SSoils Basicsoils Basics

Soil

• Horizons or layers, or . . .• The ability to support rooted plants in a natural

environment– Upper limit is air or shallow (>2.5 m) water– Lower limit is either bedrock or the limit of biological

activity– Lower limit for classification set at an arbitrary 2 m

Natural body that occurs on the land surface, occupies space, and is characterized by one or both of the following:

Nonsoil

• Badlands• Beaches• Rubble lands• Rock outcrops• Glaciers• Deepwater habitats

Factors That Influence Soil Development

• Climate• Parent material• Topographical relief• Organisms• Time• Humans

Landscape Position

• Critically influences water flow and soil formation• Most wetlands, even groundwater seeps, are on

some sort of concave surface

Divergent

Convergent

Slope Block Contour

Hill Slope Elements and Curvature

Upslope

Upslope

After Pennock et al., 1987Overland and Throughflow:

Convergent landscapes

Throughflow

Runoff

InfiltrationPercolation

Potential hydric soilzone

“Epiaquic”

“Endoaquic”

Modified from Pennock et al., 1987

Soil Catena

Summit BackslopeFootslope Toeslope

Key Soil Properties

• Color• Organic matter• Texture• Horizonation• Drainage• Permeability

Properties that are important to hydric soil development and recognition:

Aspects of Soil Color

• Hue• Value• Chroma

10R 5/8

Coloring Agents in Soil

• Organic matter – OM will mask all other coloring agents.

• Iron (Fe) – brown colors are the result of Fe oxide stains coating

individual particles

• Manganese (Mn) – resulting in a very dark black or purplish black color

• Lack of coatings– Color of the mineral soil grains (stripped)

Soil Color

Coating of Fe2 O3

Mineral grain (gray)

Remove FeBright Soil

Gray Soil

“Bright-colored” soil is bright because the gray-colored mineral grains are coated with a thin layer of “paint” formed by Fe oxides. Stripping the paint off the particles leaves the

mineral grains exposed.

“Bright-colored” soil is bright because the gray-colored mineral grains are coated with a thin layer of “paint” formed by Fe oxides. Stripping the paint off the particles leaves the

mineral grains exposed.

HueRed

Yellow-Red

Yellow

0 2.5R 5R 7.5R 10R

0 2.5YR 5YR 7.5YR 10YR

0 2.5Y 5Y 7.5Y 10Y

Value

• 10/0 - Pure White

• 5/0 - “Gray”

• 0/0 - Pure Black

The Lightness or Darkness of Color

“Neutral”Color

“Pure”Color

/0 /2 /4 /6 /8

Increasing strength of color

Increasing grayness

Chroma

What is important?

• Chroma is most important, with value close behind

• Hue is also important – Gley page hue is present– Red parent material soils– Gray parent materials

Soil Color

Munsell Soil Color Book, 7.5YR page

Munsell®

or Earth Colors ®

• The Munsell notation system is a system for recording color.

• The Earth Colors and Munsell books use the same colors.

– Differences are from personal interpretation of color, fading of pages over time, smudging of chips with use, and quality control from the factories.

Reading Soil Colors

• Optimum conditions– Natural light– Clear, sunny day– Midday– Light at right angles– Soil moist

Color Patterns in Soils

• Matrix (predominant) color• Color of redoximorphic features (mottles)• Contrast, abundance, location, and size of

redox features

Redox Feature

Matrix

Contrast

• Contrast refers to the degree of visual distinction between associated colors– Faint -- evident only on close examination– Distinct -- readily seen at arms length– Prominent -- contrast strongly

Several indicators require distinct or prominent contrast!

Defining Distinct Redox

• Different Munsell page (hue), OR• Value differs by more than 2 (i.e. 10YR 3/6

vs 10YR 6/6), OR• Chroma differs by more than 1 (i.e. 10YR

4/4 vs 10YR 4/6)

Abundance and Size of Redox

• Few -- less than 2%• Common -- 2 to 20%• Many -- more than 20%

• Fine -- < 5 mm• Medium -- 5 to 15 mm• Coarse -- > 15 mmSeveral indicators require at least 2% abundance From Munsell Color Chart & Field Book

Two Categories of Soil Material

• Organic horizons• Mineral horizons

Differences in organic matter content:

Two Categories of Soil Material

• Consist of decomposed organic material

• An organic soil has at least 16 inches of decomposed organic material in the upper 32 inches, or any thickness over bedrock

Organic horizons

Two Categories of Soil Material

• Primarily sand, silt, and clay, with varying amounts of organic matter

Mineral horizons

Soil Texture - Relative Sizes of Soil Particles

Sand (0.05-2.00 mm)

Silt (0.002-0.05 mm)

Clay (<0.002 mm)

Soil Texture Triangle

Loamy Sands?

• 70-87% sand, 0-30% silt, and 0-15% clay,• loose and single-grained,• most individual grains can be seen and felt,• are slightly cohesive when moist, and • fragile casts can more readily be formed

with them vs. sands

Specifically, What Kind of Loamy Sand?

• Loamy fine sand. 50 percent or more* fine sand; or less than 50 percent* very fine sand and a total of less than 25 percent* very coarse, coarse, and medium sand.

OR• Loamy very fine sand. 50 percent or

more* very fine sand.

• *by weight, not volume

How to Know…

• Accurate texturing procedure• Soil survey information may help• Fine sands are slightly gritty• Very fine sands will feel smooth• Sieve analyses

• - If >50% (by weight of total soil) lvfs, then loamy• - If <50% (by weight of total soil) lfs, then sandy

Crops and Soils Club Phone: 715-425-3395Department of Plant & Earth ScienceUniversity of WI – River Falls410 South Third St., River Falls, WI 54022-5001• A. Texture kit contains samples of 10 soils, listing

percentage of sand, silt and clay and includes a soil textural triangle.– $25.

• B. Sand-size card – Samples of USDA vfs, fs, s, cos, vcos mounted on a 4 1/2 x 7 1/4 6-hole punch card. $8.00

Texturing Standards

Major Horizon Designations

O horizon

A Horizon

B Horizon

C Horizon

R Horizon

Surface Mineral Layer

Subsoil

UnderlyingMaterial

Bedrock

Surface Organic Layer

Horizon Terminology

Transition Layer

Layer ofAccumulated

Lime

Bedrock

SurfaceSoil

Subsoil

UnderlyingMaterial

SurfaceLayer

SubsurfaceLayers

Oi Oe OaApA1A2

A

EAB

BA

Bt

BC

AC

Bk

C1

C2

2CRCr

A horizon

Bw horizon

C horizon

A horizon

E horizon

Bh horizon

Bs horizon

Permeability

• A measure of the ability of water or air to move through the soil profile

• Permeability is “measured” in inches per hour– Permeability is actually an estimated property

based primarily on texture and structure– Measured values are saturated hydraulic

conductivity

Permeability

Capillary Fringe

• Based upon permeability• The zone above the free

water table that is effectively saturated– Water held at tension– Theoretical values much

higher than “real life”– Difficult to measure

SaturatedSaturated

Capillary FringeCapillary Fringe

All Pores Filled with WaterAll Pores Filled with Water

Largest Pores Not FilledLargest Pores Not Filled