amount of lime.

Bkqm horizonThis horizon is called a duripan

(Plate 10). It is enriched with calciumcarbonate (k) and silica (q), and it isstrongly cemented (m).

Duripans are common in several soils ofeastern Oregon. Limited rainfall hasleached lime and silica from the upper 10to 20 inches of soil and redeposited themin the Bkqm. Thin, pinkish coatings ofopal may be present on the upper sur-faces of duripan fragments. The duripanusually is only 6 to 10 inches thick, but it isso cemented that plant roots can’t gothrough it.

A dense mat of roots spreading horizon-tally is a good indicator of a duripan.Sometimes, however, there are fracturesin the duripan that will allow some plantroots to find a way down. And if the pan isonly weakly cemented, you can break itup even more by ripping.

For soil judging contests, mark the mostappropriate color. Check none for mot-tling, NA for texture, and more than60 percent coarse fragments. The struc-ture type is massive and the grade is struc-tureless.

Cr horizonWeathered bedrock, or rock that is soft

enough to slice with a knife or a spade, iscalled Cr (Plate 13). It’s rock material, andyou often can see original rock structure,but it’s not hard enough to be desig-nated R.

When judging a Cr, first record its color,then check none for mottles, NA fortexture, and more than 60 percent coarsefragments. The structure type is massiveand the grade is structureless.

Transition HorizonsMaster horizons rarely change abruptly

from one to another. Instead, the changesoccur gradually throughout a zone thatmay be 5 or 10 inches thick. These zonesare called transition horizons. There arethree common ones in Oregon soils.

AB horizonThis transition horizon occurs between

the A and the B. It’s dominated by proper-ties of the A, but some of the properties ofthe B are evident. Dark colors associatedwith organic matter are fading becauseorganic matter is decreasing (Plate 4). Thestructure often changes from granular tosubangular blocky (see Chapter 4, “SoilStructure”).

BA horizonThis horizon also occurs between the A

and the B, but it has more of the charac-teristics of the B. Generally, the structurewill be the same type as the B, but lessstrongly expressed. The color may be alittle darker than the B (Plate 3), or theclay content may be less than the maxi-mum in the B.

BC horizonThis is a transition from B to C. Proper-

ties of the B are dominant, but someinfluence of the C horizon is evident



13

(Plates 2 and 3). Often the clay contentwill be less than the maximum in the B,but more than in the C. Or the color willbe fading. If the C is massive, the BC hasstructure, but it may have larger units andbe more weakly expressed than in the B.

Subdivisionsof Thick Horizons

Sometimes one or two of the horizonsin a soil are so thick that they need to besubdivided. Small changes in texture,color, or structure commonly are used tomake the subdivision.

Subdivisions, or vertical sequenceswithin any single kind of horizon, alwaysare indicated by a number immediatelyfollowing the letter symbol(s). Here are afew examples of some thick soil horizonsthat could be subdivided:

Thick A horizon—Al, A2 (Plate 8)Thick Bg horizon—Bg1, Bg2 (Plate 7)Thick Bt horizon—Bt1, Bt2 (Plate 5)Thick Bw horizon—Bw1, Bw2 (Plate 2)Thick C horizon—C1, C2 (Plates 3

and 4)

More Than One Kindof Parent Material

Parent material is the geologic stufffrom which soils form. It may be a riverdeposit, volcanic ash, clays weatheredfrom rock in place, or one of many otherkinds of materials. When all the horizonsof a soil have formed in a single kind ofparent material, we simply use the ordi-nary A, B, and C designations.

Some soils, however, have formed inmore that one kind of parent material. Aflooding river, for example, may depositfresh silts on top of older sands andgravels. Or volcanic ash may be depositedon top of weathered bedrock.

If soil horizons are developed in morethan one material, we place a number infront of the horizon name to indicate itsposition from the top down.

The geologic material at the surface isalways assumed to be the first one, andthe number 1 is never used.

The second geologic material is indi-cated by a 2, the third by a 3, and so on.Thus a soil developed in silt loam overgravel could have the following set ofhorizons: A-AB-B-2BC-2C.

Figure 6.—Multiple parent materials. The gray material isrecent volcanic ash resting on older, more weathered soilmaterial. Notice the abrupt contact between the two contrast-ing materials.

Subdivisions of Thick Horizons


14

Typical Horizon SequencesSeveral common Oregon soils are listed

below, followed by the names of thehorizons in a typical profile. Nearly allgeographic areas of Oregon are repre-sented by the soils on this list.

Alicel Ap-A-BA-Bw-2CBandon O-E-Bs1-Bs2-Bs3-Bs4-CBrenner Ap-A-Bg1-Bg2-BCg-CgCarney A1-A2-AB-C-2RCascade A-Bw1-Bw2-2Bx1-2Bx2-2Bx3Dayton Ap-E-2Bt-2BCt-3CDeschutes A-BA-Bw-2C-3RFordney Ap-C1-C2Gem Al-A2-BAt-Bt1-Bt2-BCt-BCtk-RHoopal A1-A2-Bw-Bkqm-CJosephine O-A-BA-Bt1-Bt2-Bt3-C-CrMalabon Ap-AB-Bt1-Bt2-BCt-2CNehalem Ap-A-Bw-CNyssa Ap-Bw-Bkq-Bkqm1-Bkqm2Oakland A1-A2-Bt1-Bt2-Bt3-BCt-CrQuincy C1-C2Ritzville Ap-BA-Bw-Bk-C1-C2Salem Ap-Bt-BCt-2CSimas A-2Bt1-2Bt2-2BCk1-2BCk2Walla Walla Ap-A-BA-Bw-BCk1-BCk2

Locating BoundariesBetween Horizons

The most useful thing you can do tohelp yourself find and name soil horizonsis to prepare a good exposure of the soilprofile. Either a good pit or a road cut willdo, but in either case you should clean upthe face of the vertical cut. Use your knifeto pick off any soil that may have fallendown from the surface or that wassmeared by a shovel or a backhoe. Afteryou have a good, fresh surface, try each ofthe following techniques.

1. Look for color changes. Where there isan obvious color change there also is ahorizon change. Color alone, however, isnot sufficient to separate all horizons.Several soils in Oregon have nearlyuniform colors extending all the waythrough the B and into the C.

2. Take a knife and gently poke the soilevery few inches from the surface downto the lower part of the pit. Often youcan “feel” the firmer consistence ofsubsoils and restrictive layers. You mayeven be able to locate a contactbetween B and C this way.

3. Starting at the top, check the soil tex-ture with your fingers every 2 to4 inches. If there is a marked increase inclay from A to B, you should detect itthis way. A decrease in clay from B to Calso should be evident.

4. With your knife, remove a handful of soilfrom the upper 4 inches of soil. Carefullybreak it apart and observe the size,shape, and strength of structural aggre-gates. Repeat this process every 4 to6 inches down through the profile.Structural changes may be a good clueto boundaries between horizons and tothe presence of transition horizons.

5. Each time you locate a tentative bound-ary, mark it with a nail, twig, or someother convenient marker. As you con-sider more and more characteristics,you may want to adjust some of theboundaries up or down.

6. When you have settled on an initial setof horizon boundaries, start lookingmore carefully at the color, texture,structure, pores, clay skins, etc. of eachhorizon. With a complete set of informa-tion, you may wish to make a finaladjustment in your horizon boundaries.



15

A fter you have marked off thehorizons in a soil, the next step isto carefully determine the proper-

ties of each horizon. We’ll discuss onlythose properties that we can observereadily in the field. The most importantones are color, texture, and structure.Others include soil consistence and thecharacteristics of horizon boundaries.

Color of the Soil MatrixSoil colors give us clues about the

nature of the root zone. Dark colors meanfavorable amounts of humus. Gray colorssuggest wetness. Brown and red colorsindicate favorable air-water relations.

The soil matrix is the main body of thesoil. In uniformly colored horizons, thematrix is the entire soil in the horizon.Some horizons, however, have two ormore colors. The matrix color is thedominant color, the one that covers thegreatest area and gives you an overallimpression of the horizon’s color.

The color of most soils depends onwhether the soil is moist or dry. Moist soilis almost always darker than dry soil. Wecan always moisten a dry soil, but we maynot always have time to wait for a moistsoil to dry out. To be consistent, there-fore, we always will judge the color of thesoil when it is moist. One or two drops ofwater from your squirt bottle will beenough to do the job.

The apparent color of a moist soil alsomay depend on the amount of sunlightstriking the sample. The color may seemto be a little darker on an overcast daythan on a sunny day, or in a shadowrather than in open sunlight. Some varia-tion is unavoidable, but you alwaysshould try to determine soil color in thegreatest amount of light available.

Properties of Soil HorizonsWe can group soil colors into four broad

classes. We use descriptive names forthese classes—even though each of ussees colors a little differently from every-one else. There is a more accurate way todescribe soil color, but it requires specialcolor books that are quite expensive. Thatmethod is described briefly at the end ofthis section.

Dark brown,Very dark brown, Black

These colors are caused by accumula-tions of organic matter in soils. Usually,the darker the color, the more the organicmatter, and the more fertile and produc-tive the soil.

Dark colors are typical of A horizons(Plates 3, 4, 5, 6, 7, 8, 10, and 12). Almostall A horizons in the soils of westernOregon will have this color.

That’s not the case in eastern Oregon,though. Organic matter contents are muchlower, and the soils are lighter colored. Ingeneral, if an eastern Oregon soil has beencultivated, and the crop residues havebeen mixed into the A, then the colorprobably is dark brown or very darkbrown. If the soil has not been cultivated,and there isn’t very much native vegeta-tion, then the A horizon is likely to have alight brown color.

Some soils have black colors extendingwell down into the subsoil. That’s usuallyan indicator of wetness. Organic matter inwet soils breaks down very slowly, and theextra organic matter accumulated darkensthe soil (Plate 8).

Some very clayey, sticky, shrink-swellsoils may be black, too. In these soils,organic matter is mixed throughout theentire shrink-swell zone, and the soil isblack, even though the organic mattercontent isn’t particularly high.

Chapter 4


16

Light brown, Brown,Yellowish brown

These are the colors of well-aerated soil.That means that air moves freely into andout of the voids, or pore spaces, of the soil.As microbes and plant roots use up oxygenin soil pores, oxygen from the air above thesoil moves in to replace it. Well-aeratedsoils, therefore, provide healthy homes forplant roots.

Brown colors are due to iron oxidecoatings on mineral grains. Chemically,they’re the same as a coating of rust on apiece of iron. These iron oxide coatingsrequire plenty of oxygen in soil pores. Ifwater should fill soil pores and remainthere for a long time, oxygen cannot reachthe iron coatings, and the soil turns gray.That’s why brown colors tell us that thesoil has good air-water relations and is notsaturated for long periods of time.

Brown colors are typical of B and Chorizons that are well-aerated (Plates 1, 2,9, 10, 11, and 12). That’s true in botheastern and western Oregon. As long asthere’s not enough organic matter todarken the soil, and there’s plenty ofoxygen to maintain iron oxide coatings, thesoil almost always will be brown (or red—see next section).

Red, Reddish brownThese colors also are caused by iron

oxide coatings, and they also indicatewell-aerated soil. The soil is red, ratherthan brown, only because the chemicalform of the iron oxide is a little different.

Most red soils are very old soils, andthey are very strongly weathered (Plate 5).They are more leached, more acid, and lessfertile than soils having brown colors.

Red soils occur in the foothills andmountains west of the Cascades from theColumbia River to the California border.Except for the A horizon, all other horizonsin these soils usually are red. Red soils arerare east of the Cascades, but there is atleast one red, clayey soil in central Oregon.

Dark gray, Light gray, WhiteDark gray soils are wet soils. When soil

pores are full of water, oxygen can’t get in.Gradually the yellow-brown coatings areremoved from mineral grains (soil par-ticles) and are leached away. The graycolor that we see is the natural gray colorof the uncoated mineral grains, darkened alittle by organic matter. Dark gray is typicalof B and C horizons in wet soils (Plates 7,8, and 16).

Light gray and white colors are charac-teristic of E horizons. They also are thecolors of uncoated mineral grains, butthere is almost no darkening with organicmatter.

Some E horizons occur in wet soils(Plates 6 and 11). Iron is reduced andleached from the soil, often when watermoves sideways on top of a tight subsoil.

Other E horizons may occur in welldrained soils (Plates 1 and 5). In thesesoils, different chemical processes causethe loss of iron oxide coatings from min-eral grains. These E horizons will havebright-colored Bs or Bt horizons belowthem.

Technical descriptions of soil colorsThe more technical method of soil color

description uses Munsell color notations.These involve symbols like 10YR 4/3. Thefirst part (10YR) designates a color hue, orpure color. The numerator of the fractionis called the value. This is an index of theamount of incident light reflected from thesoil. The denominator is called the chroma.It is an index of how much white lightdilutes the pure color.

The Munsell Color Company makessmall color chips for each combination ofhue, value, and chroma. Chips of thosecolors that are most frequently found insoils are arranged in special books of soilcolor charts.

To determine soil color in the field, yousimply match the color of a soil aggregatewith a chip of the same color. Then yourecord the symbol for that chip.

Chapter 4 • Properties of Soil Horizons


17

Technical descriptions of Oregon soils,published either in soil survey reports oras single-sheet official descriptions, usethis more precise method of evaluatingsoil colors.

MottlingSome soil horizons have spots of one

color in a matrix of a different color. Formany, many years, these spots have beencalled mottles, and the soil has been saidto be mottled. Now, soil scientists observeand describe these spots in much greaterdetail, and they are called redoximorphicfeatures—redox concentrations (brightcolored mottles), redox depletions (lowchroma mottles), and reduced matrices(strongly gleyed soils). For our work,however, the older nomenclature ofmottles still serves the purpose very well,and we’ll defer the use of the newer termsto university-level soil judging.

Some mottles appear as splotches ofreddish-brown in a gray matrix (Plates 7and 16). Others appear as gray mottles in abrown matrix. In either case, mottlesusually tell us that the soil has a highwater table during the rainy season.

A water table is the top of a zone ofsaturated soil. Beneath the water table, allthe soil pores are full of water. Without asupply of air, iron oxide coatings areremoved from soil particles, and graycolors develop.

When the water table drops, oxygenenters the soil through root channels andlarge pores. Iron changes back to theyellow-brown form and coats soil particlesin contact with the air. The result is ayellowish-brown mottle surrounded bygray soil.

The depth to mottles, and the numberand brightness of the mottles, are keys tothe degree of wetness of the soil. This willbe discussed more fully in the section oninternal soil drainage (page 42). There are,however, two situations in which mottlesdo not indicate wetness.

How to Describe Mottles

Abundance—the percentage of exposed sur-face area occupied by mottles. Classes of abun-dance are:

Few—less than 2 percent of exposed surface

Common—2 to 20 percent of exposed surface

Many—more than 20 percent of exposedsurface

Size—the approximate diameter of individualmottles. Size classes are:

Fine—diameter less than 5 mm

Medium—diameter of 5 to 15 mm

Coarse—diameter more than 15 mm

Contrast—the relative difference between themottle color and the matrix color. Classes ofcontrast are:

Faint—mottles evident only on close scrutiny.Mottle color and matrix color are very nearlythe same.

Distinct—mottles are readily seen though notstriking. Mottle color and matrix color aredifferent, though not widely so.

Prominent—mottles are so conspicuous thatthey are the outstanding visible feature of thehorizon. Mottle color and matrix color arewidely different.

Color—mottle colors are described in the sameway as colors of any horizon. The most com-mon mottle colors are yellowish-brown, darkreddish-brown, and gray.

Mottling


18

One situation is caused by the chemicalweathering of rocks. Each different mineralthat makes up a rock reacts differently tochemical processes. Some minerals turnyellow, some turn red, some turn gray, andsome are destroyed completely. The resultof rock weathering can be a mixture ofcolors that may look like drainage mottles,even though the soil is quite well drained.Many Cr horizons have this kind of “mot-tling.”

The key to avoiding this false interpreta-tion of soil colors is to study other factorsof the soil and the landscape very carefully.Concave (bowl-shaped) depressions,low-lying areas, or broad, flat terraces arelandscapes that are likely to have wet soils.Mottles in these soils are probably drain-age mottles. Soils that have horizons thatrestrict water movement also are likely tohave mottles caused by wetness.

Soils on rounded hilltops and slopinghillsides shed water. They are likely to bewell drained. Many of these soils are notvery deep to bedrock. In these soils, thelower horizons may very well containweathered rock fragments that look likemottles. The closer you get to the bedrock,the more mottled it may look.

But brown colors throughout the soil,and the shape of the landscape, should tellyou that these “mottles” don’t indicate soilwetness. You may even be able to seeremnants of layering of the original rocks,and that’s another clue that color varia-tions are not due to wetness. Check nonefor mottling on your scorecard if you seethis kind of color pattern.

Another false interpretation of colorpatterns is caused by coatings on soilaggregates. Organic matter coatings, claycoatings, or moisture films all darken thesurface of soil aggregates, particularly inB horizons.

Don’t confuse these coatings withmottles. And don’t judge the color of thesoil matrix by the color of the coatings.Always break open a soil aggregate, andjudge the color of both the matrix and themottles from a freshly exposed surface.

Mottle patterns in soils are describedusing four properties: abundance, size,contrast, and color. Standards for deter-mining each feature are given in the box onpage 17.

TextureTexture refers to the amounts of sand,

silt, and clay in a soil. Depending on howmuch sand, silt, and clay are present, wegive the texture a name like sandy loam,clay loam, or silty clay loam. Soils that alsocontain gravel or cobblestones may havenames like gravelly loam or very cobblyclay.

Texture is an important soil propertybecause it is closely related to manyaspects of soil behavior. The ease of tillingthe soil and the ease of plant root develop-ment within the soil both are influenced bysoil texture. Texture affects the amounts ofair and water a soil will hold and the rateof water movement through the soil.

Plant nutrient supplies also are relatedto soil texture. Tiny silt and clay particles

Sand.05 to 2mmfeels gritty

Silt.002 to .05 mmfeels smooth

Clayless than .002 mmfeels sticky



19

provide more mineral nutrients to plantsthan large sand grains. We can managesandy soils to improve their productivity,but they require more fertilizer and morefrequent irrigation than silty or loamysoils.

The determination of soil texture beginsby separating the soil into two broadclasses of particle size, fine earth andcoarse fragments. Fine earth includes allparticles smaller than 2 mm in diameter.This is the fraction that passes through ano. 10 sieve. Sand, silt, and clay all aresmaller than 2 mm and are the compo-nents of fine earth.

Coarse fragments include gravel andcobbles up to 10 inches (25 cm) in diam-eter. Rock fragments larger than 10 inchesare called stones and boulders. They aredescribed as a characteristic of the siteand are discussed further in Chapter 6.

Sand, silt, and clay are called the sepa-rates of the fine earth. Sand particles rangein size from .05 mm to 2 mm. They arelarge enough to see each grain with thenaked eye, and they feel gritty.

Silt particles range in size from .002 mmto .05 mm. You cannot see them without ahand lens or microscope. Silt has a smoothfeeling, like flour or corn starch. It is notsticky.

Clay particles are less than .002 mm insize. They usually are flat, or plate-shaped,and they can be seen only withhigh-powered microscopes. Clay feelssticky, and it can be molded into ribbonsor wires.

Every soil contains a mixture of sand,silt, and clay. We use a textural triangle toshow all the possible combinations. Wealso use the triangle to form groups, orclasses, of soil texture, which we thenidentify with a textural name.

Look at Figure 7. A soil that is almost allsand would lie very close to the sandcorner of the triangle. Its textural classname would simply be sand.

Similarly, a soil dominated by clay wouldlie near the clay corner of the triangle. Itwould be named simply clay.

Now consider a balanced mixture ofsand, silt, and clay. All three separates

are present, though not in exactlyequal proportions. (Actually, it takes

less clay to balance the mixturethan either sand or silt.) These

soils lie near the center of thetriangle, and they are called

loams.Now suppose we

were to upset abalanced mixture ofsand, silt, and clay byadding more sand.

The sand wouldbegin to dominate,and we’d move awayfrom the center of the

Figure 7.—Generalized textural triangle. Try thinking of each soiltexture in terms of one or more steps away from a balancedmixture of sand, silt, and clay.

CLAY

SAND SILT

LOAM

clay

clayloam

siltyclay loam

siltloam

siltsand

sandyloam

sandyclay loam

Texture


20

triangle toward the sand corner. Thetexture would change from loam to sandyloam, and ultimately to a sand.

If we were to add clay to a loam, wewould get first a clay loam, then a clay. Ifwe were to add both silt and clay, wewould move away from sand towardsomething intermediate between silt and

clay. The texture becomes a silty clayloam.

Precise boundaries between texturalclasses are shown in Figure 8. Each side ofthe triangle is a base line, or zero point, forthe separate in the opposite corner. Ascale runs from 0 percent at the middle ofeach base line up to 100 percent at the

Figure 8.—Official textural triangle. The scales of sand, silt, and clay, along with the precise boundaries betweensoil textural classes, are used to determine the correct name for the texture of a soil sample.

CLAY

clay loam

siltyclay

siltyclayloam

loam

sandyclayloam

sandyloam

loamysand

sand

siltloam

sandyclay

SAND SILT

clay



21

corner. If we know how much sand, silt,and clay a soil has, we can easily plot itslocation on the triangle and see whichtextural class it falls into.

Here’s a simple example. Suppose wehave a soil that contains 40 percent sand,45 percent silt, and 15 percent clay. Startwith the clay content. Go to the midpointof the base line running from sand to silt.Then go up to the horizontal line at15 percent. Every soil along this linecontains 15 percent clay.

Next, go to the midpoint of the base linerunning from silt to clay. This line repre-sents 0 percent sand. Move along the sandscale, down and to the left, until you reachthe 40 percent line. Then move down the40 percent sand line until it intersects the15 percent clay line. Mark that point.

If you wish, you can find the 45 percentsilt line and track it to the same point.Note, however, that it takes only twopoints to determine the texture. Thissample is a loam.

Figure 10.—Silty clay loam texture. This soilhas 3 percent sand, 68 percent silt, and29 percent clay. It forms a smooth ribbon aboutan inch long.

We determine soil texture in the field byworking the soil between our thumb andfingers and estimating the amounts ofsand, silt, and clay. Estimate sand by thegrittiness you can feel. Estimate clay bythe length of the ribbon you can form (seeFigures 9, 10, and 11). The procedure fordoing this is highlighted on page 22.

Figure 11.—Silty clay texture. This soil has 2 percent sand,54 percent silt, and 44 percent clay. You can squeeze the soilupward between your thumb and index finger to form a ribbonnearly 3 inches long.

Figure 9.—Sandy loam texture. This soil has 73 percent sand,23 percent silt, and 4 percent clay. When textured by feel, itforms only a short, broken ribbon.

Texture


22

1. Fill the palm of your hand with drysoil.

2. Moisten the soil enough so that itsticks together and can be workedwith the fingers. Don’t saturate it torunny mud. If the soil sticks to yourfingers, it’s too wet to texture. Addmore dry soil.

3. Knead the soil between your thumband fingers. Take out the pebbles,and crush all the soil aggregates.You may need to add a little morewater.

4. Continue working the soil until youcrush all the aggregates.

5. Estimate the sand content by theamount of textural grittiness youfeel.

a. More than 50 percent—Sanddominates. The textural namecontains the word sandy.

b. 20 to 50 percent—Sand is notice-ably present, but not dominant.The texture most likely is loam orclay loam, though silt loam orclay are possible.

c. Less than 20 percent—Silt andclay dominate. The textural nameis silt loam, silty clay loam, orclay.

6. Estimate the clay content bypushing the sample up betweenyour thumb and index finger toform a ribbon.

a. Less than 27 percent(Figure 9)—The ribbon is lessthan 11⁄2 inches long. Texturalnames contain the word loambut not the word clay.

b. 27 to 40 percent (Figure 10)—The ribbon is 11⁄2 to 3 incheslong. Textural names containboth the words clay and loam.

c. More than 40 percent(Figure 11)—Clay dominates.The ribbon is more than3 inches long. The texturalname contains the word claybut not the word loam.

7. Combine your estimates of sandand clay.

Texture by Feel

SAND>50 20–50 <20

>40 Sandy clay Clay ClaySilty clay

27–40 Sandy clay Clay loam Silty clayloam loam

<27 Sandy loamLoamy sand Loam Silt loamSand

CLAY



23

Four Key Texture Points

50 percent sand 27 percent clay20 percent sand 40 percent clay

Actually you need to learn only four keypoints on the textural triangle: 27 percentclay, 40 percent clay, 20 percent sand, and50 percent sand. These points don’texactly match the textural class bound-aries on Figure 8, but they’re close enoughto make good estimates.

Study the locations of these key valueson Figure 8 very carefully. Note that noneof the texture names below 27 percentclay contain the word clay. Texture namesbetween 27 and 40 percent clay containboth the words clay and loam. Texturenames above 40 percent clay contain onlythe word clay.

Similarly, soils having more than50 percent sand all have names thatinclude the words sand or sandy. If there isless than 20 percent sand, silt or siltyusually is part of the name. If the soilcontains between 20 and 50 percent sand,neither silt nor sand is part of the name.

Additional clues to the way each kind ofsoil texture feels are given in the box onpage 24. The effect of these differenttextures on things like permeability,water-holding capacity, and erosionhazard will become clearer when wediscuss specific interpretations of soilbehavior.

Coarse FragmentsCoarse fragments are soil particles that

are between 2 mm and 10 inches in size.Soil textural names based on the fine earthmust be modified if the soil contains asignificant amount of coarse fragments.

The two most common kinds of coarsefragments in Oregon soils are gravel andcobbles. Gravel refers to rounded rockfragments with a diameter between 2 mmand 3 inches. Cobbles are rounded orpartly rounded, with diameters from 3 to10 inches.

Coarse fragment names depend on thevolume of the soil mass occupied bycoarse fragments. You can estimate thevolume by looking at the vertical surfaceexposed in a soil profile. If 50 percent ofthe surface consists of coarse fragments,then 50 percent of the soil volume iscoarse fragments as well (see Plate 14).

Once you know both the percent byvolume and the dominant size of coarsefragments, find the correct modifier in thekey below. If a soil contains both graveland cobbles, at least 60 percent of thecoarse fragments must be gravel to use thegravelly term. If more than 40 percent ofthe coarse fragments are cobbles, use thecobbly term.

Key to Naming Coarse Fragment Modifiers

% by Gravel Cobblesvolume 2 mm–3 inches 3–10 inches

<15 no modifier no modifier

15–35 gravelly cobbly

35–60 very gravelly very cobbly

>60 extremely extremelygravelly cobbly

Coarse Fragments


24

Sand

— Moist sample collapses after squeezing.— Your hands don’t get dirty working the

sample.

Loamy sand

— Sample has very little body.— Moist soil barely stays together after

squeezing.— Just enough silt and clay to dirty your

hands.

Sandy loam

— Sand dominates noticeably.— Enough silt and clay to give the sample

body.— Moist soil stays together after squeez-

ing.— Hardly forms any ribbon at all.

Sandy clay loam

— Feels gritty and sticky.— Forms ribbon 1 to 2 inches long.

Sandy clay

— Feels definitely sandy.— Forms ribbon 2 to 3 inches long.— A rare texture in Oregon.

Loam

— Sand noticeably present, but doesn’tdominate.

— Sample works easily between thumb andfingers.

— Contains enough silt and clay to givesample good body.

— Sample only forms short, broken ribbons.

Silt loam

— Feels smooth, like flour or corn starch.— Tends to be nonsticky.— Only forms short, broken ribbons.

Clay loam

— Noticeably gritty, but sand doesn’t domi-nate.

— Noticeably sticky.— Noticeably hard to work between

thumb and fingers.— Forms ribbons 11⁄2 to 3 inches long.

Silty clay loam

— Feels smooth and sticky.— Contains very little sand.— Forms ribbons 11⁄2 to 3 inches long.

Clay and Silty clay

— Dry sample absorbs a lot of water beforeit is moist enough to work.

— Sample very hard to workbetween thumb and finger.

— Forms ribbon 3 to 4 inches long.

Clues to the Feel of Textural Classes



25

Soil StructureSoil structure forms when individual

grains of sand, silt, and clay are boundtogether in larger units called peds. Plantroots, soil organic matter, and clay par-ticles all provide physical and chemicalbinding agents. The shape of the pedsformed determines the type of structure.The extent of ped formation, and thestrength of each ped, together determinethe grade of the structure.

Soil structure is important because itmodifies some of the undesirable effectsof texture on soil behavior. Structurecreates relatively large pores, which favorwater entry into the soil and water move-ment within the soil. Even clayey soils,which tend to have very tiny pores, canhave good rates of water movement ifthey have well-developed A horizonstructure.

Good soil structure also means goodaeration and a favorable balance betweenpores that contain air and pores that storewater for plant use. Soils with good struc-ture are easy to work and provide idealenvironments for plant root growth. Inshort, good structure means good tilth.

Organic matter is vital to the formationand maintenance of good soil structure. Ahorizons of western Oregon soils arenaturally high in organic matter. Soilstructure in these horizons tends to bewell developed, and peds resist break-down from tillage and raindrop impact.

A horizons of eastern Oregon soils arenaturally low in organic matter. Soilstructure tends to be weakly formed andunstable. These soils are more difficult toirrigate, and they have a higher erosionhazard.

Keeping up the organic matter level isessential if you want to maintain good soilstructure. Mixing animal wastes and cropresidues into the soil is an excellent wayto do this. One of the real benefits ofconservation tillage programs is the useof crop residues to form stable soil struc-ture.

You can determine both the type and thegrade of soil structure by carefully observ-ing the soil and by gently breaking it apart.The first step is to study the pit face to seeif structural peds are evident. If you candetect the shapes of individual peds, thenthe grade is probably strong.

The next step is to fill your hand with alarge “lump” of soil. Observe how easilythe soil breaks out of the pit face and fallsinto your hand. The easier it breaks out,the stronger the structure. Observe alsothe shapes of the peds that lie in yourhand.

Then hold a large piece of the soil inboth hands and gently apply pressure tobreak the soil apart (Figure 12). If the soilbreaks easily along a natural plane ofweakness, you’ve separated it into distinctpeds. If the soil fractures randomly leavingan irregular, dull surface, you’ve simplyforced a break through a ped.

The ease with which the soil massbreaks into peds, and the amount ofunaggregated soil that remains in yourhand, together indicate the structural

Figure 12.—Determining soil structure. Hold a clod of soil inboth hands and apply gentle pressure. If the soil breaks easilyalong a natural plane of weakness, it is breaking into units ofsoil structure.

Soil Structure


26

grade. The shapes of the peds you brokeout of the soil indicate the structural type.

Structure typeCommon types (or shapes) of soil

structure include granular, platy, blocky,and prismatic. Soils lacking peds are saidto have either massive or single graintypes of structure. Each of these commonstructure types is illustrated and dis-cussed on page 27.

Structure gradeThe grade of soil structure refers to the

strength and stability of structural peds.Structure grade is described using theterms strong, moderate, and weak. Defini-tions are in the box on this page.

Strong structures are stable structures.They provide favorable air-water relationsand good soil tilth. Weak structures areunstable. Surface soils readily slake andseal when irrigated or tilled. Weak struc-tures slow down water movement intoand within the soil and increase theerosion hazard.

Two aspects of structural developmentwork together to indicate the grade:

1. How well the entire soil mass issubdivided into distinct peds

2. How well the grains in individualpeds are held together to resistbreakdown and give the peds stabil-ity

Compound structureSome soil horizons have large structural

aggregates that can be further subdividedinto smaller aggregates of a differentshape. Examples are blocks that breakinto plates and prisms that break intoblocks.

Technical soil descriptions wouldinclude both situations. For soil judging, ifone shape has a stronger grade, checkthat one. If both have the same grade,mark the smaller one.

Grades of Soil Structure

Strong—The soil mass is welldivided into distinct and easilyrecognizable peds. Structure isreadily apparent on the face of asoil pit. A handful of soil readilybreaks into distinct peds withonly very gentle pressure. Indi-vidual peds are stable and resistfurther breakdown. Very little ifany soil remains as loose grainsnot bound into peds.

Moderate—Peds are evident in apit face, and they are readilyapparent when you gently breakapart a mass of soil held in yourhands. Some grains of soil maynot be part of any aggregate, oreasily slough off larger aggre-gates. Peds are stable againstweak forces, but may breakdown under stronger pressure.

Weak—Peds are difficult to detect,even when you break soil apartin your hands. Many grains maynot be part of any aggregate.Peds easily break down whensmall forces are applied.

Structureless—The grade appliedto massive and single grain soils.



27

Granular—roughly spherical, like grape nuts. Usually1–10 mm in diameter. Most common in A horizons,where plant roots, microorganisms, and sticky productsof organic matter decomposition bind soil grains intogranular aggregates.

Platy—flat peds that lie horizontally in the soil. Most areless than 2 cm thick. Platy structure is not common inwestern Oregon soils, but it can occur in a tillage pan atthe base of an Ap or in soil horizons where water isforced to move sideways. Many soils in the low rainfallregions (less than 12 inches) of eastern Oregon do haveplaty structure in the A horizon.

Blocky—roughly cube-shaped, with more or less flatsurfaces. If edges and corners remain sharp, we call itangular blocky. If they are rounded, we call it subangularblocky. Sizes commonly range from 5–50 mm across.Blocky structures are typical of B horizons, especiallyBt horizons. They form by repeated expansion andcontraction of clay minerals.

Prismatic—larger, vertically elongated blocks, often withfive sides. Sizes are commonly 10–100 mm across.Prismatic structures occur in some B and BC horizons.

Massive—compact, coherent soil not separated into pedsof any kind. Massive structures in clayey soils usuallyhave very small pores, slow permeability, and pooraeration.

Single grain—in some very sandy soils, every grain actsindependently, and there is no binding agent to hold thegrains together into peds. Permeability is rapid, butfertility and water-holding capacity are low.

Types of Soil Structure

Soil Structure


28

Soil ConsistenceConsistence has to do with the strength

of the soil mass. Consistence describes theresistance soil offers to pressures thatcould break it or change its shape. It isalso related to a soil’s ability to supportthe weight of traffic and structural loads.

When soil resistance is low, the soil iseasily manipulated. Energy requirementsfor plowing or tillage are low, and plantseedlings can easily push their way upthrough the soil.

When the resistance is high, the soil ismuch more difficult to work. Cloddyseedbeds are more likely, and seedlingemergence is more difficult.

You can determine consistence by takinga handful-sized mass of soil and applyingpressure between the thumb and fingers.Because moisture content greatly affectsthe strength of the soil, consistence isdetermined at three different moisturelevels: moist, dry, and wet.

In western Oregon, moistconsistence is the one mostfrequently determined. Ineastern Oregon, it is dry consis-tence. You can determine wetconsistence for any sample byadding the proper amount ofwater.

Moist consistenceMoist consistence is deter-

mined when the soil is at, or alittle drier than, field capacity.Usually when soil is found to bemoist—but not saturated—inthe field, the soil is near fieldcapacity, and we can determinethe moist consistence. Commonclasses of moist consistenceinclude loose, friable, firm, andextremely firm.

Classes of Dry Consistence

Loose—The soil does not sticktogether in a mass.

Slightly hard—The soil crusheseasily under gentle pressurebetween thumb and forefinger.

Hard—The soil is barelycrushable between thumb andforefinger, but it can be brokenwith the hands without muchdifficulty.

Extremely hard—The soil is soresistant to deformation that itcan’t be broken in the hands.

Classes of Moist Consistence

Loose—The soil does not stick together ina mass.

Friable—Soil material crushes easily undergentle pressure between thumb andforefinger.

Firm—Soil can be crushed between thumband forefinger, but considerable pres-sure is required to overcome naturalresistance.

Extremely firm—Soil can’t be crushedbetween thumb and forefinger, and canonly be broken apart bit by bit.



29

Dry consistenceThis is determined for soils that are air

dry, even in the field. Although it may bedescribed for any soil, it usually is thepreferred system for eastern Oregon soils.Classes of dry consistence include loose,slightly hard, hard, and extremely hard.

Wet consistenceWhen the soil is a little wetter than field

capacity, two kinds of soil behavior areevaluated, stickiness and plasticity. Sticki-ness is a measure of how much the soilclings to a foreign object like a shovel or atractor tire or your hands. Plasticity is ameasure of the extent to which a soilclings to itself and can retain a shape afterpressure is removed.

Stickiness is evaluated by squeezingwet soil between your thumb and fingers,then slowly pulling your thumb away. Thestickier the soil, the more it will stretchand pull apart before breaking, leavingsoil sticking to both thumb and fingers.The relative degree of stickiness isexpressed using the terms nonsticky,slightly sticky, sticky, and very sticky.

Plasticity is determined by rolling thesoil between your hands to form a thinwire. The longer and thinner the wire youform, and the more resistant it is to whip-ping back and forth, the more plastic thesoil. Terms used to describe plasticity arenonplastic, slightly plastic, plastic, and veryplastic.

Horizon BoundariesThe boundary between any two hori-

zons can vary both in distinctness and inform. Some boundaries are very sharp.Others merge very gradually into thehorizon below. The nature of the boundarymay provide clues to soil development andto certain aspects of soil behavior. Anabrupt boundary, for example, may indi-cate a sudden change to another kind ofmaterial, either geologic or formed by soildevelopment. Such a change may limitroot penetration, or it may signal a differ-ent rate of water movement through thesoil. Gradual boundaries, on the otherhand, may indicate a very young soil, or adeep, highly weathered, old soil.

Classes of Soil Stickiness

Nonsticky—Wet soil doesn’tstick to fingers.

Slightly sticky—Wet soil sticksto one finger only.

Sticky—Wet soil sticks to bothfingers and thumb, and itstretches a little before break-ing as fingers are separated.

Very sticky—Wet soil stronglysticks to both fingers andthumb and it stretches consid-erably before breaking.

Classes of Soil Plasticity

Nonplastic—Can’t form a wire byrolling between hands.

Slightly plastic—Only short wires(<1 cm) will form.

Plastic—Long wires (>1 cm) willform, but wire breaks whenwhipped back and forth.

Very plastic—Long wires (>1 cm)will form, and wire withstandswhipping back and forth with-out breaking.

Horizon Boundaries


30

The form, or shape, of horizon bound-aries also may be described. Evaluation ofthis characteristic, however, requirescareful examination all along a pit face orroad cut to be sure that you have discov-ered the true relationships between soilhorizons.

Terms used to describe boundarydistinctness are abrupt, clear, gradual, anddiffuse. Terms used to describe boundaryform are smooth, wavy, irregular, andbroken. Both sets of terms are defined inthe boxes below.

Special Featuresof Soil Horizons

Some horizons have unique propertiesand deserve special emphasis. Thesefeatures are important because they havea significant impact on the behavior of thewhole soil. Most of them restrict the flowof air and water through the soil. Theyalso limit the depth of rooting.

As a result, soils that contain any ofthese special features are likely to requirespecial kinds of management practices inorder to overcome the limitations.

Some special features, namely thefragipan, duripan, and Cr, are nothingmore than special kinds of master hori-zons. These are discussed in detail inChapter 3. Tillage pans are special fea-tures of some Ap horizons, and they occuronly in the lower part of these horizons.Slickensides are a special feature of clayeysoils that shrink and swell a great deal.

Each of these special features isdescribed below. Obviously if a soil doesnot contain any of these special features,the correct answer to mark on yourscorecard is none.

Tillage panA tillage pan is a compacted zone at the

base of a plow layer, or Ap horizon. Itforms by repeated plowing or cultivationof the soil when it is too wet. The plowsmears the wet soil, breaking down thenatural structure. The result is a zone ofsoil 1 or 2 inches thick that is eithermassive or has a coarse platy structure. Ineither case, the tillage pan restricts themovement of water to deeper parts of thesoil profile.

Some tillage pans also have a thin layerof undecomposed straw right at the top ofthe compacted zone. When a field contain-ing a lot of straw residue is plowed, thestraw is buried under 6 or 7 inches of soilwithout any real mixing. In this position,air can’t get to the straw, and it just staysin the soil without decaying.

Classes of HorizonBoundary Distinctness

Abrupt—The boundary is lessthan 1 inch (2 cm) wide.

Clear—The boundary is 1 to2 inches (2 to 5 cm) wide.

Gradual—The boundary is 2 to5 inches (5 to 15 cm) wide.

Diffuse—The boundary is morethan 5 inches (>15 cm) wide.

Classes of HorizonBoundary Form

Smooth—Nearly a plane.

Wavy—Shallow pockets arewider than they are deep.

Irregular—Pockets are deeperthan their width.

Broken—Parts of the horizon areunconnected with other parts.



31

It is important that you recognize atillage pan, because it creates unfavorableconditions for plant growth. Root penetra-tion may be limited, but the greatestproblem is that a tillage pan restrictsdownward water movement. This is aserious problem in dryland areas, becauseit prevents the subsoil from receiving allthe water it could and storing it for laterplant use.

In wetter areas, the pan may cause theAp horizon to fill up with water. Thisreduces the rate of water entry into thesoil and increases the hazard of runoff anderosion.

The best way to deal with tillage pans isto prevent them from forming. One way todo that is to avoid plowing or tilling thesoil when it is too wet. Ideally, the soilshould be allowed to dry for several daysafter a heavy rain before working it again.

Another way to prevent the pan fromforming is to use minimum tillage or no-tillpractices. Tillage pans that do exist usu-ally can be broken up by ripping ordeep-plowing the soil.

FragipanA fragipan is a massive, dense subsoil

Bx horizon (see definition on page 11). It isnot cemented. The fragipan illustrated inPlate 11 is so dense that water cannotmove through it. Instead, water builds upon top of the fragipan and moves side-ways, forming the white E horizon. Notealso the gray streaks of silt that are typicalof fragipans.

Soils with fragipans generally are notgood agricultural soils, and they posesevere limitations for homesite develop-ment as well. In most soils it is not practi-cal to break the pan up by ripping, or todrain the soil with tile lines placed into orbeneath the pan. Sometimes tile linesplaced above the pan help remove someof the excess water, but in most soilswetness remains a severe limitation.

DuripanA duripan is a massive Bkqm horizon

that is cemented with both silica andcalcium carbonate (see definition onpage 12). Duripans are quite common ineastern Oregon.

The duripan shown in Plate 10 is sostrongly cemented that it does not sloughoff an exposed bank as easily as the hori-zons above and below it. As a result, theduripan sticks out from the face of theexposure. Duripans that are less stronglycemented may be broken up by ripping.

CrA Cr horizon consists either of strongly

weathered bedrock or of naturally verysoft rock (see definition on page 12). Ineither case, you can slice Cr material witha knife or a spade.

The Cr horizon shown in Plate 13 is softsandstone bedrock in the Coast Range.Other Cr horizons may be multicoloredbecause of weathering. Don’t confuse theircolors with mottles caused by wetness(see page 18). Soils that contain Cr hori-zons usually occur on sloping, hilly land-scapes.

SlickensidesSlickensides (Figure 13) are polished,

shiny surfaces caused by the movement oftwo masses of soil past each other. Theyare characteristic of special kinds of soilscalled Vertisols.

Vertisols are soils that have a very highclay content, and they are very sticky andvery plastic. When the clays wet up, theyexpand, causing the soil to swell andheave.

As Vertisols dry out, the clay particlesshrink, and the soil develops numerousopen cracks. These cracks may extend to adepth of 2 feet or more, and they may be1 to 4 inches wide at the surface. In thisdry condition, the soil may appear to havestrong blocky structure.

Special Features of Soil Horizons


32

When a Vertisol is dryand cracked, soil from thesurface often tumbles intothe open cracks and partlyfills them. Then as the soilwets up during the nextrainy season, the clayparticles expand. But sincethere is more material therethan before, the swellingcreates strong pressuresthat cause masses of soil toslide past each other. Thisshearing movement formsthe polished surfaces calledslickensides. Swelling alsomay result in a bumpy,irregular soil surface.

Wet, swollen soil has nostructure and is bestdescribed as being “massivewith intersecting slicken-sides.” This is true even if the soil is dryand cracked and appears to be blocky. Ifslickensides are present, you shouldrecord the structure type of the horizon asmassive and the structure grade as struc-tureless.

Slickensides, then, are important specialfeatures because they indicate the

presence of Vertisols, which have seriousmanagement limitations. Vertisols havevery slow permeability, and they usuallyare poorly drained. Both the drainagefeasibility and the irrigation suitability arepoor. The most intensive crop under bothirrigated and dryland conditions is perma-nent pasture.

Figure 13.—Slickensides. Shiny, polished, and wavy surfacesare formed when two masses of very clayey soil move pasteach other. Pressures generated by swelling clays cause thismovement. Slickensides are characteristic of Vertisols.



33

Color, texture, and structure arethe primary properties of soilhorizons. They are observed

directly in the field.Properties of the whole soil, however,

are not so easy to observe directly. It ispossible to make field measurements ofwater content, rates of water movement,or erosion hazard—but they require agreat deal of time, talent, and expensiveequipment.

We can still judge these aspects of soilbehavior, though. Instead of making directmeasurements, we will use information oncolor, texture, and structure to estimatewater-holding capacity, permeability,internal drainage, and several otherproperties. These estimates will enable usto learn a great deal about how the soilwill respond to agricultural use andmanagement.

The sections that follow describeseveral key properties of the soil and tellhow and why each is important. They alsotell you how to use color, texture, andstructure to estimate important kinds ofsoil behavior.

Many interpretations, both in thischapter and in Chapter 7, can be con-densed into a tabular guide. The proce-dure is explained fully in Appendix B. Besure you know how to use these interpre-tation guides, for they allow you to use afew basic soil properties to find thecorrect answer to a given interpretation.

Effective Depth of RootingMany plants extend roots to depths well

beyond 3 feet, provided there is no physi-cal barrier to root growth. Soils that allowdeep rooting are potentially very produc-tive—plants that grow in them can use thegreatest possible volume of soil in searchof water and nutrients.

Soils that have restricted rooting depthsare droughtier and may require morefrequent irrigation. They may requiremore fertilizer as well.

The effective depth of rooting is simplythe distance from the ground surface tothe top of any soil horizon that preventssignificant root penetration. Very dense orcemented horizons, and very gravelly orcobbly horizons, all limit root develop-ment.

Fragipans and claypans are notcemented, but they are so dense and havesuch poor structure that roots can’tpenetrate very far. In a duripan, the poresare actually filled with a hard cement thatbinds soil grains together into a rocklikehorizon. Bedrock, of course, is an obviouslimit to root development.

Subsoil horizons of very gravelly orcobbly sands are effective barriers to rootgrowth. Roots will develop in finer-textured soil above these layers, but theydon’t expand into the gravelly soil. Thereason is that the gravelly soil often ismuch drier than the fine-textured soil, androot growth is confined to the moist soilabove it. If the soil is sandy and gravellyright at the surface, though, plant rootswill extend throughout the coarse-textured soil material.

Soil color, texture, structure, and den-sity each provide clues for judging theeffective depth. Soils that have brown orred colors throughout usually allow deeprooting. These colors indicate gooddrainage and good aeration, both of whichfavor deep root penetration.

Gray colors usually indicate soil wet-ness. Roots of many agricultural cropsdon’t grow well in soil that is saturated forlong periods of time. But if the water tableis not present during the growing season,or if it has been lowered with artificial

Properties of the Whole Soil

Chapter 5


34

drainage, then gray colors don’t necessar-ily indicate a limitation to root develop-ment. Be suspicious of gray colors, butcheck the texture, structure, and densitybefore you make a decision.

Soil texture limits root growth only ifthe texture changes abruptly from onehorizon to another. Silt loam over clay, orloam over gravelly sand, are commonexamples of root-limiting textures.

Textures that are nearly uniformthroughout, even in clayey or gravellysoils, are not likely to prevent rootgrowth. Other factors, however, may beresponsible for limited rooting in suchsoils.

Structure and density work together toinfluence rooting depth. Moderate andstrong structures always favor root devel-opment. Weak structures and massive soilhorizons may or may not limit rooting,depending on the density.

In a fragipan, for example, massive,dense silt loam restricts rooting. In manyother soils, however, silt loam C horizonsare open and porous and do not preventrooting at all.

Evaluate the effects of structure anddensity by using your knife to probe thesoil. If it probes easily and breaks offreadily, structure and density probablydon’t limit roots. The opposite response isa sure sign of trouble.

The roots themselves may give us otherclues to the effective depth of rooting. Ifroots are clearly visible and easy to findthroughout the soil, then there is no limitto the effective depth.

Sometimes, though, only shallow-rootedplants are growing in the soil, even thoughthe effective depth is unlimited. Absenceof roots, therefore, doesn’t necessarilymean there is a root depth limitation.

On the other hand, some plants may beable to send a few very fine roots partlyinto a fragipan, a claypan, or a gravellyhorizon. Even so, most of the volume ofthese horizons can’t support plantgrowth. So if you find one tiny root down

at the bottom of the profile, that doesn’tnecessarily mean the effective depth isunlimited.

The pattern of root growth also mayhelp you. Duripans, fragipans, and someclaypans may be so firm that plant rootsstart growing horizontally along the top ofthem. A horizontal root mat is goodevidence of the limit of the effectiverooting depth.

Available Water-holdingCapacity

Available water-holding capacity(AWHC) refers to the amount of water asoil can store for plants to use. Since soilprovides the only reservoir of water forplants to draw upon, the size of thatreservoir is one of the most importantproperties of the whole soil.

Soils having high AWHC’s are potentiallyvery productive. Soils having low AWHC’sare droughty, more difficult to irrigate,and generally less productive.

Only a portion of the total amount ofwater contained in a soil is available toplants. We think in terms of three classesof soil water: gravitational, available, andunavailable.

Classes of Effective Depth

Deep — over 40 inches(>100 cm)

Moderately — 20 to 40 inchesdeep (50 to 100 cm)

Shallow — 10 to 20 inches(25 to 50 cm)

Very shallow — less than 10 inches (<25 cm)

Chapter 5 • Properties of the Whole Soil


35

Classes of Soil AWHC

High More than 8 inches

Medium 5 to 8 inches

Low 2 to 5 inches

Very low Less than 2 inches

Gravitational water fills large poreswhen the soil is saturated. It drains awayquickly as soon as the water table dropsor the rain stops. Plants can’t make use ofgravitational water.

Available water is held in smaller soilpores against the force of gravity. Plantscan exert enough force to remove thiswater and use it.

Unavailable water is held so tightly intiny soil pores that plant roots can’tremove it. When a soil is so dry that onlyunavailable water remains, plants wilt anddie, even though there is still some mois-ture left in the soil. Some soils, especiallyclays, contain large amounts of water thatis unavailable for plant use.

The available water-holding capacity ofa soil depends mainly on its texture,coarse fragments, and effective depth ofrooting. Together they determine thevolume of soil pores that are the right sizefor storing available water.

Both structure and organic matterincrease the volume of water-storingpores a little bit. For most soils, however,we can make a good estimate of the AWHCby evaluating the texture and coarsefragments of each horizon within theeffective depth of rooting.

Think of a soil as a giant sponge. Sup-pose we could squeeze this soil-spongejust enough to remove all the availablewater. If we could catch all this water in apan with the same bottom area as oursoil-sponge, then the depth of water in thepan is a measure of the AWHC of the soil.

Values of AWHC range from 1 or2 inches of available water in a verygravelly soil to 12 inches or more in adeep, well drained silt loam.

Each class of soil texture has a charac-teristic AWHC. We express that AWHC asinches of water per inch of soil depth. Tocalculate the AWHC for any single horizon,multiply the inches per inch AWHC timesthe total thickness of the horizon.

To determine the AWHC for the wholesoil, repeat this calculation for each hori-zon using the appropriate AWHC. Coarsefragments can’t store water, so for horizonsthat contain coarse fragments, multiplyAWHC x Depth x Percent Fine Earth. Thetotal AWHC for the whole soil then is thesum of the AWHC’s for each horizon withinthe rooting depth.

AWHC Rates

Texture Inches/Inch

Sand .06Loamy sand .06

Sandy loam .12

Loam .20Clay loam .20Silt loam .20Silty clay loam .20

Clay .15Silty clay .15Sandy clay .15Sandy clay loam .15

Available Water-holding Capacity


36

Field Procedurefor Estimating Soil AWHC

1. Identify the horizons present in the soil profile.

2. Measure the thickness of each horizon.

3. Determine the effective depth of rooting.

4. Determine the texture and the coarse fragmentcontent.

5. Find the AWHC rate that corresponds to thetexture of each horizon.

6. Multiply AWHC x Depth x Percent Fine Earth.

7. Total the AWHC’s for all horizons within theeffective depth.

PermeabilityPermeability is a term that describes the

rate of water movement through the soil.Because water moves through the pores ofthe soil, (the spaces between the grains ofsand, silt, and clay), the rate of watermovement depends on the amount of porespace (porosity), the size of the pores, andthe connections between the pores.

We can’t measure these characteristicsof pores directly. We do know, however,that porosity and permeability are closelyrelated to soil texture and structure. Thus,we can estimate permeability in the fieldby carefully observing the texture andstructure of soil horizons.

Soil layering is another important factorthat affects water movement through thesoil. Any time the texture, structure, ordensity change abruptly, the rate of watermovement changes, too.

Sample Calculations of AWHC

% Coarse Thick- FractionHor. Depth Texture Fragments AWHC ness Fine Earth AWHC

A 0–12 Silt loam 0 0.2 x 12 x 1.0 = 2.4

BA 12–20 Silt loam 0 0.2 x 8 x 1.0 = 1.6

Bt 20–36 Silty clay loam 0 0.2 x 16 x 1.0 = 3.2

BC 36–48 Silty clay loam 0 0.2 x 12 x 1.0 = 2.4

C 48–60 Silt loam 0 0.2 x 12 x 1.0 = 2.4

Total Soil AWHC = 12.0AWHC is high

A 0–4 Loam 0 0.2 x 4 x 1.0 = 0.8

BA 4–10 Clay loam 0 0.2 x 6 x 1.0 = 1.2

Bw 10–18 Grav. clay loam 30 0.2 x 8 x 0.7 = 1.1

Bkqm 18–28 (Duripan) 100 — — — 0

Ck 28–40 Loam 10 — — — 0

Total Soil AWHC = 3.1AWHC is low



37

A tillage pan, for example, has poorerstructure and much smaller pores thanthe soil in the Ap above it. Water can’tflow through the pan as fast as it canthrough the soil above. That’s why,during heavy rainfall or irrigation, all thepores in the soil above the pan may fillwith water. Any additional water that fallson the soil surface must run off, and thatincreases the hazard of erosion.

Subsoil layers that affect permeabilityinclude claypans and fragipans. Bothhave very small pores, and little or nostructure to create larger pores. Watermoves very slowly through these layers,and water moving through the soil abovethem tends to build up, or perch on topof them.

The change in permeability caused bythese layers often forces water to movesideways on top of them. That’s why weoften see leached E horizons immediatelyabove claypans and fragipans.

Because permeability depends on theamount and size of soil pores—and onhow well interconnected they are—anysoil property that increases any of thesefactors increases permeability.

Sandy and gravelly soils have large,well-connected pores and rapid perme-ability. Clayey soils have tiny pores. Theirpermeability is slow, unless well-devel-oped structure creates some larger pores.Silt loams and clay loams tend to havemoderate permeability, especially if thestructure is moderate or strong.

Even if the structure is weak, thepermeability can be moderate as long asthe soil is loose and porous. But if the soilis very dense and difficult to break outwith a knife, the soil is not porous, andpermeability is likely to be slow.

Slickensides indicate very slow perme-ability. Any horizon that contains slicken-sides will be so swollen when wet and willhave pores so small that water can hardlymove at all.

Because soil layering creates differentpermeability rates in different parts of thesoil, we’re going to make separatejudgments of surface soil permeability andof subsoil permeability. In both cases,we’ll judge permeability as rapid, moder-ate, slow, or very slow according to thestandards in the box on page 38.

Surface soil permeabilityThe rate of water movement through

surface soil directly affects irrigation,runoff, and erosion. Soils best suited forcenter pivot irrigation should have rapidpermeability in the surface horizon.Otherwise water applied at the outer endof the pivot may run off. Soils that haveslower surface soil permeability requiredifferent kinds of irrigation systems thatwill deliver water at acceptable rates.

Slow permeability of the surface soilalso increases the hazard of erosion. Ifwater applied to the surface, either duringa rainstorm or during irrigation, can’t getinto the soil, then it must run off over thesurface.

Figure 14.—Surface soil permeability. Characteristics of thesurface soil affect the soil’s ability to absorb irrigation water.Furrow irrigation is a good method for soils that have slowpermeability in the surface horizon.

Permeability


38

Besides causing erosion, runoff wasteswater that could be stored in the soil forplant use. In dryland wheat country, everyinch of water that runs off costs about7 bushels of wheat. Clearly, it pays tomaintain the best rates of surface soilpermeability possible.

One way to maintain good permeabilityis to incorporate crop residues so as tokeep up the amount of organic matter inthe surface soil. Organic matter encour-ages the formation of soil structure andimproves the grade of the structure. Inthis way organic matter helps createrelatively large and well-connected pores,which allow good rates of water move-ment through the soil.

Soils that are low in organic matter,especially silt loam and silty clay loamsoils, are very susceptible to slaking, orstructural breakdown. The impact offalling raindrops on weak peds in thesesoils causes the peds to disintegrate.Individual particles of silt and clay thenclog up larger pores.

The result of slaking is a very thinsurface crust that greatly reduces waterentry into the soil. Runoff and erosion areserious problems in these soils.

Another way to maintain good perme-ability of surface soils is to stay off themwhen they’re wet.

Driving over wet soils compacts them—it reduces the total pore space, anddestroys the large pores needed for goodpermeability.

Tillage implements drawn through wetsoil smear and compact the soil at thebase of the Ap, forming a tillage pan. Ifyou must plow the soil under wetter thanideal conditions, you should at least try tovary the depth of plowing from year toyear. In some cases, you may still have tobreak up a tillage pan by ripping. And inall cases, adding crop residues always is agood way to help rebuild good soil struc-ture.

When you judge the permeability of thesurface soil, consider the effects of tex-ture, structure, and porosity in all parts ofthe surface horizon. It’s easy if theyremain the same throughout.

Often, however, the A or Ap horizonmay contain two or three different struc-tures. For example, the top inch or so mayhave a strong granular structure becauseof the roots of sod-forming crops. Themiddle may be compacted from traffic,and the lower part may have the massiveor platy structure of a tillage pan.

In any case, the permeability of theentire horizon can never be greater thanthe part with the slowest permeability. If a

Guide for Determining Soil Permeability

Rapid Moderate Slow Moderate Slow Moderate Slow Very Slow

Texture Sand Sandy loam Sandy clay loam Sandy clayLoamy sand Silt loam — Silty clay loam — Silty clay — —

Loam Clay loam Clay

Porosity Any Porous Not Porous Not Porous — Notporous porous porous

Structure Any Any Any Any Weak, Strong Moderate, Massive,grade Massive Weak Vertisol



39

tillage pan is present, the permeabilityusually will be slow, regardless of thetextures and structures above the pan.

Subsoil permeabilityWater movement through B and C

horizons affects soil drainage, leaching ofsalts and fertilizers, and performance ofseptic tank drainfields.

Slowly permeable soils are difficult todrain with tile lines. Water moves soslowly toward the drain lines that theymust be closely spaced in the soil, andthat is expensive.

Slowly permeable soils also are bad forseptic tank drainfields because the soilnear the distribution pipes is likely tobecome saturated and cause drainfieldfailure.

Rapidly permeable soils are readilyleached. Soluble salts, especially nitrogenfertilizers, are easily lost from the soilwithout doing the crops any good. Theyalso contaminate groundwater.

Rapidly permeable soils don’t makegood waste disposal sites, either. Effluentfrom a septic tank drainfield is likely toreach groundwater too quickly to receiveadequate biological treatment. Similarly,sanitary landfills placed on rapidly perme-able soils increase the hazard of leachingdangerous chemicals into the groundwater.

Because the texture, structure, andporosity may change from horizon tohorizon in the subsoil, you must evaluatethe permeability of each horizon individu-ally. The overall permeability of thesubsoil is that of the least permeablehorizon within the subsoil.

It is standard practice to base theevaluation of soil permeability only onhorizons of porous soil material. MostB and C horizons, including fragipansand claypans, are considered in evaluat-ing subsoil permeability. But if the soilhas an R, a Cr, or a Bkqm (duripan)horizon, don’t consider them. Base thepermeability evaluation only on those

subsoil horizons above R, Cr, or Bkqmhorizons.

Normally we would evaluate the perme-ability of all subsoil horizons down to adepth of 60 inches. For some managementpractices, however, restricted permeabil-ity deep in the soil may not have muchinfluence on the way we manage the soilfor crop production.

We should be able to drain a wet soileasily, for example, as long as the soil hasmoderate permeability down to a depth ofat least 30 inches. For this reason, we willuse one more rule to judge subsoil perme-ability. Base your permeability evaluationonly on soil between the bottom of the A orAp horizon and a depth of 30 inches.

Water Erosion HazardThe hazard of soil erosion by water is

an important concern for management ofcultivated soils. Erosion damages both theproductivity of the soil and the quality ofwater in rivers and streams.

Erodibility is closely related to the slopeof the soil and the amount of runoff. Asboth increase, so does the erosion hazard.

Runoff is difficult to measure. But it isdirectly related to the texture and thepermeability of the surface horizon. Thesetwo properties, plus the slope of the soil,are therefore used to evaluate the watererosion hazard. Use the “Guide for Deter-mining Water Erosion Hazard” (page 40) tocomplete your evaluation.

Of all the soil textures, silt loam is themost erodible. That’s because the size ofsilt particles is just right for water toloosen and carry over the soil surface.Sand particles are too big to easily dis-lodge and move. Clays are so small andflat that they are not easily dislodged,either.

One other soil factor that may affecterosion is soil depth. Soils that are shal-low to restrictive layers (slowly perme-able horizons) like bedrock, fragipans, orclaypans are more erodible than deep

Water Erosion Hazard


40

soils. Their capacity to hold water is solow that extra water quickly runs off. Thisincreases the length of time during whichadditional rainfall could cause damagingerosion.

Vegetation and climate are not soilfactors, but they do influence the erosionhazard. Natural forest provides the besterosion protection, even on very steep

Guide for Determining Water Erosion Hazard

Low Moderate High Low Moderate High Very high

Texture Sand Sandy clay loamLoamy sand Silty clay loamSandy loam — — Clay loam — — —Sandy clay Silt loamSilty clay LoamClay

Slope Permeability 0–12/Any 12–20/Any >20/Any 0–3/Any 3–7/Slow, 7–12/Slow, >20/Any of surface 3–7/Rapid, V. Slow V. Slow soil Mod. 7–12/Rapid, 12–20/Any

Mod.

Depth In western Oregon, increase the erosion hazard by one classif the soil is less than 20 inches deep.

Figure 15.—Water erosion. Water flowing over unprotected soilcuts rills and gullies into the soil and leaves sediment depositsat the base of the hill. Lost topsoil reduces soil productivity,and sediment deposits damage growing crops.

slopes, because the O horizons are highlypermeable.

Solid cover crops like pasture and hayalso reduce erodibility. They promotewater entry, absorb the impact of fallingraindrops, reduce the velocity of flowacross the surface, and tend to bind soilparticles together and hold them in place.

Forest clearcuts and clean-tilled rowcrops tend to increase the erosion hazard.In both cases, water drops strike soilparticles directly, causing some break-down of soil structure and reducing therate of water entry into the soil. Andwithout vegetation, there are no roots tohold the soil in place and no stems andleaves to slow down the velocity of waterrunning over the soil surface.

Climate affects erosion through theintensity of individual rain storms. Thetotal amount of yearly rainfall is far lessimportant than the intensity of eachstorm. Many storms are not particularlyerosive because the rate of rainfall is slowenough to allow it all to soak in.

Occasionally, however, it rains so hardthat the soil cannot absorb it fast enough.Runoff starts almost immediately, and thevolume of runoff can be large. When thishappens, erodible soils not protected byvegetation can lose tremendous volumes



41

of soil. The result can be economic loss tofarmers and degradation of the environ-ment for all of us.

It is standard practice to judge thehazard of soil erosion under the worstpossible conditions. That means bare soil,without vegetation, and without any kindof soil conservation practice to slow downwater running over the surface.

Most soils you see won’t be managedthis poorly. But the erosion hazard isalways present, and it’s essential that wemanage erodible soils to protect themfrom damage by erosion. Judging theerosion hazard in its most susceptiblecondition is therefore a measure of theconservation task required.

Wind Erosion HazardClimatic conditions in eastern Oregon

favor soil blowing. Wind erosion is costlyto agriculture because valuable topsoil islost. Blowing sand also is abrasive andcan seriously damage young crops.

Wind erosion is costly to the environ-ment because it reduces air quality andcreates a dust nuisance. Road ditches mayquickly fill with sediment and requirefrequent cleaning.

Wind erosion also can be costly tohuman lives. Blowing sand and silt reducevisibility markedly. A few years ago,blowing sand was the direct cause of anaccident on I-84 that involved several

vehicles and resulted in a spectacular fireand the loss of several lives.

Besides a climate that favors strong,steady winds, the major soil factor thataffects wind erosion is surface texture.Loam and silt loam soils will blow, butsandy loams and loamy sands are worse.That’s because fine sand particles areeasily picked up by the wind, and they aremore abrasive.

To judge wind erosion hazard, considerfirst the location in Oregon, then thetexture of the surface horizon. As withwater erosion, evaluate the soil in theworst possible condition—with no vegeta-tion cover.

Figure 16.—Wind erosion. Blowing soil removes valuabletopsoil, fills road ditches, and accumulates along fence rows.

Guide for Determining Wind Erosion Hazard

Low Low Moderate High

Location Western OR Eastern OR — —

Texture Any Sandy clay loam Loam SandSandy clay Silt loam Loamy sandClay loam Sandy loamSilty clay loamSilty clayClay

Wind Erosion Hazard


42

Internal Soil DrainageWhen all the pores of a soil are full of

water, we say the soil is saturated. The topof a zone of saturated soil is called a watertable. The height of the water table, andthe length of time that the soil remainssaturated, determine the internal drainageof a soil.

We use five classes to describe internalsoil drainage. Rapidly permeable soils thatare never saturated are called excessivelydrained. Moderately permeable soils thatare rarely saturated are well drained.

Soils that are periodically saturated inlower horizons are either moderately welldrained or somewhat poorly drained,depending on the depth to the watertable. Soils that are thoroughly saturatedfor long periods of time are called poorlydrained soils.

Soil drainage is important because itaffects the environment of plant rootgrowth. For most agricultural crops, abouthalf the pores in the soil should containwater. The other half should contain air.For wetland plants and ecosystems,poorly drained soils that remain saturatedfor long periods of time are the preferredhabitat.

When the soil is saturated, roots ofagricultural crops quickly become oxygen-starved, and prolonged saturation killsmany plants. That’s why the choice ofcrop plants is severely limited in poorlydrained soils.

Wet soils also are cold soils. Earlyspring growth is slower because it takeslonger for the soil to warm up. Wet soilscan’t be plowed or cultivated as early inthe year, so planting dates may have to bedelayed. Nitrogen fertilizers aren’t used asefficiently in wet soils, and wetness causesthe loss of some of the nitrogen to theatmosphere. Root rots and other plantdiseases also are more serious in soilsthat are not well drained.

Saturated soils are poorly suited forhomesites, too. Conventional septic tankdrainfields can’t be installed in these soils,

because waste water doesn’t receiveadequate treatment. No one wants waterin the basement, so special precautionsmust be taken when houses are built onpoorly drained soils.

Wet soils are extremely valuable compo-nents of wetland ecosystems. These wetsoils are called hydric soils. These soilssupport plants that provide cover, food,and nesting sites for wildlife. Hydric soilshelp clean up water by enhancing theremoval of nitrogen, phosphorus, andsediments. And they help absorb theenergy of overflowing floodwater.

For these reasons, hydric soils and thewetlands they support are valuable natu-ral resources. That’s why special effortsare made to determine their locations andmake sure they are not further destroyedto support commercial developments.

It usually happens that we study soils inthe field during dry seasons, after watertables have disappeared. Rarely do weobserve water tables directly as theyfluctuate throughout the year. That makesit necessary to determine the drainageclass from clues we find in the permanentproperties of the soil. Color, permeability,kinds of horizons present, soil pH, andlandscape position all enter into ourjudgment of soil drainage.

The most important clues come fromcolor and mottling. Brown, yellow, and redcolors are characteristic of well-oxidized,well drained soils. These soils rarely aresaturated—and when they are, it’s onlyfor very short periods of time.

Dark gray or bluish-gray colors reflectintense reduction, which can only becaused by long periods of saturation.Poorly drained soils have these colors.

Mottles indicate that a soil undergoesrepeated cycles of saturation and oxida-tion. They often represent the effects oftemporary water tables that may beperched above slowly permeable layers.Mottles are present in moderately welldrained and somewhat poorly drainedsoils.



43

Permeability is another clue to internalsoil drainage. It is not the same as drain-age, though. Some rapidly permeablesandy soils may be poorly drained if theylie in a depression that has a permanentlyhigh water table. Some slowly permeablesoils may have good drainage if they areon rounded upland hills.

Slow permeability, though, does suggestthat excess water can’t escape quickly bymoving through the soil. This situationoften leads to buildup of at least tempo-rary water tables.

Subsoil permeability is especiallyvaluable when combined with evidence ofrestrictive layers like fragipans or clay-pans. Because these layers are so slowlypermeable, water does tend to build upabove them, creating perched watertables.

These water tables are temporary, andtheir presence usually is indicated bymottling just above and in the upper partof the restrictive layer. The closer theselayers are to the surface, the more fre-quent and prolonged the perched watertable will be.

On the other hand, restrictive layersdeep in the soil may have little effect oninternal drainage, particularly if the soilabove them is moderately permeable.

Landscape position also providesvaluable information on soil drainage.Soils on convex (rounded, archingupward) uplands tend to lose water bothby runoff and by flow within the soil. Theygenerally are well drained.

Soils lower on the slope, or on concave(saucer-shaped) footslope positions, tendto receive extra water both as runoff andas seepage from higher soils. Water tablesin these landscapes are likely to be closeto the surface periodically. If the soils inthese positions also contain slowly per-meable horizons, they are sure to besomewhat poorly drained or even poorlydrained.

Soils in low-lying areas, or on broad,level landscapes, or in depressions, may

have permanent water tables just a fewinches beneath the surface. They arepoorly drained. Again, slow permeabilitycompounds the problem, although poordrainage conditions can form without it.

Climate affects the internal drainageclass, too. In western Oregon, total rainfallfar exceeds soil capacity to store water.The excess must either run off or movedown through the soil—or else the soilwill have restricted drainage.

In eastern Oregon, the limited amountof rainfall rarely creates this kind ofexcess. Many of the soils are well drained.Only in some of the basins that havenaturally high water tables will we findsoils with restricted drainage. These soilsoften are sodic or alkaline soils as well.

The five classes of internal drainage aredefined below. Soil color and depth tomottles are the primary keys to the rightdrainage class. Remember that some soilsmay have color patterns or coatings onaggregates that are not related to internaldrainage.

The other factors—landscape, perme-ability, restrictive layers, soil pH, andclimate—all are used as supporting evi-dence when determining drainage class.

Excessively drained (ED)These soils have sand to sandy loam

textures and often are gravelly or cobbly(Plate 14). The permeability is rapid. Theyhave the brownish colors of well-oxidizedsoils, and they are not mottled.

Well drained (WD)These soils may have any texture,

though silt loam, silty clay loam, loam,clay loam, and sandy loam are mostcommon. Clayey Vertisols that occur ineastern Oregon also are considered to bewell drained.

Soil colors are various shades ofyellowish-brown and reddish-brown, allindicating well-aerated soil (Plates 1, 2, 3,4, 5, 9, 10, 12, and 13).

Internal Soil Drainage


44

Well-drained soils generally have mod-erate subsoil permeability. They do nothave high water tables, and there are nomottles within 40 inches (1 meter) of thesurface. Occasionally, however, the lowerpart of the soil may be saturated for a dayor two at a time. Thus, the soil below40 inches may have a few gray mottles.

Moderately well drained (MWD)These soils may have any texture.

Subsoil permeability usually is moderateor slow. There may be a deep, restrictivelayer that temporarily perches water(Plate 11). The upper part of the soil isbrown, yellowish-brown, or reddish-brown.

The lower part of the soil is mottledsomewhere between 24 and 40 inches(60 and 100 cm). The mottles may be graymottles in a brownish matrix, or they maybe yellowish-brown mottles in a grayish-brown matrix. It takes only a few gray orbrown mottles to indicate enough wetnessto drop a soil into a moderately well-drained class.

Some clayey Vertisols inwestern Oregon are consid-ered moderately well drained.They occur on convex uplandsand have dark brown colors.

Somewhat poorlydrained (SWP)

These soils may have anytexture and any permeability.In western Oregon, they usu-ally occur in flat or low-lyingpositions that have seasonallyhigh water tables. Some havefragipans or claypans as well.Even a few upland soils aresomewhat poorly drainedbecause they have shallowrestrictive layers.

Somewhat poorly drainedsoils in western Oregon arerarely saturated all the way to

the surface for long periods of time. Theyare not mottled in the Ap or A horizon. Thewater table is high enough to cause somemottling of the soil somewhere between 8and 24 inches (20 and 60 cm).

The second horizon may or may not bemottled—but if it is, the matrix usually isbrown, and the mottles are no more thancommon. A few soils have a gray E horizonthat contains dark mottles (Plate 6). Atincreasing depths, the mottlingbecomes more noticeable.

In eastern Oregon, the mottling patternmay not be as easy to detect. Two kinds ofsomewhat poorly drained soils may occur:1. Black or very dark brown colors

extend throughout the soil. Theseeither have yellowish-brown or reddish-brown mottles below 12 inches (30 cm),or they have a few grayish mottlesthroughout starting right below the Ap.

2. Soil pH values are greater than 9.0. Thisoccurs when the soil has a high sodiumcontent. It indicates that water from ashallow water table rises toward thesurface and evaporates (Figure 17).

Figure 17.—Somewhat poorly drained sodic soil. In semi-arid areas, slick spots of salt accumulation are goodindicators of shallow water tables and somewhat poorlydrained soils. Salt grass in the foreground and greasewoodin the background are other clues to the presence of sodicsoils.



45

Summary of Drainage Class Characteristics

DepthColor to mottles Permeability pH Other

Excessive Brown, Red None Rapid ≤ 8.2

Well Brown, Red > 40 inches Moderate ≤ 8.2 E. Oregon Vertisols

Moderately well Brown, Red 24–40 inches Moderate ≤ 8.2 W. Oregon Vertisols onconvex uplands

Somewhat poorA. Western Oregon Gray subsoil 8–24 inches Mod. or Slow ≤ 8.2B. Eastern Oregon 1. Black 8–12 inches Mod. or Slow ≤ 8.2 2. Black, Brown None Mod. or Slow ≥ 9.0 Sodic soil

PoorA. Western Oregon Black, Gray 0–8 inches Mod. or Slow ≤ 8.2 Gleyed soil may not be

mottled; black Vertisolsin low-lying landscapes.

B. Eastern Oregon Black, Gray Variable Mod. or Slow ≤ 8.4 Gleyed soil may not bemottled.

In both cases, low-lying landscapepositions and dark soil colors indicateperiodically high water tables.

Poorly drained (PD)These soils are saturated at or near the

surface for long periods each year. Theyeither occupy low-lying or depressionalareas that have permanently high watertables, or they have restrictive layersclose to the surface—or both. Any textureor permeability can occur, but fine tex-tures and slow permeabilities are mostcommon.

In western Oregon, poorly drained soilshave either mottles or small black concre-tions (hard pellets) right in the Ap or A. Ina few soils, black colors (caused by highorganic matter content) may completelymask the mottles in surface horizons(Plate 8).

Gleyed horizons are common in poorlydrained soils, but they aren’t required.The gleyed soil may occur right below theAp or it may be a little farther down. Itmay or may not be mottled (Plate 7), but if

it is mottled, common mottling (2 to20 percent) is the norm.

Many western Oregon Vertisols arepoorly drained. They occur in low-lyinglandscape positions and have blackcolors.

Poor drainage is harder to evaluate ineastern Oregon. In all cases, prolongedwetness delays the decomposition oforganic matter, so the soils tend to beblack throughout.

If mottles are present, they are mostlikely to be faint. A few poorly drainedsoils have yellowish-brown or reddish-brown mottles in the dark matrix immedi-ately below the Ap or A.

Others are truly gleyed, so that the soilbelow the A is a gray or dark gray thatbecomes noticeably light gray upondrying. This gleyed soil may or may notbe mottled, but if it is, the mottles tend tobe darker than the matrix and gray orgrayish-brown in color.

Landscapes are low-lying to depres-sional in every case. Poorly drained soilsare not sodic soils, and the pH always isless than 9.0.

Internal Soil Drainage



Plate 1.-Well-drained sandy soil with a black 0 horizon (4-0") , a white E horizon (0-11 ") , a yellowish brown Bs horizon (11-18"), and a light brown C horizon (18-30").

Plate 2.-Well-drained silty soil with an Ap horizon (0-11 "), a thick Bw horizon (11-36"), aBC horizon (36-45"), and a C horizon (45-65") . The dry colors shown are all brown. Note the coarse prismatic structure of the Bw horizon and the massive structure of the C horizon.

Plate 3.-Well-drained silty soil with a black A horizon (0-15"), a brown BA horizon (15-23"), a grayish brown Bt horizon (23-34") , a light brown BC horizon (34-48") , and a light brown C horizon (48-60"). Note the strong prismatic structure in the Bt horizon.

Plate 4.-Well-drained silty soil with a black A horizon (0-12"), a very dark grayish brown AB horizon (12-18"), a brown Bw horizon (18-30"), and a light brown C horizon (30-60").

Plate 1 Plate 2

Plate 3 Plate 4


Plate 5.-Well-drained, strongly weathered soil with a thin A horizon (0-3"), a white E horizon (3-7") , a thick, reddish brown Bt horizon (7-26"), and a BC horizon (26-40").

Plate 6.-Somewhat poorly drained soil with a very dark brown Ap horizon (0-10''), a gray E horizon (10-27"), and a brown Bt horizon (27-45") .

Plate 7.-Poorly drained soil with a black A horizon (0- 15"), a gray Bgl horizon (15-22") that has many coarse prominent yellowish brown mottles , and a gray Bg2 horizon (22-26") that is not mottled.

Plate 8.-Poorly drained soil with a very thick, black A horizon (0-24"), a black, gleyed Bg horizon (24-33"), and a dark gray, gleyed Cg horizon (33-48") .

Plate 5 Plate 6

Plate 7 Plate 8


Plate 9.-Well-drained, partly leached soil with a dark brown Ap horizon (0-5"), a brown Bt horizon (5-16"), a white Bk horizon enriched with calcium carbonate (16-36"), and a light brown C horizon (36-42").

Plate 10.-Well-drained soil containing a duripan. This soil has a brown A horizon (0-4"), a brown Bt horizon (4-14"), and a white Bkqm horizon (14-24") that is thoroughly cemented with calcium carbonate and silica. Note how the cemented duripan sticks out from the face of the soil exposure.

Plate 11.-Moderately welldrained soil containing a fragipan. The horizons are a dark brown A (0- 4") , a brown Bw (4-26"), a white E (26-30"), and a brown Bx (30-60") . Note the wedges of white silt that divide the massive Bx horizon into very coarse prisms.

Plate 12.-Well-drained soil with an R horizon. The soil has a very dark brown Ap horizon (0-8") and a yellowish brown Bw horizon (variable thickness) resting directly on consolidated bedrock.

Plate 9 Plate 10

Plate 11 Plate 12


Plate 13.-Well-drained soil with a Cr horizon. The light brown material at the end of the spade is soft sandstone.

Plate 14.-Excessively drained soil high in coarse fragments. The gravelly A horizon contains about 30 percent coarse fragments . The extremely gravelly C horizon is structureless and single grained, and it contains about 75 percent coarse fragments.

Plate 15.-Strong blocky structure. Notice how natural planes of weakness divide the soil both vertically and horizontally. The darker coatings on ped surfaces are clay skins.

Plate 16.-Gieyed, mottled soil. The soil matrix has a gray color, and the mottles are yellowish brown spots.

Plate 13 Plate 14

Plate 15 Plate 16

I

l


51

Figure 18.—Uplands. The tree-covered hill and the convex, roundedslopes are characteristic of upland landforms.

Site Characteristics

Soils are more than the horizons thatmake up their profiles. They alsoare part of the landscape. The posi-

tion of the soil in the landscape can tell ussomething about the age of the soil andthe kind of geologic materials from whichthe soil developed.

Site characteristics affect runoff, erod-ibility, and internal drainage. They alsoaffect management decisions about irriga-tion, choice of crop, and conservation.

Site evaluation, then, is just as importantin judging soils as the description of theproperties of each horizon. Five major sitecharacteristics are used in soil judging:landform, parent material, stoniness orrockiness, slope, and aspect.

LandformLandforms are distinct parts of the

landscape that have characteristic shapesand are produced by natural geologicprocesses. To evaluate landforms, youneed to cast your eyes in everydirection around a site to assessthe general lay of the land.Consider both slope steepnessand slope shape (convex, linear,or concave). The parent materialalso may be a guide, althoughthe correlation between land-form and parent material is notperfect.

There are many specific kindsof landforms. We will use only sixrather general classes of land-forms that commonly are foundin Oregon.

UplandUplands are the higher parts of the land

surface. Most uplands are hilly, thoughsome may be nearly level plains. Uplandsinclude gently rounded foothills(Figure 18) and steep mountain slopes.Uplands are likely to have residual orcolluvial parent materials.

Uplands also include very old, highlydissected stream terraces. Because ofdissection, the terrain is all sloping, butthe parent material is old alluvium.

Nearly level ridge crests and hilltopsalso are uplands. So are basalt plateaus ineastern Oregon. Some of the soils on theseplateaus have a mantle of old alluvium orof wind-blown silts; others formed inresiduum.

Sand dunes are uplands, but their parentmaterials are wind-blown sand. There areother situations, but the keys are thegeneral position and shape of the surfacein relation to the surrounding landscapeand the amount of slope.

Chapter 6


52

Foot slopeFoot slopes occur at the base of upland

hillslopes. They mark the change fromupland to stream terrace or floodplain.Foot slopes usually are concave. Theycollect more water than they shed byrunoff.

Foot slope soils often have colluvialparent materials that accumulate bygravity movement down the uplandslopes. Some foot slopes, however, mayhave parent materials that consist ofresiduum, old alluvium, or wind-blownsands or silts.

Alluvial fanFans also occur at the junction between

sloping uplands and nearly level valleyfloors. They form where a rapidly flowingstream emerges from a narrow valley ontoa terrace, floodplain, or lake plain.

The fan is narrow and sloping at theupstream point of origin. It broadens andflattens as the stream spreads sedimentonto the valley floor (Figure 19). Thesurface of a fan is gently rounded. Theparent material is alluvium.

Sometimes, where several parallelstreams emerge from a mountain slope, awhole series of fans will overlap to form acontinuous landform of intermediate slopebetween the upland and the lowland.

FloodplainFloodplains are the nearly level surfaces

next to stream channels. Every time astream overtops its banks, the excess waterflows out onto the floodplain (Figure 20).

Any given flood may not cover the entirefloodplain, but all parts of the floodplainwill be covered with water at least onceevery 100 years.

Floodplains usually are nearly level, butsome have slopes up to 5 or 6 percent.Scouring by water in overflow channelsmay give the floodplain a rolling, hum-mocky appearance, especially right next tolarge rivers. The parent material on afloodplain is recent alluvium.

Stream terraceStream terraces are abandoned flood-

plains. When a river cuts down through itsexisting floodplain, it establishes a newfloodplain at a lower level. The old

Figure 20.—Floodplain. The soils on a floodplain usually arevery productive, but frequent flooding or long periods ofstanding water may restrict agricultural use of the soil.

Figure 19.—Alluvial fan. The gently sloping, fan-shaped land-form at the base of the mountains is an alluvial fan. The dissectedlandforms in the background are uplands. In the foreground,near the fence, the fan merges gradually with an old lake plain.

Chapter 6 • Site Characteristics


53

floodplain is left high and dry as a terracethat is no longer subject to flooding.Terraces have the same nearly level shapeas floodplains, but they are at higherelevations.

At the junction between a terrace and afloodplain is a rise, called an escarpment.In some places the escarpment is veryobvious. At others it may be a very smallrise that requires close observation todetect.

Some rivers may have two or threeterrace levels, each separated by anescarpment (Figure 21). Such compoundlandforms resemble a giant set of stairs.

Lateral erosion by a river may removeall of the floodplain on one side. The riverthen flows at the base of a terrace that itnever overtops. In such cases, the flood-plain on one side of the river will be dis-tinctly lower than the terrace on the otherside.

Because terraces have been abandoned,their parent materials are old alluvium.Soils on terraces are older and moredeveloped than soils on floodplains.

Lake plainLake plains are broad, nearly level

surfaces that once were at the bottom oflarge lakes. They are common in theintermountain basins of Klamath, Harney,and Lake counties (Figures 17 and 22).There also are some old lake basins incentral Oregon, in the Columbia Basin, andin Baker, Union, and Umatilla counties.

The Willamette Valley once held a largelake, too, but there it’s very hard to tell thedifference between an old lake bottom anda stream terrace. Detailed, regional geo-logic studies are necessary to be sure.

The parent material of lake basins islacustrine sediments.

Figure 22.—Lake plain. This lake plain is the levelfloor of a basin surrounded by mountains. Wave-cutterraces on the surrounding slopes often mark thepositions of the shorelines of the ancient lakes.

Figure 21.—Terraces. Three terrace levels are appar-ent. In the foreground is a low terrace. Beyond thefenceline an escarpment rises to a higher, nearlylevel terrace surface. In the background, a thirdterrace stands at an even higher elevation.

Landform


54

Parent MaterialsParent material is the geologic material

from which A and B horizons have devel-oped. Some soils form in place by weather-ing of bedrock. We call them residual soils,and the parent material is called residuum.Colluvium is similar, but it has been moveddownslope by the pull of gravity.

Other soils form in loose materialstransported by wind, water, or ice. We callthese loose materials sediments, and thereare several kinds. Sediments carried by ariver and deposited on a floodplain or afan are called alluvium. Sediments depos-ited on the bottom of a lake are calledlacustrine. Silty sediments carried by windare called loess. Wind-borne volcanicdebris is called ash.

Parent materials are determined bycomparing A and B horizons with C andR horizons. The problem is that once Aand B horizons are fully developed, thecharacter of their original parent materialsmay no longer be clear.

Usually, we can assume that the C hori-zon is still pretty much like the originalparent material of the A and B. This oftenis the case in deep soils that have nearlyuniform textures.

In some soils, properties of the A and Bare very different from those of the C andR horizons beneath them. These differ-ences, combined with abrupt changesfrom B to C, or B to R, suggest two differ-ent kinds of parent materials. For thesesoils, it’s not correct to say that the C orR horizons are like the original parentmaterials of the A and B.

Because many Oregon soils do have acomplex geologic history, we try to deter-mine all parent materials that have influ-enced the entire soil profile. If all horizonsin a soil profile appear to have developedfrom a single kind of parent material,simply mark that answer.

Alluvium, lacustrine sediments, andvolcanic ash may be distinctly layered.The layers result from different episodes of

flooding or volcanic eruption. Even thoughthese layers may have different textures,they all form from the same process, andthey represent a single kind of parentmaterial, too.

Two or more kinds of parent materialsinclude loess resting directly on basaltrock, layers of volcanic ash mixed withlayers of alluvium, and layers of lacustrinesediments on top of residuum or collu-vium. Abrupt changes in color, texture,and coarse fragments provide the majorclues to these situations. Mark only theresponse for two or more classes for thesesoils.

There is a close relationship betweenthe kind of parent material and the land-form of a soil. Uplands usually haveresidual or colluvial parent materials. Soilson floodplains and stream terracesdevelop from alluvium. Old lake basinscontain lacustrine sediments.

Sometimes, however, it is difficult todistinguish between two similar kinds ofparent materials unless you’re very famil-iar with the geology of an area. That’s whysome of the parent materials describedseparately in the manual are groupedtogether on the scorecard. Seven kinds ofparent materials are common in Oregon.

ResiduumThis is earthy material that accumulates

by weathering of bedrock in place. Differ-ent kinds of rocks give rise to differentkinds of residuum.

Basalt, andesite, and volcanic tuffweather to clayey soils. Granite and coarsesandstones weather to loamy and sandysoils. Siltstones and fine-grained sand-stones weather to silt loam and silty clayloam textures.

In the strictest sense, residual materialshave not been moved from the place ofweathering. If we didn’t allow any excep-tions, there would be very few trulyresidual soils in Oregon.



55

Small movements—due to tree throw,animal mixing, soil creep, and localizedslumping—occur almost everywhere,particularly in the upper foot or so of soil.Such movements shouldn’t exclude amaterial from residuum, as long as we canbe reasonably sure that the entire profileformed from material weathered from therock that underlies it.

Generally, as depth increases in aresidual material, there is a gradualincrease in the amount and size of rockfragments, and they are less and lessweathered. Some soils, however, may haverelatively unweathered rock fragmentsthroughout, especially in eastern Oregon,where low rainfall limits rock weathering.

Residuum is common on uplands and onsome footslopes. It also may be buriedbeneath a thin deposit of alluvium onstream terraces or fans.

ColluviumThis is loose, earthy material that is

transported down steep slopes and depos-ited at the base of these slopes. Gravity isthe main force, but water helps by weaken-ing the strength of a soil mass upslope, orby local, unconcentrated runoff.

In western Oregon, colluvial depositsmay have relatively unweathered angularrock fragments mixed uniformly through-out. In eastern Oregon, this standard maynot help much to distinguish colluviumfrom residuum.

Colluvium is a common parent materialon footslopes and on steep uplands.

Recent alluviumAlluvium refers to sediments deposited

by running water. Recent alluvium refersspecifically to materials on the floodplainsof modern rivers. All streams and riverscarry a load of suspended sediments,particularly during periods of heavyrunoff. Every time a river floods, fresh

sediments are added to the recent allu-vium of the floodplain.

When a river floods, water moving overthe floodplain flows much more slowlythan water in the main channel. Suspendedsediments then have a chance to settle outon the floodplain. The coarsest sedimentssettle out nearest the river. Fine-texturedsediments are carried farther away andsettle out in quiet, backwater areas.

Repeated episodes of flooding result ingradual accumulation of thick deposits ofsediment on the floodplain. These depos-its may have fairly uniform textures, orthey may be distinctly stratified into layersof widely different textures. If any of thelayers contain coarse fragments theyusually are rounded because of abrasionover long distances of stream transport.

Recent alluvium is also the parentmaterial of an alluvial fan being formed bya modern stream emerging from steepterrain. It, too, often is stratified. Sand andgravel layers are not unusual.

Soils developed in recent alluvium areyoung soils, and may have nothing morethan A/C profiles. If the period betweendeposition of fresh sediments is longer, thesoil may have a simple A/B/C profile.

Old alluviumThis is alluvium associated with a

stream terrace or an abandoned alluvialfan. The alluvium once was laid down asrecent alluvium on a floodplain, but aban-donment of that floodplain means thatthere are no further deposits of freshsediments.

If an old terrace or fan has been upliftedand highly eroded to form hills and val-leys, then the landform might be an uplandor a footslope, but the parent material stillis old alluvium.

Soils developed in old alluvium usuallyhave much more distinct horizon develop-ment than soils in recent alluvium.

Parent Materials


56

Lacustrine sedimentsThese are sediments deposited origi-

nally at the bottom of a lake. They usuallyare finely stratified layers of silts andclays. Some sandier deposits may occurnear the shoreline of an old lake. Lacus-trine deposits often exhibit distinct lamina-tions that are apparent as platy structure.

Lacustrine sediments are the parentmaterials associated with lake plain land-forms. In some of the old lake basins insouth-central Oregon, white layers of siltydiatomaceous earth may be interstratifiedwith other lacustrine sediments.

Lacustrine sediments may occur onuplands if the old lake plain has beenuplifted and dissected. They also may beon uplands, footslopes, or fans if anancient lake temporarily covered anexisting landscape of hills and valleys.

Wind-blown sands and siltsWind-blown sands take the form of

dunes. They occur primarily along thecoast, in the Columbia Basin, and in a fewother isolated areas throughout Oregon.

The sand was derived originally from abeach deposit or from a lacustrine orfloodplain deposit. Since it has beenreworked by the wind, however, it iscustomary to refer to the material aswind-blown, or eolian sand.

Wind-blown silt is called loess. The siltsoriginally were deposited in a lake basin oron a floodplain. Later the wind pickedthem up, transported them, then redepos-ited them on top of existing hills andvalleys.

Loess is a common parent material onuplands and footslopes overlying basalt onthe Columbia Plateau in north-centralOregon. It is thickest near the ColumbiaRiver and thins southward. The texture ofloess also changes from a fine sandy loamat the river to silt loams and silty clayloams with increasing distance.

Soils developed in loess can be quiteproductive, but they are particularlysusceptible to erosion.

Volcanic ashVolcanic ash is fine-grained material

ejected from a volcano, carried by wind,and deposited in varying thicknesses (seeFigure 6). It consists of a mixture of threekinds of particles: glass shards, silt andsand-sized pumice fragments, and rockfragments torn off the pre-existing wall ofthe volcano.

Strictly speaking, ash particles are lessthan 2 mm in diameter, the same as fineearth. For our purposes, we will includewith volcanic ash those larger fragments,commonly referred to as “popcorn pum-ice,” that were deposited close to thesource.

Some ash deposits are the result ofwind-borne transport over several tens orhundreds of miles. Closer to the volcano,mixtures of hot gases and ash can literallyflow over the surface of the ground with-out being sorted by the wind. Such a flowis called a glowing avalanche. We willinclude glowing avalanche parent materi-als with volcanic ash, as well.

Figure 23.—Very stony soil. Numerous large stones on thesurface of the soil make tillage almost impossible. Manage-ment for improved permanent pasture, however, is practical.



57

Ash commonly has silt loam or sandyloam textures, becoming coarser close tothe volcano. Ash typically has a very lowdensity and tends not to compact underapplied loads.

Volcanic ash usually occurs on uplandsand footslopes, but it may occur as dis-tinct layers in alluvial or lacustrine sedi-ments.

Stoniness and RockinessStoniness refers to the amount of indi-

vidual rock fragments larger than 10 inches(25 cm) in diameter exposed at the soilsurface (Figure 23).

Rockiness refers to the amount of theland surface that consists of bedrockoutcrops (Figure 24).

Neither stones nor rock outcrops areconsidered as part of the soil, becausethey are so large as to exclude both fineearth and pore space from a large volumeof the soil. Both are important characteris-tics of the site, however, because theyinfluence cultivation and other forms ofagricultural management.

Stoniness and rockiness are judgedaccording to the percentage of the soilsurface covered by detached stones orrock outcrops. Four general classes are

Classes of Stoniness and Rockiness

Not stony/Not rocky—No stones or rocks arepresent, or there are too few to interfere withtillage.

Stones cover less than 0.01 percent of thearea.Rock outcrops cover less than 0.01 percentof the area.

Stony/Rocky—There are enough stones or rockoutcrops to interfere with tillage, but not to makeintertilled crops impractical.

Stones cover 0.01 to 0.1 percent of thearea—stones about 1 foot in diameter arespaced 30 to 100 feet apart.Rock outcrops cover 0.01 to 2 percent of thearea—outcrops are more than 300 feet apart.

Very stony/Very rocky—There are so manystones or rock outcrops that tillage between croprows is impractical. The soil still can be workedfor hay crops or improved pasture.

Stones cover 0.1 percent to 3 percent of thearea—stones about 1 foot in diameter arespaced 5 to 30 feet apart.Rock outcrops cover 2 to 10 percent of thearea—outcrops are about 100 to 300 feetapart.

Extremely stony/Extremely rocky—Stones orrock outcrops are so widespread that no agricul-tural improvements are possible. Some soils stillhave limited value as native pasture or range.

Stones cover more than 3 percent of thearea.Rock outcrops cover more than 10 percentof the area.

Figure 24.—Extremely rocky soil. Outcrops ofbasalt bedrock are so numerous that soilmanagement is limited to use of the soil fornative pasture or range.

Stoniness and Rockiness


58

used here. Each is defined not only quanti-tatively but also in terms of impact onagricultural management.

Because stones tend to be randomlyscattered over the soil surface—whereasrock outcrops are concentrated in smallareas—the numerical limits of stoninessclasses are different from the limits ofrockiness.

SlopeSlope, or slope gradient, refers to the

steepness of the land surface. We measureslope in percent as the amount of verticalchange in elevation over some fixed hori-zontal distance. Slope always is measuredin the same direction as water would runover the surface.

We often think of slope as the “rise overthe run.” If two points are 100 feet apart(the run), and one point is 10 feet higherthan the other (the rise), then the slope is10 over 100, or 10 percent.

The slope of a soil is important becauseit affects use and management of the soil.It is directly related to soil erosion hazard,

and it influences a farmer’s choice of cropsand conservation practices. Irrigationbecomes more difficult on steeper slopes,and so does operation of farm machinery.In general, as slope gradient increases,agricultural suitability decreases.

Slope has some other characteristicsbesides steepness. One is slope length.Long, uniform slopes are easier to operateequipment on than short, broken slopes.But long slopes allow runoff water togather more speed as it flows over thesurface.

Long slopes are, therefore, more subjectto erosion than short slopes. Diversionterraces are an effective erosion controlpractice specifically because they breakup long slopes into several short segments(see Figure 32).

Another slope characteristic is slopeshape. Slopes may be linear, concave, orconvex.

Linear means flat, or without curvature.Imagine a clipboard sitting on your desk.The surface of the clipboard is flat, and asit sits on your desk it is level. Now tilt it bylifting one end. The surface is still flat, butit now slopes at some gradient.

Figure 25.—Judging soil slope. Posted elevations on two stakes a known distance apart allow easy calculation ofthe slope gradient.

➤

➤

100 ft

➤

➤

8 ft

elev. = 1,028'

elev. = 1,036'

% Slope = 1,036'–1,028'100'

RiseRun

8'100'

= 8%= =



59

Concave means saucer-shaped orbowl-shaped. The slope gradient progres-sively decreases over a concave slope.Convex means just the opposite. Rounded,convex slopes get progressively steeper asyou go downslope. Many hills have convextops, linear sides, and concave footslopes.

We can measure slope gradient with anumber of surveying instruments. Soilscientists usually carry an Abney level or aclinometer to measure slope. With prac-tice, they often can estimate the slopegradient within a percent or two.

Since you probably don’t have theinstruments, and your experience atestimating slopes is limited, we’ll use adifferent method for judging (Figure 25).

At each site we’ll drive two stakes intothe ground. We’ll tell you the elevation ateach stake and the horizontal distancebetween stakes. You’ll have to calculatethe difference in elevation, the rise overthe run, and the percent slope. The stakeswon’t always be 100 feet apart, so be sureto adjust your answer to percent.

The final step is to determine the properslope class according to the chart.

AspectOne final site characteristic is the aspect

of the slope. Aspect is the compass direc-tion that the slope faces.

Southerly aspects are more nearlyperpendicular to the sun’s rays thannortherly aspects. They are exposed to thesun for longer periods of time each day.Soils on southerly aspects tend to behotter and droughtier than soils on north-erly aspects.

The vegetation on northerly aspects ismore likely to be trees, whereas southerlyaspects may have only grasses. Because ofthe vegetation difference, soils on south-erly aspects are more susceptible toerosion, and they often are shallower thansoils on northerly exposures.

For all these reasons, soils on southerlyaspects tend to have considerably lowerproductivity potentials than soils onnortherly aspects.

The only drawback to northerly aspectsis that, at a given elevation, the growingseason may be quite a lot shorter, and ittakes longer for the soil to warm up in thespring.

Easterly and westerly aspects are inter-mediate in their response to sunlight andmoisture supply. In general, an easterlyaspect will be more like a northerly aspect,and a westerly aspect will be a little morelike a southerly aspect.

Classes of Slope Gradient

Nearly level 0–3%Gently sloping 3–7%Sloping 7–12%Moderately steep 12–20%Steep >20%

Aspect



61

The first steps in evaluating soilsinvolve learning how to identifyhorizons, how to describe their

characteristics, and how to evaluate thebehavior of the whole soil. Having donethat, we now can focus our attention onuse and management of soil resources.

Management interpretations sometimesare difficult to judge, because (1) theydepend on interactions among several soilfactors, (2) they depend on specific cropsbeing grown, and (3) they are affected byclimatic conditions.

In order for this Manual to havestate-wide use, we’ve had to generalizeconsiderably. The guidelines given in thesections that follow should be adequatefor soil judging contests, but you shouldrecognize that there may be exceptions toevery one of them in specific cases. Oregonhas such a diversity of soils, climates,types of agriculture, and managementpractices, that a general manual simplycannot account for every possibility.

As we did in Chapter 5, we’ve organizedthe interpretations into tabular guides forsequential evaluation. Refer to the Guide inAppendix B for instructions on the use ofthose keys.

Feasibility ofArtificial Drainage

Soils that are less than well drainedoften can be improved for commercialcrop production by artificial drainage.Ditches, subsurface drains, or combina-tions of the two may be required.

The feasibility of such measures, andtheir design, installation, and cost, dependon the properties of the soil being drained.Moderately well-drained soils with goodpermeability are easy to drain. Clayeysoils, soils with shallow restrictive layers,and soils that are saturated for very longperiods of time are difficult to drain.

Another factor that affects drainagefeasibility is the availability of outlets—having someplace to discharge waterremoved from the soil. Outlets may not beavailable in low-lying depressions or onbroad, level surfaces where the water tableis at or near the surface much of the time.

Outlet availability can’t always be evalu-ated from the characteristics of the soilprofile or of the immediate site. Regionalstudies and seasonal observations may berequired. For these reasons, informationon the availability of outlets will be pro-vided at each judging site.

Management Interpretations

Guide for Determining Drainage Feasibility

Good Fair Fair Poor Poor Poor

Outlets Available — — — — Notavailable

Effective depth Deep — Mod. deep — Shallow Any

Drainage Subsoil MW/Any — MW/Any —class permeability SWP/Rapid, SWP/Slow, SWP/Any — Any Any

Mod. V. slow PD/Rapid, PD/Slow, Mod. V. slow

Chapter 7


62

The key word in this interpretation isfeasibility. This term refers specifically tothe relative ease or difficulty of draining asoil. It is not the same as drainage effec-tiveness.

Soils with good drainage feasibility mayrequire nothing more than tile drains at60- or 80-foot spacings. Good feasibilityimplies that it is logical and practical toinvest in tile drainage, because effectivedrainage can be accomplished easily, andit will be profitable.

Soils with poor drainage feasibility mayrequire tile lines at 20- or 30-foot spacings,as well as sumps, pumps, and valves.Technology exists to drain them effec-tively, but it may not be practical or eco-nomical to do so.

The first soil characteristic to consideris the internal drainage class. Excessivelydrained soils and well-drained soils don’t

have a wetness problem. Drainage is notneeded, and that’s what you should checkon your scorecard.

The next factor to consider is whetheror not outlets are available. If they are,then drainage feasibility can be as good asother soil properties permit. If they arenot, drainage feasibility is poor, regardlessof any other soil qualities.

The effective depth of rooting is anindicator of the depth of soil that can bedrained effectively. In deep soils, theeffective depth is not a limitation to drain-age feasibility. If the effective depth ismoderately deep, the drainage feasibility isno better than fair.

If there is a shallow, massive, clayeylayer in the soil, then only a small volumeof soil above the claypan can be drained,and the feasibility is poor.

If the soil does need drainage, then thefeasibility also depends oninteractions between thedegree of wetness and thesubsoil permeability. Moder-ately well-drained soils don’thave a serious wetnessproblem, so they can beslowly permeable and stillhave good drainage feasibil-ity.

Somewhat poorly drainedsoils must have at leastmoderate permeability tohave good drainage feasibil-ity. If the permeability isslow, they can have no betterthan fair drainage feasibility.

Poorly drained soils are sowet that they always will bedifficult to drain. If they haveat least moderate permeabil-ity, however, they can havefair drainage feasibility.Poorly drained soils withslow permeability have poordrainage feasibility.

Figure 26.—Tile drainage. Specialized trenching machines install continuousPVC drain tiles into the soil. The gradient of the tile lines is carefully con-trolled with laser beams to ensure good water flow. The depth of the tilelines and the spacing between them depend on the properties of the soil andthe depth to the water table.

Chapter 7 • Management Interpretations


63

Suitability for IrrigationThe best soils for irrigation are deep,

nearly level, well-drained soils with goodsurface soil permeability and highwater-holding capacity. Departures fromthese conditions may lower the irrigationsuitability.

When we judge irrigation suitability, weare judging only the soil’s ability to beirrigated. Availability of water for irrigationis not a factor to be considered. Neither isthe specific crop to be irrigated.

The primary factors influencing irriga-tion suitability are slope, surface soilpermeability, available water-holdingcapacity (AWHC), drainage class, anddrainage feasibility. These are the soilproperties used to determine irrigationsuitability in the guide below.

Effective depth also is important, but itis so closely related to AWHC that we don’tneed to consider it separately. Subsoilpermeability is another important factor,but its effect is accounted for in drainagefeasibility.

Slope affects the choice of an irrigationsystem. Flood or furrow irrigation workswell only on soils having slopes less than3 percent. Wheel lines and center pivotswill work on slopes up to 7 percent, butwith increasing difficulty. Above 7 percent,solid sets or hand-move systems may berequired. Irrigation generally is not feasibleon slopes greater than 20 percent.

Surface soil permeability interacts withslope. Moderate or rapid rates are best,but slow permeability can be tolerated onnearly level slopes. As the slope increases,however, restricted rates of water absorp-tion reduce the rates at which water canbe applied.

Increasing slope/permeability restric-tions limit the choice of irrigation system,the amount of equipment required, and thekind of crops that can be grown. Theefficiency of irrigation also decreases, andthe potential for runoff of excess waterincreases.

Silt loam soils under center pivotsillustrate this problem. At the outer end ofthe circle, water must be applied at rateshigher than the soil’s ability to absorb it.The excess water runs off, which not onlywastes water and energy, but also cancause erosion.

Available water-holding capacity deter-mines the amount and frequency of irriga-tion. Deep soils with high AWHC’s cansupport plant growth over relatively longintervals between irrigation. These soilsrequire the least amount of equipment andoffer the greatest flexibility in choice ofcrop and irrigation scheduling.

Shallow soils, sandy soils, or soilscontaining coarse fragments all requiremore irrigation. Farmers either have toinvest in more equipment, or accept thefact that some parts of their fields may beseriously water-stressed before they can

Guide for Determining Irrigation Suitability

Excellent Good Fair Poor Non-irrigable

Slope Permeability 0–3/Any 3–7/Slow 7–12/Slow 3–12/V. slow >20/Anyof surface soil 3–7/Rapid, Mod. 7–12/Rapid, Mod. 12–20/Any

Minimum AWHC High Medium Low Very low Very low

Internal Drainage WD WD WD Any Anydrainage feasibility MWD/Good MWD/Good, Fair MWD/Any

SWP/Good SWP/Good, Fair SWP/AnyPD/Fair

Suitability for Irrigation


64

get back to them with the next applicationof water.

The internal drainage of a soil is not aserious limitation as long as the soil can beeasily drained. A somewhat poorly drainedsoil may have excellent irrigation suitabil-ity if it is feasible to drain it.

High water tables restrict plant growth,however, so irrigation will be less efficientif water tables persist. The effect of drain-age, therefore, depends on both the naturalwetness of the soil and the feasibility oflowering high water tables through artifi-cial drainage.

Most Intensive CropThis interpretation is really a measure of

the potential productivity of a soil. Soilcharacteristics, plus climate, determinethe most intensive kind of cropping systemthat is feasible.

Deep, well-drained, fertile soils that havean adequate water supply and a longgrowing season can be used for a widevariety of crops. Shallow, stony soils that

receive only a few inches of rain may bebest suited for range management.

Between these extremes, various combi-nations of texture, depth, drainage, slope,elevation, and rainfall influence both thechoice of crops and potential yields.

Most farmers manage their fields in acrop rotation. As a result, a soil well-suitedfor potatoes (a row crop) may be usedperiodically for wheat to control soil-bornediseases. A soil suitable for wheat may berotated with alfalfa (to build up nitrogenand organic matter) or with grass seed (tocontrol disease and erosion). These andother kinds of crop rotations mean thatthe most intensive crop may not be thecrop currently grown on the soil.

Some farmers deliberately use a highlyproductive soil for a less-intensive crop.Irrigated pasture and hay grown for anintensively managed cattle operation is anexample. This doesn’t necessarily repre-sent under-use of soil potential. Rather it’san intentional decision to take advantageof the quality of soil resources in a differ-ent way.

Other reasons for choosing crops oflower intensity may include a lack of goodmarkets and low prices.

The opposite situation sometimesoccurs—using a soil at an intensity beyondits potential. Row-cropping land with ahigh erosion hazard is risky business. Afarmer may be able to get away with it fora while, but severe erosion over severalyears may ultimately ruin the productivityof soil resources.

There are about 150 different kinds ofcrops grown in Oregon. We’re going togroup them into eight very general catego-ries of crop intensity. Obviously, there willbe some local exceptions to the guidelinesgiven here.

When that happens, try to find out fromyour instructor or local agriculturalexperts what the reasons for the differ-ences are. Remember that good soilmanagers may be able to use their soilresources more intensively than the

Figure 27.—Irrigation suitability. Sandy soils are well-suitedfor center-pivot irrigation because the soil absorbs waterreadily. Low water-holding capacities, however, require waterapplications in small amounts at frequent intervals. The overallirrigation suitability, therefore, is only fair.



65

guidelines suggest. Poor managers, on theother hand, may not be able to take advan-tage of the productivity that really exists.

Selecting the most intensive cropdepends on whether or not the soil can beirrigated. Two things are required. First,the soil must have at least poor irrigationsuitability. Second, irrigation water mustbe available.

Under these conditions, row crops orspecialty crops are the most intensive. Ifrow crops are not feasible because oflimitations of climate, slope, or soil drain-age, irrigated legume hay is the next mostintensive. If the soil is poorly drained, orsodic, irrigated pasture may be the onlyirrigated crop that can be grown.

Soils that are too steep to irrigate, andirrigable soils for which no irrigation wateris available, must be used for drylandcrops.

Winter wheat is the most intensivedryland crop. Except for a few very wet orvery alkaline soils, moisture, slope, andtemperature are the major factors affectingwheat production.

In western Oregon, there’s alwaysenough moisture to grow dryland wheat.But high rainfall and steep slopes increasethe erosion hazard. If wheat cannot begrown without eroding the soil, permanentpasture is the next most intensive. Verysteep slopes that receive high rainfall inthe Cascade, Klamath, and Coast Rangemountains are used for forestry.

Wheat production in eastern Oregonrequires a balance between rainfall and thewater-holding capacity of the soil. Asrainfall decreases, the soil’s AWHC mustincrease to compensate.

Many soils that are suitable for wheatalso are suitable for dryland hay, espe-cially if the AWHC is medium or high. Inaddition, some soils that are at elevationstoo cold for wheat still can be used for hay.

If moisture supply or slope eliminateboth wheat and hay, timber grazing is thenext most intensive crop managementsystem. There must be enough moisture to

support tree growth, but slopes must notbe too steep for cattle grazing.

Where low rainfall and soil AWHC areseverely limiting, rangeland grazing is thebest choice. On very steep, north-facingslopes of the Blue Mountains, forestry isthe most intensive crop.

Most intensive crop—irrigatedThe guide on page 66 summarizes the

requirements for each category of irri-gated crops. More information on eachcrop choice is given below. Remember thatthe crop you see on the land at any giventime may not be the most intensive crop.

Information on location, pH, and avail-ability of irrigation water will be providedat each site. Use your own information onslope, irrigation suitability, AWHC, anddrainage feasibility to complete yourevaluation.

Row crop/Specialty crop. This categoryincludes all the vegetable crops in Oregon—corn, beans, onions, potatoes, carrots,garlic, and many others. Soils with excellent

Figure 28.—Irrigated row crops. This deep, nearly level soilhas moderate permeability and high water-holding capacity.Irrigation water is available, and the soil is well-suited forgrowing a variety of row crops. The crop shown is sugar beetseed.

Most Intensive Crop


66

or good irrigation suitability are best forrow crops.

Soils with fair irrigation suitability areacceptable if they are on 0–3 percentslopes. Soils with sandy loam or loamysand textures and fair irrigation suitabilityare acceptable even on slopes up to12 percent.

Irrigable soils at high elevations inHarney, Lake, Baker, and Wallowa countiesaren’t suitable for row crops because thegrowing season is too short.

Soils in the fog belt along the OregonCoast generally aren’t suitable for rowcrops, either. Cool marine air throughoutthe year prevents many row crops fromfully maturing.

Some irrigable soils are used intensivelyfor orchards, grapes, cranberries, lilybulbs, or other specialty crops. In thesecases, choose row crop as the most inten-sive crop regardless of irrigation suitabilityor climatic restrictions, as long as such

use doesn’t cause a decline in soil produc-tivity.

Legume hay. If row crops are unsuitable,irrigated alfalfa or clover hay is the nextmost intensive crop. Legume hay can begrown on slopes up to 20 percent as longas the AWHC is at least low and poorlydrained soils have fair drainage feasibility.

Legume hay also can be grown at highelevations with short growing seasons. Itdoes very well in the coastal fog belt.

Keep in mind that a lot of legume hay isgrown on soils that could support rowcrops. Usually, this hay is part of a croprotation or an intensive cattle operation.

Permanent pasture/Grass seed. Somesoils are so limited by texture, depth,drainage, or pH that it is not feasible to usethem for cultivated crops. Extremelyshallow or stony soils, poorly drainedclayey soils with poor drainage feasibility,and sodic soils with very high pH are themost common situations. With irrigation,

Guide for Determining Most Intensive Irrigated Crop

Row crop or Perm. pasture/Specialty crop* Legume hay Grass seed

Location All except— Any —High basins in Baker, Harney, Lake, Wallowa counties;Coastal fog belt

pH <8.4 <8.4 Any

Slope 0–12 0–20 0–20

Irrigation Excellent Any Anysuitability Good

Fair on 0–3% slopeFair on sandy soils up to 12% slopes

AWHC High-Low High-Low Any

Drainage Good-Fair Good-Fair Anyfeasibility

*Grapes, cranberries, and lily bulbs are examples of specialty crops. Disregard any other limitations if these or otherspecialty crops are being grown.



67

however, these soils can at least be man-aged for pasture production.

In western Oregon, several poorlydrained clayey soils on nearly level ter-races or floodplains are used for grassseed production rather than permanentpasture. These grasses can withstand longperiods of wet soil, and farmers have takenadvantage of that fact to get a little moreproduction from these soils.

Grass seed is not an irrigated crop, butbecause irrigation water generally isavailable to these soils, it is placed in thesame category as irrigated permanentpasture.

Most intensive crop—drylandThe guides on page 69 will allow you to

select the most intensive dryland crop foryour soil and site conditions. Note thatthere are separate guides for westernOregon and for eastern Oregon. You mustprovide information on soil drainage,drainage feasibility, AWHC, and slope.Information on soil pH, rainfall,and elevation will be posted ateach site.

Winter wheat. Dryland wheatcan be grown on a wide varietyof soils. The only exceptionsare poorly drained clayey soils,soils on slopes over 20 percent,and a few soils receiving eithertoo much or too little rain.

Wet, clayey soils are toodifficult to drain for wheatproduction. Soils on slopesgreater than 20 percent are tooerosive under western Oregonrainfall conditions. Excessiverainfall makes fall plantingdifficult and may interfere withripening and harvesting. Someareas of southwestern Oregonmay be too droughty for wheatproduction.

In eastern Oregon, wheatgenerally is grown at elevations

below 4,100 feet. At higher elevations, coldtemperatures may cause either excessivewinter kill or freezing of flowers in thespring.

Wheat won’t do well on highly alkaline,sodic soils, so the pH must be 8.4 or less inorder for wheat to be the most intensivecrop.

If there are 12 inches or more of rain, thesoil needs to have at least low AWHC, andwheat should be grown only on soils ofless than 20 percent slope to avoid erosion.

With less than 12 inches of rain, soilwater-holding capacity becomes an essen-tial factor, so the soil must have at leastmedium AWHC. Erosion is less of a prob-lem, so wheat can be grown on slopes upto 35 percent.

Dryland hay. Wheat and dryland hayare about equally intensive crops, and inmany areas soil conditions are suitable foreither one. This is particularly true if thesoil water-holding capacity is medium orhigh.

Figure 29.—Dryland wheat. Without irrigation water, dryland wheat is themost intensive crop. Many different soils are well-suited to wheat produc-tion. Only severely limiting conditions of slope, droughtiness, excessivewetness, or soil pH mean that a soil can’t be used to grow wheat.

Most Intensive Crop


68

In eastern Oregon, however, dryland hayis the most intensive crop at elevationsabove 4,100 feet, because wheat will notdo well in such cold areas. As long as theAWHC is medium or high, total rainfall isnot a serious limitation.

If soil AWHC is low, then dryland hayneeds at least 18 inches of rainfall. If soilAWHC is very low, dryland hay is not apractical crop. Hay production alsorequires nonsodic soils (pH 8.4 or less)and slopes less than 20 percent.

Permanent pasture/Grass seed. Inwestern Oregon, soils unsuited for wheatproduction may be suitable for drylandpasture. Poorly drained clayey soils, soilson 20 to 35 percent slopes, and soils onstream terraces in high rainfall (>60 inches)regions can be used for pasture. Many ofthe poorly drained soils also are used forgrass seed production.

Timber grazing. In eastern Oregon,many soils unsuited for wheat or hay canbe used for forestry and grazing combined.Careful management is required, for standsof trees must be thinned enough to allowadequate forage growth.

There must be at least 12 inches ofrainfall to support tree growth.

AWHC generally should be at least low, butsome soils with very low AWHC may beable to support timber grazing if therainfall exceeds 18 inches.

Elevation and slope do not offer seriouslimitations, except that some steep moun-tain slopes at high elevations may bebetter suited for forestry.

Rangeland grazing. Where rainfall,AWHC, and slope exposure (aspect), eitheralone or in combination, provide too littlemoisture for either wheat, hay, or trees,rangeland forage production is the mostintensive crop. Rangeland grazing also isthe only choice for dryland use of sodicsoils having pH values above 8.4.

Many rangeland soils can be managedfor improved forage production with suchpractices as brush removal, seeding ofadapted grasses, rotational grazing, anddevelopment of stock ponds.

Forestry. Managed forests are the mostintensive crop on steep slopes in highrainfall regions. In general, as the amountof rainfall increases, the more likely for-estry is, regardless of slope.

Figure 31.—Rangeland. Rangeland in good conditionprovides an ample supply of grasses for livestockgrazing. Good rangelands must be managed carefullyto avoid overgrazing, however, for the supply offorage is easily reduced and replaced with lessvaluable shrubs.

Figure 30.—Timber grazing. Combined use of the soilto grow trees and provide livestock forage is verygood management. The forage produces a short-termreturn while the timber provides a long-term return.



69

Some soils, particularly in the CoastRange, are very productive forest soils.Managed tree production can be quiteprofitable, and it is a wise use of the soilresource.

Guide for Determining Most Intensive Dryland Crop—Western Oregon

Permanent Permanent Permanentpasture/ pasture/ pasture/

Grass Grass GrassWheat seed Wheat seed Forestry seed Forestry

Slope <12 — 12–20 — — 20–35 Any

Rainfall <60 Any 30–45 <30 >60 30–45 Any45–60

Drainage Drainage WD Any WD Any Any Any Anyclass feasibility MW/Any MW/Any

SWP/Any SWP/AnyPD/Good, Fair PD/Good, Fair

In eastern Oregon, forestry is the properchoice only on steep, north-facing slopesat high elevations. Forestry is consideredless intensive than permanent pasture orrangeland grazing only because of the longintervals between harvests.

Guide for Determining Most Intensive Dryland Crop—Eastern Oregon

Elevation less than 4,100 feetTimber

Wheat grazing Grazing Grazing Wheat Grazing

pH <8.4 — — — — Any

Rainfall ≥12 — — — <12 Any

AWHC Low–High — — V. low Med.–High Any

Slope/Aspect <20/Any ≥20/N ≥20/S Any <35/Any Any

Elevation above 4,100 feetTimber Timber Timber Timber

Hay Hay grazing Forestry grazing Grazing grazing Grazing grazing Grazing

pH ≤ 8.4 — — — — — — — — Any

Slope/Aspect <20/Any — — ≥20/N — <20/Any — — ≥20/S Any≥20/N

AWHC Med.–High Low — Low–High — Low–High V. low — Low–High Any

Rainfall Any ≥18 12–18 ≥15 12–15 <12 ≥18 <18 ≥15 Any

Most Intensive Crop


70

Erosion Control PracticeThe need for erosion control practices

depends on a soil’s erosion hazard and themost intensive crop that can be grown. Forsome crops, such as hay and pasture, thecrop itself provides much of the protectionneeded. For others, such as row crops andwheat, specific management practices areneeded to prevent soil erosion.

Regardless of the size of the erosionproblem, conservation is something thatfarmers must be aware of—any soil, ifmanaged improperly, is subject to erosionthat reduces the quality of the soilresource for future generations.

The right kind of erosion control prac-tice for any particular field, farm, or forestcan be determined only by an onsiteinspection. There are no magic formulas orprescriptions that fit every situation.Sometimes adequate erosion control canbe achieved by simple application of oneof the general practices described below.In other cases, two or three different kindsof practices may be needed. In still others,there may be measures not included inthis list that are appropriate.

The important thing is to recognize theproblem, know the various kinds of ero-sion control practices available, and use ahealthy dose of common sense in manag-ing soils so as to minimize erosion.

For the contest, use the guide on page 73to select from the practices below the onethat is the most important, even thoughadditional practices might provide evenbetter protection.

Water controlThis refers specifically to using the right

amount of water when irrigating rowcrops. The danger is that if too muchwater is applied, or if water is applied toofast, the excess will run off.

If sloping soils are irrigated by channel-ing water across the field in furrows, orcorrugates, great care must be taken to

control the flow of water so that it doesn’tstart cutting rills and washing away thetopsoil. This can be a serious problem ifthe permeability of the surface soil is slow,as in many Ontario area soils, which arelow in organic matter.

If soils are irrigated with sprinklersystems, good management means keepingthe water-application rate below the rateat which the soil can absorb the water. Thegreater the slope and the slower thepermeability of the surface horizon, thegreater the need for careful water manage-ment to avoid soil erosion.

Cover cropMany soils used for row crops are left

bare during the winter growing season. Fallplanting of a quick-growing crop like aspring cereal can help hold the soil inplace. In the spring, the crop can easily beplowed or disked into the soil. The covercrop protects the soil from erosion, and ithelps to maintain soil organic matter aswell.

Cover crops are appropriate for control-ling water erosion on sloping soils used forrow crops and on floodplain soils used forrow crops. They also are appropriate forcontrolling wind erosion on all soils thathave a moderate or severe wind-erosionhazard.

Standing stubbleThis may be a necessary practice for

some irrigated row crop soils subject toerosion by either wind or water during thewinter. Usually a cover crop would beprescribed to control this erosion. Butsome crops in some parts of the state maybe harvested so late in the season thatthere is no time left to plant a cover crop—and expect it to grow enough to do anygood.

In these situations, it is best to leave thecrop residue in place to slow down thespeed of wind or water moving across thesoil surface.



71

Stubble mulchThis is a method of residue management

that partly mixes wheat straw or otherstubble into the surface soil. Trashy fallowand minimum tillage may refer to this samepractice. The idea is to create a roughsurface that slows down water runoff orwind speed, and improves the absorptionof water into the soil.

Even if there is very little crop residueavailable, leaving the soil surface in arough, cloddy condition helps accomplishthe same thing.

Stubble mulch is a good practice forerosion control in most wheat fieldsthroughout the state.

Diversion terracesThese are channels built on the contour

with a ridge on the downslope side. Theyare designed to catch water coming downthe slope before it starts to flow fastenough to erode the soil. On long slopes,two, three, or even four terracesmay be necessary so that waternever has the opportunity toflow overland very far.

Many terraces are designed totrap the water behind the ridgeso that it can soak into the soil.This not only conserves soil butalso saves water and improvescrop yields. Other terraces aredesigned to divert water flowacross the slope into a grassedwaterway, where it can run ondown the slope without erodingthe soil.

Diversion terraces are mostcommon on steep slopes usedfor wheat production in northcentral Oregon. Stubble mulch-ing is a good practice to com-bine with diversion terraces.

No-tillNo-till means exactly what it says. A

crop is planted directly into the stubblefrom a previous crop, or into a sod orcover crop, without tilling the soil first.The soil isn’t cultivated during the growthof the crop, either. Instead, weeds arecontrolled with chemical sprays.

No-till is a very effective erosion controlpractice because bare soil is neverexposed at the surface. In addition, rootsfrom the sod or the previous crop helpkeep the soil anchored in place. No-tillhelps maintain high organic matter levels,and it eliminates the possibility of forminga tillage pan.

All these things combine to maximizewater absorption into the soil, rather thanrunning off. Erosion control thus increasesthe amount of water stored in the soil forlater plant use. This extra benefit permitsannual cropping in some parts of Oregonthat are currently restricted to wheat-fallowrotations under conventional tillage.

Figure 32.—Diversion terraces. Benches cut across long slopes reducethe erosion hazard by shortening the distance over which water canflow. When water reaches the terrace, it either sinks in or flows sidewaysinto a grassed waterway, which conducts the water safely down the restof the slope.

Erosion Control Practice


72

No-till won’t work in every situation, andit does require some special equipmentand different kinds of fertilization andweed control practices. But in many areasof Oregon, no-till can be both an effectiveerosion control practice and an effectivesoil management practice to improveyields.

Strip cropWith limited rainfall, dryland wheat can

be grown in a given field only every otheryear. During the intervening, or fallow year,the soil is left bare—or stubble-mulched—so that the soil’s reservoir of availablewater can be restored to its maximum.

Where the rainfall is 10 inches or less,fields are managed in alternating strips ofwheat and fallow to control wind erosion.The strips are perpendicular to the domi-nant wind direction.

Standing wheat slows down the windand partly shelters the adjacent strip

downwind. By keeping the strips narrow,there is much less opportunity for sus-tained winds to travel far enough overbare ground to pick up much soil. And ifsoil does blow, the next wheat strip willtrap some of it.

Controlled grazingThis is the proper practice for perma-

nent pasture, timber grazing, and range-land grazing. In most situations thepurpose is to prevent overgrazing, whichreduces the natural vegetative protectionand increases soil erosion. You can pre-vent overgrazing by using proper stockingrates and by using fences and stock pondsto allow rotational grazing.

On subclover pastures in western Oregon,controlled grazing means intentionalheavy stocking rates. This reduces compe-tition from other pasture plants and pro-vides the right conditions for subclover toset seed for the next year’s crop.

Watershed managementErosion control is just as important in

forestry as it is in agriculture. Unfortu-nately, there are no categories of erosioncontrol practices that apply generally tospecific situations. Each timber site mustbe assessed individually.

The key ingredient is to maintain maxi-mum permeability of the surface soil.Avoiding compaction is important, as isthe proper location and design of loggingroads.

Choosing a suitable harvest method canaccomplish much of the needed protec-tion. Clearcutting is quite acceptable formany soil/slope combinations, but it canbe quite damaging in others.

Watershed management means a sys-tems approach, in which the soil, land-scape, and vegetation all are considered indeveloping a suitable forest managementscheme.

Figure 33.—No-till. Special drills allow planting directly intothe standing stubble of a previous crop. By leaving the cropresidue in place, more water is absorbed into the soil andmuch less runs off. Water saved increases crop yields, andreduced runoff lowers erosion substantially.



73

Reaction CorrectionA neutral soil has a pH of 7.0. Acid soils

have lower pH values; alkaline soils havehigher values. For most crops the opti-mum pH is in the range from pH 6.6 to 7.3.Slight changes from this range aren’t tooserious, however, so we’ll broaden therange a little before soil pH calls for amanagement correction.

Correction of soil acidity (low pH) isrecommended if the pH of the surface soilis less than 6.2. Excess soil acidity makesmany nutrients less available for plantgrowth. Some elements may even beharmful to plants.

Add lime to correct soil acidity. Limeneutralizes the acidity and raises the pH.The amount of lime to apply may be asmuch as 4 or 5 tons per acre, depending

on how much acidity needs to be neutral-ized, the crop to be grown, the purity andfineness of the liming material, and theextent of mixing into the soil.

Correction of alkalinity (high pH) isappropriate if the pH of the surface soil ishigher than 8.4. Excess alkalinity is causedby sodium in the soil. Sodic soils havevery poor structure. Permeability is low,and the soil has a very poor physicalcondition for plant growth.

Several practices are necessary tocorrect alkalinity. Gypsum or sulfur isapplied to replace the excess sodium.Irrigation is necessary to leach out thesodium, and drainage is needed to carrythe sodium-laden leaching water com-pletely out of the soil system. This lowersthe pH and improves soil structure andpermeability.

Guide for Determining Erosion Control Practice

Western Oregon

Water control, Stubble Controlled Watershed Cover crop mulch grazing management

Most intensive crop Row crop Wheat Pasture ForestryLegumeGrass seed

Eastern OregonWater Water-

Stubble Diversion Strip Strip Stubble Diversion Stubble control, shedmulch, terraces, crop, crop, mulch, terraces, mulch, Cover Controlled man-No-till No-till No-till No-till No-till No-till No-till crop grazing agement

Crop Wheat — — — — — — Row crop Timber Forestrygrazing,Range

Rainfall ≤10 in. — — — >10 in. — — Any Any Any

Wind Low Low, Mod. High Low, — High Any Any Anyerosion Mod. Mod.

Water Low, High, Low, Any Low, High, Any Any Any Anyerosion Mod. V. high Mod. Mod. V. high

Reaction Correction


74

Two situations prevent reaction correc-tion. Without irrigation water, alkalinitycan’t be corrected. If irrigation water isn’tavailable, check none even if the pH ishigher than 8.4.

Acidity in forest soils is not severelylimiting to tree growth, and it’s not practi-cal to correct forest soil acidity, anyway. Ifforestry is the most intensive crop, checknone, regardless of soil pH.

Limitation for SepticTank Drainfields

Any home built in an area not served bypublic sewers must have some kind ofonsite waste disposal system. The mostcommon system combines a septic tankwith a soil drainfield. All household wastesgo first into the septic tank. Solid wastessink to the bottom, and waste water isdrawn off the top for discharge into thesoil.

The drainfield consists of a series ofperforated plastic tile drains laid intrenches in the soil. Waste water flowing

through the tile lines trickles into the soil,where further treatment and disposaloccurs.

The adequacy of these systems dependsmore than anything else on the propertiesof the soil. Soil must do three things for adrainfield to function properly: accept thewaste water, treat the waste, and disposeof the water. All three depend heavily onsubsoil permeability.

Waste treatment is a biological process,and it requires plenty of oxygen. Wastewater must move through the soil fastenough to prevent a buildup of saturatedconditions, but slowly enough for microor-ganisms to do an effective job.

Most drainfields work satisfactorily indeep, well-drained, permeable soils. But ifthe soil has a slowly permeable layer, or aperiodically high water table, septic tankwaste water may not receive adequatetreatment—and it may be the source of ahealth hazard.

Soils that are less than well drained haveseasonal water tables that interfere withproper drainfield operation. Shallow soils

don’t have enough volume ofsoil available for treatment, andthe danger of effluent breakingout at the surface is increased.

Sloping soils are poorly suitedfor drainfields because wastewater may concentrate at theends of tile lines and eitherbreak out at the surface or flowtoo rapidly downslope in thesoil.

Floodplains generally are notsuitable places for drainfieldsbecause floodwaters canbecome contaminated withsewage waste water. In easternOregon, however, floodplains ofintermittent streams or gulliesare flooded so rarely, or forsuch short periods of time, thatseptic tank drainfields can besafely installed.

Figure 34.—Septic tank drainfield. A network of perforated pipes distrib-utes sewage effluent evenly throughout the entire area of the drainfield.Each pipeline is surrounded by crushed rock, covered with paper, thencovered with soil. This drainfield is on a slope, and pipelines are installedacross the slope.



75

Soil scientists help land use plannersand prospective homebuilders by ratingsoil limitations for septic tank drainfieldperformance. Categories of slight, moder-ate, and severe limitations indicate therelative suitability of a soil for drainfieldinstallation.

Soils with a slight limitation generallycan be used for drainfields without modifi-cations. Soils with moderate limitationsgenerally can be used for drainfields, butsome special practices may be needed.Increasing drainfield size, installing drain-age to remove seepage water, or adding alittle extra soil to the surface are examplesof the kinds of things that may be needed.

Soils with severe limitations can’t beused for conventional drainfields. Thereare, however, other ways of dealing withhousehold sewage on these soils. Sandfilters, deep tile drainage, even pumping toa remote site all are possibilities.

All of these alternatives are much moreexpensive than conventional drainfields,so a severe rating is really an indicator ofthe amount of engineering, and the cost,required to dispose of wastes properly.

Ratings of slight, moderate, and severelimitations for septic tank drainfields areinterpretations based on the soil proper-ties that affect drainfield performance.Determine the correct rating for your soilusing the accompanying Guide.

Guide for Rating Limitations for Septic Tank Drainfields

Slight Moderate Severe

Effective depth ≥48 inches ≥36 inches <36 inches

Subsoil Moderate Rapid to Anypermeability moderate

Internal drainage WD WD, MWD Any

Slope 0–7 0–12 Any

Landform All except All except Anyfloodplains floodplains

of perennialstreams

Limitation for Septic Tank Drainfields



77

Appendixes



79

Interpretation guides for permeability,water erosion, and wind erosion in Chap-ter 5, and for drainage feasibility, irrigationsuitability, most intensive crop, erosioncontrol practice, and septic tank drainfieldsin Chaper 7, all are set up the same way.

In each case, the factors that affect theinterpretation are listed down the left side.The different ratings, or classes, that arepossible within each interpretation arelisted across the top. Soil or site condi-tions that are acceptable in each ratingclass are given in the body of the table.This format is illustrated above.

To use these guides, always start in theupper left corner of the table. See if yoursoil satisfies the conditions given for thefirst factor in the first column.

If it does, then move down, and checkthe requirements for the next factor. If itdoesn’t, then move across the table untilyou come to the first column that doeshave acceptable conditions.

Continue moving down, and to the rightif necessary, until you have evaluated allthe factors. The rating at the top of thecolumn you end up in is the correct inter-pretation for your soil.

Here’s an example. Suppose we have amoderately deep, poorly drained soil. Thesubsoil permeability is slow, but drainageoutlets are available.

Start in the upper left corner of the tablein the row labeled “Outlets” and the col-umn labeled “Good.” The table indicatesthat to be rated “Good,” a soil must haveoutlets available. Our soil does, so wemove down the “Good” column to evaluate“Effective depth.”

Here we see that “Good” drainage feasi-bility requires a deep soil. Our soil failsthis standard because it’s only moderatelydeep. So we move across the table until wereach the first column that allows moder-ately deep soils. That’s the third columnover, and it’s labeled “Fair.”

Having met the depth standard in the“Fair” column, we continue moving downthat column to evaluate “Drainage class/Subsoil permeability.”

Here we find that moderately well andsomewhat poorly drained soils are accept-able regardless of their subsoil permeabil-ity, but poorly drained soils are acceptableonly if they have rapid or moderate perme-ability.

Our soil is poorly drained and slowlypermeable, so it fails this requirement. Wemove to the right again, and find thatpoorly drained soils with slow subsoilpermeability are permitted in the nextcolumn over.

This column, however, is labeled “Poor.”Thus the overall drainage feasibility forthis particular soil is poor.

How to Use Interpretation Guides

Drainage Feasibility GuideFeasibility Rating

Factor Good Fair Fair Poor Poor Poor

Outlets Available — — — — Notavailable

Effective depth Deep — Mod. deep — Shallow Any

Drainage Subsoil MW/Any — MW/Any —class permeability SWP/Rapid, SWP/Slow, SWP/Any — Any Any

Mod. V. slow PD/Rapid, PD/Slow, Mod. V. slow

Appendix A



81

This information is intended primarily tohelp those who may be asked to organize acontest, prepare the pits at the contestsite, and do the official judging and scor-ing. Instructors and students, however,may wish to read this section so they willhave a better idea of what to expect whenthey arrive at a contest.

Locating a contest siteAn official contest has four soil pits. One

is used as a practice or demonstration pit.The other three are used as contest pits.Each pit should represent a distinctlydifferent soil. If at all possible, the pitsshould all be within a short (5- to10-minute) walk of each other.

Local soil scientists in the NaturalResources Conservation Service, the U.S.Forest Service, the Bureau of Land Man-agement, or the OSU Extension Serviceshould be able to provide assistance insuggesting possible contest areas and inselecting precise locations for each pit.

These same people may be willing toserve as official judges, as well. Theyshould not, however, be asked to arrangefor the pits to be dug.

How big should the pits be?Ideally pits should be big enough for

everyone in a group to get into the pit atthe same time. For a group of 40 people, anideal pit would be 5 feet wide, 30 feet longat the bottom, and deep enough to see allthe horizons that are to be judged. Thisallows 20 people to use each face of the pitand provides about 11⁄2 feet of pit wallspace for each person.

In addition, both ends should be taperedup to the ground surface, bringing the totallength of the pit to about 42 feet. If the soilis loose or subject to slumping, it’s advis-able to keep the pits shallow enough toprevent cave-ins, rather than to try andshore up a deeper pit.

When digging the pits, try to keep bothwalls as straight as possible and thebottom as level as possible. A good back-hoe operator can dig a 30-foot pit in 11⁄2 to2 hours.

If the pit has water in it, use suctionpumps to keep it dry while it is beingjudged. Wooden pallets placed in thebottom provide a very good surface tostand on while judging wet soils.

Circumstances often require the use ofpits, road cuts, or streambanks that aresmaller than ideal. If this is the case, thereare a couple of things you can do to makeit easier for the students to judge the soil.

One helpful technique is to place soilfrom each of the four horizons to bejudged into large pans. Label all pansclearly, and place them on the ground nearthe pit.

Students can use the soil in these pansto judge color and texture, and they can dothis while others are examining the soil inthe pit. They still will have to get into thepit to determine structure, coarse frag-ments, horizon names, and effective depth,however.

Another helpful procedure is to rotatesmall groups of students into and out ofthe pit at 5- or 10-minute intervals, to giveeach student an equal opportunity to be inthe pit. With large groups, it may be neces-sary to allow a longer total judging time inorder to make sure that everybody hasadequate time to study the soil profile.

How To Set Up and Run A Soil Judging Contest

Appendix B


82

Setting up and judgingthe official profile

The official judge(s) should select asingle area on the pit face within which alldecisions regarding scorecard entries willbe made. This area should be about 11⁄2 to2 feet wide, and it should be plainly markedwith colored ribbons on each side runningfrom the top to the bottom of the pit. Eachboundary between horizons should bemarked with a ribbon or string, too.

If more than four horizons are present,place a small numbered card in the middleof each horizon for which the students areto record answers on their scorecards.

Suppose, for example, a soil has anAp-BA-Bt1-Bt2-BC-C profile, and the stu-dents are to judge the first, third, fifth, andsixth horizons. Place card number 1 in theAp, card number 2 in the Bt1, card number3 in the BC, and card number 4 in the C.

If fewer than four horizons are present,students normally will judge each horizonin the profile. In any case, the officialjudges always have the right to specifywhich horizons are to be judged if for anyreason they do not want students to judgea particular horizon.

It is imperative, however, that studentsbe able to easily determine which horizonsthey are to judge and how the horizonsmarked in the soil profile correspond tothe four horizons listed on the scorecard.

Official judge(s) should use only thesame kinds of resources for making deci-sions as are available to the students. Thatis, colors should be estimated according tothe guidelines in the Manual without theaid of a Munsel color chart. Texturesshould be determined by feel withoutresorting to laboratory data. Coarsefragments should be estimated by eyerather than using a sieve.

The judge(s) may decide to allow morethan one correct answer if the texture,color, structure, or any other property orinterpretation is very close to the bound-ary between two choices.

Information that must beprovided at each pit site

Each pit needs to be clearly identified,either with a letter, a number, or acolor-coded scorecard. Post this identifica-tion in a prominent place, to minimize thepossibility that students will record theiranswers on the wrong card for that pit.

Additional information that the studentsneed for judging each site includes thefollowing:• The upper and lower depth limits for

each horizon• The pH of each horizon• The amount of rainfall at each site• The elevation at each site• Whether or not irrigation water is

available• Whether or not drainage outlets are

availableWrite this information in bold, clear

letters on a large piece of posterboard andmount it on a stake at the pit site. You alsocan mount the pit identification card onthe same stake.

Finally, you must provide the elevationsneeded for slope determination. Select anarea near the pit that is to be used forjudging the slope of the site. Drive twostakes into the ground, one directlydownslope from the other, at a horizontalseparation distance of 25, 50, or 100 feet.

Write the lower elevation on a card andtack it to the lower stake. Write the higherelevation on a card and tack it to the upperstake. Be sure that the elevation differ-ence, when divided by the separationdistance, does in fact calculate out to bethe slope gradient intended.

Appendix B • How To Set Up and Run a Soil Judging Contest


83

Supplies and equipmentThe host school should provide all the

stakes, posters, ribbons, pumps, andpallets needed to set up the contest pits.

They also should have enoughscorecards and interpretation guides onhand to provide four scorecards and oneguide for each student. Reproduce copiesof the scorecard and interpretation guidefrom this manual (at back of book).

Students should come dressed for theweather and should bring field equipmentnecessary to judge the soil. Here’s a list:1. Warm clothing and a warm coat2. Hat and gloves3. Rain gear4. Rubber boots5. Clipboard6. Clear plastic bag to cover your clip-

board and keep your scorecards dry7. Two no. 2 pencils (don’t use harder

pencils—the judges won’t be able toread your answers)

8. Pocket knife9. Water bottle for moistening your tex-

ture samples

Ground rulesAt the beginning of a contest, remind all

students of the rules that always apply andannounce any special conditions that mayalso apply. Ground rules include (butaren’t necessarily limited to) these eight:1. The time allowed for judging each pit is

30 minutes. Local officials may extendthis limit if necessary to allow enoughtime to rotate small groups into and outof the pits.

2. Allow 10 minutes after each pit hasbeen judged (and the scorecards havebeen collected) for an official judge toreview the correct answers for that pit.

3. Allow 5 or 10 minutes, as necessary, forrotation between pits after the pitreviews have been completed.

4. The official profile, between the rib-bons, is reserved as a reference area. Itis not to be disturbed in any way by anyof the contestants.

5. Discuss local conditions and/or localdeviations from the guidelines in theManual with the entire group at thepractice pit. Examples include specifickinds of landforms and parent materi-als, exceptions to drainage class crite-ria, location within or outside of thecoastal fog belt, different degrees oferosion hazard, or specific kinds oferosion control practices that may beappropriate.

6. Students must record their answers onthe scorecard as the number for thecorrect response. Enter one and onlyone answer. The official judge(s) maydecide to accept more than one answer,but in no case should the students givemore than a single number.

7. If an answer falls right on a class bound-ary (for example 15 percent coarsefragments, 8 inches AWHC, 7 percentslope), always mark the next higherclass.

8. Interpretation guides are allowed, andstudents should use them during thecontest. Students aren’t expected tomemorize all the criteria required toreach a correct decision. However, theyshould be familiar with the proper wayto use the guides.

ScoringThe most important rule in scoring is to

do it consistently. The official judge(s)should provide completed cards for eachof the contest pits, from which additionalkeys can be made up. The scorecards aredesigned so that the edge of the key cardcan be placed right alongside the columnof answers on a contestant’s card.

It doesn’t matter whether you mark rightanswers or wrong answers, as long as

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84

everyone who is helping with the scoringdoes it the same way. Similarly, it doesn’tmatter if you total up right answers orwrong answers, as long as everyone doesit the same way.

In any case, each answer, on each side ofthe scorecard, is worth 1 point.

Scorecard graders usually enter thenumber of points earned in each of theboxes at the bottom of the scorecard.Some may prefer, however, to enter thenumber of points missed, and determine

the outcome of the contest on the basis ofthe lowest, rather than the highest score.

Again, it doesn’t matter, as long as it isdone consistently.

If you have enough time, it’s a good ideato double-check some of the scoring. Afterall cards have been graded, and teamscores have been compiled, you couldrescore all cards for the top 10 teams andcheck all the arithmetic. This ensures thatthe ranking of the winning teams is notaffected by inadvertent errors in scoring.

Appendix B • How To Set Up and Run a Soil Judging Contest


85

In 1981, the procedure for naming soilhorizons was revised. The new procedureis a good one, and that’s why we’ve used itin Chapter 3 of this Manual.

But if you look at soil profile descrip-tions in almost any soil survey reportpublished before 1985, you’ll find thatsome of the names are different. To helpyou match the older names with thepresent ones, we present here all thenames used in Chapter 3, along with theircounterparts in the older system.

Master Special Transitionhorizons horizons horizonsOld New Old New Old NewO O Ap Ap A3 ABA1 A B2t Bt B1 BAA2 E B2g Bg B3 BCB2 B Bir BsC C B2 BwR R Bx Bx

Bca BkCcasim BkqmCca CkCr Cr

More than oneThick horizons parent materialOld New Old NewA11 A1 A1 AA12 A2 A3 AB

B2 BB21g Bg1 IIB3 2BCB22g Bg2 IIC 2CB21t Bt1B22t Bt2B21 Bw1B22 Bw2C1 C1C2 C2

Typical Horizon SequencesFor each of the soils listed, the sequence

of old names is listed first. Underneath theold names are the equivalent names in thenew system.

Horizon Names Used Before 1983

Alicel Ap A12 B1 B2 IICAp A BA Bw 2C

Bandon O A2 B21ir B22ir B23ir B24ir CO E Bs1 Bs2 Bs3 Bs4 C

Brenner Ap A12 B21g B22g B3g CgAp A Bg1 Bg2 BCg Cg

Carney A11 A12 A3 C IIRA1 A2 AB C 2R

Cascade A1 B21 B22 IIBx1 IIBx2 IIBx3A Bw1 Bw2 2Bx1 2Bx2 2Bx3

Dayton Ap A2 IIB2t IIB3t IIICAp E 2Bt 2BCt 3C

Deschutes A1 B1 B2 IIC IIIRA BA Bw 2C 3R

Fordney Ap C1 C2Ap C1 C2

Gem A11 A12 B1t B21t B22t B31t B32tca RA1 A2 BAt Bt1 Bt2 BCt BCtk R

Hoopal A11 A12 B2 Clcasim C2A1 A2 BW Bkqm C

Josephine O A1 B1 B21t B22t B23t C1 C2rO A BA Bt1 Bt2 Bt3 C Cr

Malabon Ap A3 B21t B22t B3t IICAp AB Bt1 Bt2 BCt 2C

Nehalem Ap A12 B2 CAp A Bw C

Nyssa Ap B2 C1casim C2casim C3casimAp Bw Bkq Bkqm1 Bkqm2

Oakland A11 A12 B21t B22t B23t B3t CrA1 A2 Bt1 Bt2 Bt3 BCt Cr

Quincy C1 C2C1 C2

Ritzville Ap B1 B2 C1ca C2 C3Ap BA Bw Bk C1 C2

Salem Ap B2t B3t IICAp Bt BCt 2C

Simas A1 IIB21t IIB22t IIB3ca IICcaA 2Bt1 2Bt2 2BCk1 2BCk2

Walla Ap A12 B1 B2 C1ca C2ca Walla Ap A BA Bw BCk1 BCk2

Appendix C



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This glossary provides simple defini-tions of many of the words that are pecu-liar to the language of soil science. It is nota thorough document, as terms that arecompletely defined in the text are notalways included here. In many cases,reference is given to the chapter or sectionin which you’ll find additional informationabout the term.

AWHC—Available water-holding capacity. Themaximum amount of water a soil can store forplant use. The amount of available wateralways is less than the total amount of water asoil can hold. See Chapter 5.

Abrasion—The wearing down of rock particlesby friction as rocks and sand grains grindagainst each other during transport by a river.

Acid soil—Soil that has a pH below 7.0. Acidsoils are leached and do not contain free limeor soluble salts.

Aeration—The movement of air back and forthbetween the atmosphere and the pores of asoil. See “Well-aerated.”

Aggregate—A single unit of soil structure. Seealso “Ped,” “Structure.”

Aggregation—The formation of peds of soilstructure. Organic matter (humus) and soilclays are important agents of soil aggregation.

Air dry—A soil that has been exposed to airand sunlight long enough to remove almost allof the water in the soil. Air dry soils do containa very small amount of water, but plants arenot able to utilize any of it.

Alkaline soil—Soil that has a pH higher than7.0. Alkaline soils frequently contain either freelime or soluble salts or both.

Artificial drainage—Intentionally lowering thewater table in wet soils by digging ditches orputting perforated tile lines in the soil. Artifi-cial drainage improves the soil environment forroot growth and makes possible more intensiveuse of the soil for crop production. SeeChapter 7, “Feasibility of Artificial Drainage.”

Aspect—The exposure of a slope, or thecompass direction that the soil faces. SeeChapter 6.

Bacteria—Tiny (microscopic), one-celledplants in the soil. Bacteria, along with fungi, arethe main organisms that carry out decomposi-tion of plant residues and the formation ofhumus.

Bedrock—Solid rock that is underneath thesoil.

Clay—A single particle, or grain of soil, that isflat, or plateshaped, and less than 0.002 mmacross. Clay particles make soils sticky andhelp bind other particles into aggregates. Clayalso refers to a specific class of soil texture. Aclay soil contains more than 40 percent byweight particles of clay size. See Chapter 4,“Texture.”

Clay skin—A thin, waxy-looking coating of clayparticles deposited on the surface of a soilaggregate in a B horizon. Clay skins can beseen on the surfaces of blocky peds inBt horizons.

Clayey—A general term that refers to a soilthat has a relatively high content of clayparticles. The specific soil textural classesnamed clay, silty clay, and sandy clay all arecalled clayey soils. Some silty clay loams andclay loams with clay contents near the upperpart of the allowed range also may be calledclayey soils.

Claypan—A compact, dense soil horizon thathas over 40 percent clay and poorly developedstructure. Claypans have slow or very slowpermeability, and they limit root penetration.Perched water tables are likely to saturate thesoil above a claypan during rainy seasons.

Coarse fragment—Individual grains of soil thatare larger than 2 mm in diameter. The mostcommon coarse fragments in soils are gravel(2 mm to 3 inches) and cobbles (3 to 10 inches).See Chapter 4.

Coarse-textured—A general term that refers tosoils that have a high sand content and a lowclay content. Sandy loams, loamy sands, andsands all are coarse-textured soils. See also“Light.”

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Cobblestone—A coarse fragment, or piece ofrock in the soil, that is between 3 and 10 inchesin size. Cobblestones may be rounded if theyhave been carried by a river, or angular if theyhave broken off exposed bedrock.

Concave—A landform surface that is curvedlike the inside of a saucer or a bowl. Concavelandscapes collect water and often have wet soils.

Concretions—Small, hard nodules in the soilthat form chemically by localized concentra-tion of free lime or iron oxide.

Conservation—Planned management of soilresources to prevent loss of their productivequality by erosion. Conservation practicesinclude control of irrigation water, cover crops,stubble mulch, strip cropping, no-till, diversionterraces, and controlled grazing. See Chapter 7,“Erosion Control Practice.”

Consistence—A soil property indicating thesoil’s resistance to rupture or shear. Consis-tence is related to the amount of energyrequired to manipulate the soil. Different termsare used to describe moist, dry, and wetconsistence. See Chapter 4, “Soil Consistence.”

Convex—A landform surface that is curvedlike the surface of a ball. Convex slopesbecome progressively steeper as you walkdownslope. Soils on convex slopes usually arewell drained.

Creep—Very slow movement of soil materialdownslope under the pull of gravity. Over aperiod of several years, creep contributes tothe colluvial parent material that collects infoot slope landscape positions.

Decomposer—A general term for any organismin the soil that decays plant residues. Bacteriaand fungi are the primary decomposers insoils.

Decomposition—The slow decay of plantresidues in the soil, changing them ultimatelyinto humus. Decomposition is carried out bytiny soil plants (bacteria and fungi) that usefresh residues as a food source and bringabout the change into humus.

Density—The amount, or weight, of soilmaterial in a fixed volume of soil. Soils withhigh density are compact and have poorstructure. They prevent root penetration andwater movement through the soil. Soils withlow density are open and porous. Water, air,and roots all move easily through them.

Diatomaceous earth—A layer of lake-laidsediment composed of the shells of micro-scopic, single-celled plants called diatoms.Diatomaceous earth is nearly pure silica, and itusually is white.

Dissected—A landscape that has been cut bystream erosion into hills and valleys.

Drainage class—The degree of wetness of asoil, as determined by the depth to a watertable in the soil and the length of time the soilremains saturated. Common drainage classesinclude excessively drained, well drained,moderately well drained, somewhat poorlydrained, and poorly drained. See Chapter 5,“Internal Soil Drainage.”

Droughty—Inability of a soil to store enoughwater to meet plant requirements. Sandy andgravelly soils are droughty because they havelow water-holding capacities. Soils that areshallow to bedrock or to a dense soil horizonare droughty because they don’t provideenough volume of soil for roots to grow in.

Duripan—A special kind of B horizon that isvery hard because it has been cemented withsilica and free lime. See Chapter 3, “SpecialKinds of A, B, and C Horizons.”

Effluent—The fluid discharged from a septictank after solid wastes have settled to thebottom of the tank. Effluent flows into perfo-rated tile lines of a drainfield, and from thereinto the soil, where soil microorganisms treatand purify the wastewater.

Erosion—Loss of valuable topsoil by the actionof wind or water flowing over an unprotectedsoil surface.

Fallow—A field that is left uncropped during agrowing season so that water can be stored inthe soil for the next year’s crop. Fallow fieldsmust be kept free of weeds so that none of thestored moisture is removed.

Field capacity—The total amount of water inthe soil when it is holding the maximum amountof plant-available water. If a soil becomessaturated by a rainstorm or by irrigation, ittakes about 2 days for the free water, orgravitational water, to drain away. The mois-ture content at that time is the field capacity.

Fine earth—Individual soil grains that aresmaller than 2 mm in diameter. Fine earth isthat which will pass through a No. 10 sieve.

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Fine-textured—A general term that refers tosoils that have a high clay content. Clay, siltyclay, silty clay loam, and clay loam are soilsthat may be considered as fine-textured. Seealso “Heavy.”

Floodplain—The nearly level surface next to ariver that is covered with water when a riverfloods.

Foot slope—A landscape position at the baseof uplands that usually has a concave shapeand collects excess water by runoff and byseepage from higher positions. Soils onfootslopes often are somewhat poorly orpoorly drained.

Fragipan—A special kind of B horizon that isnot cemented but is very dense and compact.Neither water nor plant roots will penetratevery far into a fragipan. See Chapter 3, “SpecialKinds of A, B, and C Horizons.”

Free lime—Calcium carbonate (CaCO3) in thesoil. Free lime is white, and it bubbles violentlywhen a drop of hydrochloric acid (HCl) isplaced on it. Free lime occurs in soils that areonly partially leached, either as part of theoriginal parent material, or as a subsoil accu-mulation in a Bk horizon. The pH of soilscontaining free lime usually is above 7.6.

Friable—A soil that crumbles easily undergentle pressure. Friable soils require a mini-mum of energy to plow or to cultivate, andthey make ideal seedbeds for seedling emer-gence and root expansion. Friable soils are saidto be in good tilth.

Fungi—Plants that live in the soil and obtaintheir energy from the decomposition of plantresidues. Fungi often have branched formswith many long, thin, white strands of tissue.Molds and mushrooms both are fungi. Fungiand bacteria are the primary decomposers insoils and change plant residues into humus.

Glass shard—A silt- or sand-sized fragment ofvolcanic ash that is made up almost entirely ofsilica and is commonly needle-shaped.

Gleyed—Soil that is very wet for long periodsof time. Gleyed soils usually have dark graycolors, with or without mottles. See Chapter 3,“Special Kinds of A, B, and C Horizons (Bg).”

Heavy—A general term used to refer to a soilthat is high in clay content. The specific soiltextures named clay, silty clay, silty clay loam,and clay loam sometimes are called heavysoils. A heavy silt loam, or a heavy silty clayloam, indicates that the clay content of the soilis near the upper limit of the amount of claypermitted in that particular textural class.

Horizon—A layer of soil that is approximatelyparallel to the earth’s surface. See Chapter 3.

Humus—A product of microbial decomposi-tion of plant residues that resists furtherdecomposition and accumulates in the soil asorganic matter. Humus has a black color, andits incorporation in the surface soil darkensthe A horizon. Humus acts as a glue whichhelps form stable soil aggregates that promotegood air-water relations in soils.

Infiltration—The rate at which water entersthe soil. Infiltration depends on the texture,structure, and layering of the surface horizon.In this Manual we use the evaluation of surfacesoil permeability to indicate relative rates ofinfiltration.

Intermittent stream—A gully or small riverthat carries water only during rainy seasons. Itis dry for several months each year.

Interstratified—Deposited as a layer of sedi-ment between two other layers of sediment.The term usually refers to sediments depositedin thin layers at the bottom of a lake (lacustrinesediments), but it may refer to alluvial sedi-ments deposited on a river’s floodplain as well.

Laminations—Very thin layers in rocks formedby the deposition of sediments at the bottomof a lake. Laminations are visible as a wholeseries of thin lines in cuts through such rocks.

Leaching—Removal of soluble minerals fromthe soil by movement of water through the soilover long periods of time. The more solublethe soil minerals, the faster they are leachedout of the soil. Free salts are first to go, thenfree lime. Water-soluble fertilizers also aresubject to loss by leaching.

Light—A general term used to refer to a soilthat has a very low clay content and a rela-tively high sand content. Sands, loamy sands,and sandy loams all are light-textured soilsbecause they are high in sand and low in clay.

Lime—Calcium carbonate. See “Free lime.”

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Linear—A landscape surface that is flat buttilted, like a ramp. The middle parts of manyhillslopes have a linear shape between aconvex hilltop and a concave footslope.

Litter—Accumulation of dead leaves and twigson a forest floor.

Loam—A specific class of soil texture thatcontains a balanced mixture of sand, silt, andclay. Generally the sand content is between20 and 50 percent, and the silt content isbetween 30 and 50 percent. The clay contentmust be less than 27 percent. Loams haveenough sand to be able to feel some grit, andthey have enough clay to give the soil somebody, but the properties and behavior of thesoil are not dominated by either sand or clay.

Mantle—A thin layer of soil resting on top of adifferent kind of soil or geologic material.Examples include loess on top of basalt andlake-laid sediments on top of older residual soils.

Matrix—A soil color term that refers to thedominant color of the soil. In mottled soils, thematrix color is the one that covers the largestsurface area, and the mottle color is the onethat occurs in little spots scattered throughoutthe matrix.

Medium-textured—A general term used torefer to a soil that has relatively low contentsof sand and clay and relatively high contents ofsilt. Loams and silt loams often are calledmedium-textured soils, although some clayloams and silty clay loams at the low end oftheir clay ranges also may be called medium-textured soils. The term loamy soil sometimesmay be used to mean the same thing as amedium-textured soil.

Microbes—See Microorganisms.

Microorganisms—Tiny, microscopic plantsand animals that live in the soil. Also calledmicrobes, they decompose dead plant andanimal residues and convert them into humus.Some microorganisms cause plant and animaldiseases. Others are a source of antibiotics tocombat diseases. Still others help make soilnutrients more available for plant use.

Mineral—A naturally formed chemical com-pound that has a nearly uniform compositionand crystal form. Rocks are aggregates of oneor more different minerals. In soils, the miner-als quartz, feldspar, and mica are common inthe silt and sand fractions. Common clayminerals are kaolinite, illite, and montmorillonite.

Mineral grains—Individual particles of rocks,or of the minerals that make up rocks(e.g. quartz, feldspar, or mica) that become thesand, silt, and clay particles of soils. Except fora small amount of organic matter, soils arecomposed almost entirely of mineral grains.

Moderately well drained—A soil that has atemporary water table for short periods oftime in the lower part of the subsoil. The soil ismottled somewhere between 24 and 40 inches.See Chapter 5, “Internal Soil Drainage.”

Mottles—Splotches of colored soil in a matrixof a different color. Mottles may be yellowishbrown in a grayish matrix, or gray in a brownmatrix. In either case, mottles indicate that thesoil is periodically saturated for several daysor weeks at a time. See Chapter 4, “Mottling.”

Muck—Black, highly decomposed plantmaterial that accumulates at the bottom of abog or a shallow lake. Muck is so completelydecomposed that the original plants cannot beidentified. Accumulations of muck and peat,when exposed by draining the lake or loweringthe water table, form special kinds of soilscalled Organic soils.

Nitrogen—A chemical element that also is anessential nutrient needed by plants in largequantities. In natural soil-plant systems, almostall of the nitrogen in the soil comes frommicrobial decomposition of plant residues, andit’s stored in the soil as soil organic matter. Inagricultural systems, we supplement the nativesupply of soil nitrogen by adding nitrogenfertilizers.

Nodules—Small, hard accumulations of chemi-cal compounds in the soil. Nodules composedmainly of iron oxides usually are sphericalpellets 2 to 5 mm in diameter. Nodules ofcalcium carbonate usually are irregularlyshaped and measure up to an inch across.

Nutrients—Chemical elements in the soil thatare required by plants to live and grow. The“big three” nutrients are nitrogen, phosphorus,and potassium. Others needed in fairly largequantities are calcium, magnesium, and sulfur.Many others are needed, but in very smallquantities.

Opal—A mineral composed of silica that formswhen silica-bearing leaching water evaporatesand leaves the silica behind as a deposit in thesoil. Opal often occurs in horizontal layers atthe top of a duripan. See also “Silica,” “Duripan.”

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Organic matter—The sum of all plant andanimal material, living or dead, that is mixedinto the soil. Living microorganisms are part ofsoil organic matter, and so is the humus theyproduce. So are living and dead plant roots,both large and small. Organic matter promotesgood soil structure, improves rates of move-ment of air and water through the soil, increasesthe storage of water for plant use, and providesnitrogen needed by plants.

Oxidation—The chemical change of iron in thesoil to a yellow-brown form that is just likerust. Oxidation requires free exchange of airbetween the soil and the atmosphere. Oxidizedsoils are well aerated and well or moderatelywell drained.

pH—A number on a scale from 1 to 14 thatindicates the relative degree of acidity oralkalinity of the soil. A pH of 7 indicates aneutral soil, one that is neither acid noralkaline. Smaller numbers indicate acid soils.Most western Oregon soils have pH valuesbetween 5 and 7. Larger numbers indicatealkaline soils. Most eastern Oregon soils havepH values between 7 and 9. Sodic soils have pHvalues of 9 or above.

Parent material—The original geologic mate-rial from which the horizons of a soil areformed. Parent material includes all kinds ofbedrock, sediments deposited by a stream orin a lake, and wind-blown silts or volcanic ash.See Chapter 6.

Peat—Brown, partly decomposed plant mate-rial that accumulates at the bottom of a bog ora shallow lake. Peat is fibrous, and some of thefibers can still be recognized. Peat and muck,when exposed by draining the lake or loweringthe water table, form special kinds of soilscalled Organic soils.

Ped—A single unit of soil structure. Pedshapes include granular, platy, blocky, andprismatic. Ped sizes may vary froml-mm granules to 10-cm prisms. See Chapter 4,“Soil Structure.”

Perched water table—The top of a zone ofsaturated soil that lies above a very slowlypermeable soil horizon. The soil in andbeneath the slowly permeable horizon may notbe saturated. Because water moving downthrough the soil can’t enter the restrictivelayer, it builds up in the soil above, creating asaturated zone perched in the soil well above apermanent water table.

Perennial stream—A stream or river that haswater in it almost all of the time.

Permeability—The rate that water moveswithin the soil. Permeability depends on thetexture, structure, and density of soil horizons.For the soil as a whole, the permeability ratingis that of the least permeable horizon present.See Chapter 5.

Poorly drained—A soil that is saturated at ornear the surface for long periods of time.Poorly drained soils are gleyed, and they oftenhave mottles in the A or Ap horizon. SeeChapter 5, “Internal Soil Drainage.”

Pores—Spaces between the mineral grains of asoil. Pores are relatively large in sandy andgravelly soils. They may be extremely small inclayey soils. The size, shape, and arrangementof soil pores determine the rates of water andair movement into and throughout the soil.They also control the amount of availablewater that a soil can store for plant use.

Porosity—The total amount of pore space in asoil. Porous soils have plenty of pore space. Asa result, they have low densities and rapid ormoderate permeability. Compact, dense soilshave low total porosity. They have slow or veryslow permeability.

Profile—A vertical section of soil that allowsyou to see all the horizons that are present.

Reduction—The opposite of oxidation. Thechemical change of iron in the soil to a formthat is more soluble than the oxidized formand can be leached from the soil. Reductionusually occurs when the soil is saturated anddeprived of oxygen. Loss of iron oxide coatingsfrom mineral soil grains by reduction andleaching result in a gray soil color. Reducedsoils often are called gleyed.

Restrictive layer—A general term for any soilhorizon that is slowly or very slowly perme-able. Restrictive layers slow down the rate ofwater movement in the soil. They also impedeplant root penetration.

Ripping—Breaking up compacted soil, tillagepans, or duripans by pulling a curved steelshank through the soil at depths ranging from12 to 36 inches.

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Sand—A specific grain size in the soil. Sandgrains range in size from 0.05 to 2.0 mm indiameter. Sand grains are big enough to seewith the naked eye, and they feel gritty. Sand isalso the name of a specific class of soil texturethat has more than 90 percent sand and almostno clay. See Chapter 4, “Texture.”

Salt—A chemical compound like ordinary tablesalt (sodium chloride) that is very soluble inwater and is readily leached out of soils. Othersoluble salts include magnesium chloride andcalcium chloride.

Saturated—Completely filled with water. Whenall the pores of a soil are full of water, the soilis saturated. Saturated soils prevent air fromgetting into soil pores, and are said to bepoorly aerated. Saturated soils lead to theprocesses of reduction and gleying.

Sediment—Loose grains of mineral materialdeposited by a river as alluvium on a flood-plain or a fan. Sediment also refers to similardeposits that accumulate on the floor of a lake.See Chapter 6, “Parent Materials.”

Separate—A subdivision of the fine earthaccording to the sizes of the individual soilgrains. The three separates of fine earth aresand, silt, and clay. See Chapter 4, “Texture.”

Shrink-Swell soil—A soil that has clay par-ticles that expand when they absorb water andcontract when they dry out. Shrink-swell soilsare called Vertisols, and they usually containmore than 50 percent clay. Slickensides arefeatures of shrink-swell soils, as well. See also“Slickensides.”

Silica—Mineral material in soil that consists ofa chemical combination of the elements siliconand oxygen. Most sand and silt grains arecomposed of silica. Silica is very slowlysoluble; but under some conditions, it can bemoved downward in the soil, where it isdeposited again in the pores between soilparticles. These silica deposits cement the soiltogether into a very hard layer called aduripan. See Chapter 3, “Special Kinds of A, B,and C Horizons.”

Silt—A specific grain size in the soil. Silt grainsrange in size from 0.002 to 0.05 mm in diameter.They are too small to see with the naked eye,and they feel smooth, like flour or corn starch.See Chapter 4, “Texture.”

Slaking—The breaking down of structuralaggregates, or peds, under the impact of fallingdroplets of water. Individual grains of silt andclay are washed off the peds and tend to clogup nearby soil pores. A thin surface crustforms that seals the soil, prevents infiltration,and increases the erosion hazard. Silty soilsthat are low in organic matter and have weakstructure are very susceptible to slaking.

Slickensides—Polished shiny surfaces causedby the movement of two masses of clayey,shrink-swell soil past each other. Slickensidesare characteristics of Vertisols. See Chapter 4,“Special Features of Soil Horizons.”

Sodic soils—Soils that have high concentra-tions of sodium salts in them. Sodium causesthe soil to have very poor structure and a pHvalue of 9 or above. Sodic soils occur ineastern Oregon, where evaporation causeswater to move upward in the soil from ashallow water table. Salt deposits on thesurface form a white crust that may be called aslick spot.

Soluble—Capable of being dissolved. Tablesalt is very soluble in water. The same salt isvery soluble in soil. Free lime in soil is quitesoluble, but less so than salts like table salt.Silt and sand-sized grains of rocks and mineralsin soils are only very slowly soluble. It takes avery long time to remove these minerals fromthe soil by leaching.

Somewhat poorly drained—A soil that issaturated in the upper part of the subsoil forsignificant periods of time during the rainyseason. The soil usually is gray and mottledbetween 10 and 24 inches. In eastern Oregon,sodic soils having pH values above 9.0 also aresomewhat poorly drained. See Chapter 5,“Internal Soil Drainage.”

Stratified—Deposited in distinct layers on ariver floodplain or at the bottom of a lake.Differences in the texture or color of a sedi-ment make these layers visible.

Structure—Arrangement of individual grains ofsand, silt, and clay into larger units calledaggregates or peds. Plant roots, humus, andclay minerals all help to hold the grainstogether. Structure is characterized by thetype, or shape, of the peds and by their grade,or degree of development. See Chapter 4.

Glossary


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Terrace—A landform that resembles the treadof a giant stair. Terraces formed originally asriver floodplains or ocean beaches. Later, asrivers cut down or ocean levels dropped, theoriginal landform was abandoned and left at ahigher level. Terraces are no longer coveredwith flood waters. See Chapter 6, “Landform.”

Texture—The amounts of sand, silt, and claythat make up a soil sample. Names are given tospecific combinations of sand, silt, and clay toform textural classes such as loam, silt loam,sandy loam. See Chapter 4.

Tilth—The physical condition of the surfacesoil. Good tilth requires plenty of organicmatter, good soil structure, and favorableair-water relations. Soils in good tilth are easyto work, easy for plant seedlings to emergefrom, and easy for plant roots to grow in.

Very poorly drained—Soils that are saturatedthroughout, almost all of the time. They arevery wet soils in swamps, bogs, and tidallowlands.

Water-holding capacity—See AWHC.

Water table—The top of a zone of saturatedsoil.

Weathering—The changing of rocks into soils.Physical weathering breaks rock fragmentsdown to smaller fragments. Chemical weather-ing destroys some original rock minerals andcreates new kinds of soil minerals. SeeChapter 2.

Well-aerated—Soil that allows easy exchangeof air between the soil and the atmosphere.Well-aerated soils have plenty of pores that arebig enough and sufficiently interconnected toprovide pathways for air movement. They haveplenty of organic matter and good structure,and they usually are well or moderately welldrained.

Well drained—A soil that is rarely saturatedabove a depth of 40 inches. Well-drained soilsare well aerated and have brown oryellow-brown colors. They have no mottlesabove 40 inches. See Chapter 5, “Internal SoilDrainage.”

Glossary


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95

Contestant ______________________________________ Site ________________________________

School __________________________________________________________________________________

Scorecard: Oregon State Soil Judging

First Horizon( __ to __ inches)

_______ Color

_______ Mottles

_______ Texture

Coarse_______ Fragments

Structure_______ Type

Structure_______ Grade

Special_______ Features

Horizon_______ Name

Second Horizon( __ to __ inches)

_______ Color

_______ Mottles

_______ Texture

Coarse_______ Fragments

Structure_______ Type

Structure_______ Grade

Special_______ Features

Horizon_______ Name

Third Horizon( __ to __ inches)

Color________

Mottles________

Texture________

CoarseFragments________

StructureType________

StructureGrade________

SpecialFeatures________

HorizonName________

Fourth Horizon( __ to __ inches)

Color________

Mottles________

Texture________

CoarseFragments________

StructureType________

StructureGrade________

SpecialFeatures________

HorizonName________

Color Mottles1 = Dark brown, 1 = None

Very dark brown, 2 = Few to commonBlack 3 = Many

2 = Light brown, Brown,Yellowish brown

3 = Red, Reddish brown4 = Dark gray,

Light gray, White

Texture Coarse Fragments1 = Sand, Loamy sand 1 = <15%2 = Sandy loam 2 = 15 to 35%3 = Loam, Silt loam 3 = 35 to 60%4 = Sandy clay loam, 4 = >60%

Clay loam,Silty clay loam

5 = Clay, Silty clay,Sandy clay

6 = NA—Duripan, Cr, R, O

Structure type Structure grade1 = Granular 1 = Structureless2 = Platy 2 = Weak3 = Blocky 3 = Moderate4 = Prismatic 4 = Strong5 = Massive, Single grain

Special features Horizon name1 = None 1 = O 5 = C2 = Tillage pan 2 = A 6 = R3 = Fragipan 3 = E 7 = AB,4 = Duripan 4 = B BA, BC5 = Cr6 = Slickensides

Total Front Total Back Grand Total

This camera-ready master is provided for your convenience. Please copy it as needed.


96

Total Back

}


Properties of the Whole Soil_______ Effective Depth Water Erosion Hazard _______

1 = Deep ( >40 inches) 1 = Low2 = Moderately deep (20–40 inches) 2 = Moderate3 = Shallow (10–20 inches) 3 = High4 = Very shallow(<10 inches) 4 = Very high

_______ Available Water-holding Capacity Wind Erosion Hazard _______1 = High (>8 inches) 1 = Low2 = Medium (5–8 inches 2 = Moderate3 = Low (2–5 inches) 3 = High4 = Very low (<2 inches) Internal Drainage _______

_______ Surface Soil Permeability 1 = Rapid 1 = Excessive2 = Moderate 2 = Well3 = Slow 3 = Moderately well

_______ Subsoil Permeability 4 = Very Slow 4 = Somewhat poor5 = Poor

Site Characteristics_______ Site Position Stoniness/Rockiness _______

1 = Upland 1 = None2 = Foot slopes or Fans 2 = Stony/Rocky3 = Floodplain 3 = Very stony/Rocky4 = Stream terrace or Lake plain 4 = Extremely stony/Rocky

_______ Parent Material Slope ______1 = Residuum or Colluvium 1 = 0–3%2 = Recent alluvium 2 = 3–7%3 = Old alluvium or Lacustrine 3 = 7–12%4 = Wind-blown sands or silts 4 = 12–20%5 = Volcanic ash 5 = >20%6 = Two or more classes

Management Interpretations_______ Drainage Feasibility Erosion Control Practice ________

1 = Not needed 1 = Water control, Cover crop,2 = Good Standing stubble3 = Fair 2 = Stubble mulch, No-till4 = Poor 3 = Diversion terraces, No-till

_______ Irrigation Suitability 4 = Strip crop, No-till1 = Excellent 5 = Controlled grazing2 = Good 6 = Watershed management3 = Fair Reaction Correction _______4 = Poor 1 = None5 = Non-irrigable 2 = Correct acidity

_______ Most Intensive Crop 3 = Correct alkalinity1 = Row crop/Specialty crop Limitations for Septic2 = Legume hay Tank Drainfields _______3 = Dryland wheat 1 = Slight4 = Dryland hay 2 = Moderate5 = Permanent pasture/Grass seed 3 = Severe6 = Timber grazing7 = Rangeland grazing8 = Forestry


97Interpretation Guide for Oregon Soil Judging

Stoniness/Rockiness

None Stny/Rcky Very Ext

Stones (%) <.01 .01–.1 .1–3 >3

Rocks (%) <.01 .01–2 2–10 >10

Drainage Feasibility

Good Fair Fair Poor Poor Poor

Outlets Available — — — — Not available

Effective depth Deep — Mod deep — Shallow Any

Drainage Subsoil MW/Any — MW/Any —class perm. SWP/Rpd, SWP/Sl, SWP/Any — Any Any

Mod V sl PD/Rpd, Mod PD/Sl, V sl

Wind Erosion Hazard

Low Low Mod High

Location W. OR E. OR — —

Texture Any Sacl Sicl L SaSac Sic Sil LsaCl C Sal

Water Erosion Hazard

Low Moderate High Low Moderate High Very high

Texture Sa Sac Sacl LLsa Sic Sicl ClSal C — — Sil — — —

Slope / Perm. of 0–12/Any 12–20/Any >20/Any 0–3/Any 3–7/Slow, 7–12/Slow, >20/Any Sfc soil 3–7/Rapid, Mod V Slow V Slow

7–12/Rpd, Mod 12–20/Any

AWHC Rates (In/In)

Sa, Lsa .06Sal .12L, Cl, Sil, Sicl .20C, Sic, Sac, Sacl .15

Permeability—Surface Soil and Subsoil

Rpd Mod Slo Mod Slo Mod Slo V Slo

Texture Sa Sal Sacl SacLsa Sil — Sicl — Sic — —

L Cl C

Porosity Any Por Not Por Not Por — Notpor por por

Structure Any Any Any Any Wk, Str Mod, Mass, Grade Mass Wk Vert

Mottles

Few 0–2%

Common 2–20%

Many >20%

Irrigation Suitability

Excellent Good Fair Poor Non-irrigable

Slope Permeability 0–3/Any 3–7/Slow 7–12/Slow 3–12/V slow >20/Anyof surface soil 3–7/Rpd, Mod 7–12/Rpd, Mod 12–20/Any

Minimum AWHC High Medium Low Very low Very low

Internal Drainage WD WD WD Any Anydrainage feasibility MWD/Good MWD/Good, Fair MWD/Any

SWP/Good SWP/Good, Fair SWP/AnyPD/Fair

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98

Most Intensive Dryland Crop—Eastern Oregon<4,100 feet

Wheat TG Graz Graz Wheat Graz

pH <8.4 — — — — Any

Rainfall ≥12 — — — <12 Any

AWHC Low–High — — V low Med–High Any

Slope/Aspect <20/Any ≥20/N ≥20/S Any <35/Any Any

Erosion Control Practice—Western Oregon

Water ctrl, Stbl Ctrl Wtrshd Cover crop mulch grazing mgt

Most intensive crop Row crop Wheat Pasture ForestryLegumeGrass seed

Most Intensive Crop—Irrigated

Row crop or Legume Perm pasture/Specialty crop hay Grass seed

Location All except— Any —High basins in Baker, Harney, Lake, WallowaCoastal fog belt

pH <8.4 <8.4 Any

Slope 0–12 0–20 0–20

Irrigation Excellent Any Anysuitability Good

Fair—0–3% slopeFair—sandy soils upto 12% slope

AWHC High-Low High-Low Any

Drain. feas. Good-Fair Good-Fair Any

Erosion Control Practice—Eastern Oregon

Stubble Diversion Strip Strip Stubble Diversion Stubble Water ctrl,mulch, terraces, crop, crop, mulch, terraces, mulch, Cover Controlled WatershedNo-till No-till No-till No-till No-till No-till No-till crop grazing Mgt

Crop Wheat — — — — — — Row crop Timber Forestrygrazing,Range

Rainfall ≤10 in. — — — >10 in. — — Any Any Any

Wind erosion Lo Lo, Mod Mod Hi Lo, Mod — Hi Any Any Any

Water erosion Lo, Mod Hi, V hi Lo, Mod Any Lo, Mod Hi, V hi Any Any Any Any

Most Intensive Dryland Crop—Eastern Oregon >4,100 feet

Timber Timber Timber TimberHay Hay grazing Forestry grazing Grazing grazing Grazing grazing Grazing

pH ≤8.4 — — — — — — — — Any

Slope/Aspect <20/Any — — ≥20/N — <20/Any — — ≥20/S Any≥20/N

AWHC Med–High Low — Low–High — Low–High V low — Low–High Any

Rainfall Any ≥18 12–18 ≥15 12–15 <12 ≥18 <18 ≥15 Any

Septic Tank Drainfields

Slight Moderate Severe

Effective depth ≥48 in ≥36 in <36 in

Subsoil perm. Mod Rpd to mod Any

Internal drainage WD WD, MWD Any

Slope 0–7 0–12 Any

Landform All except All except Anyfloodplains floodplains

of per. streams

Most Intensive Dryland Crop—Western Oregon

PP/ PP/ PP/Wheat GS Wheat GS For GS For

Slope <12 — 12–20 — — 20–35 Any

Rainfall <60 Any 30–45 <30 >60 30–45 Any45–60

Drain. Drain.WD Any WD Any Any Any Anyclass feas. MW/Any MW/Any

SWP/Any SWP/AnyPD/Good, Fair PD/Good, Fair



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Manual for Judging Oregon Soils

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