Soil Quality: The view through the prism of Soils 101 D.W. Johnson Natural Resources and...

Soil Quality: Soil Quality: The view through the prism of Soils 101The view through the prism of Soils 101

D.W. JohnsonD.W. Johnson

Natural Resources and Environmental ScienceNatural Resources and Environmental Science

University of Nevada, RenoUniversity of Nevada, Reno

Soil Quality: Who or What defines it?Soil Quality: Who or What defines it?

•Soil Scientists?Soil Scientists?•Plants? Plants? •Water quality?Water quality?•Lawyers?Lawyers?•Farmers?Farmers?•Conservationists?Conservationists?

Will one definition fit all?Will one definition fit all?Not likelyNot likely

Will the various definitions conflict?Will the various definitions conflict?Almost certainlyAlmost certainly

Factors of soil formationFactors of soil formation Hans Jenny:

time

Soil = ∫f(parent material, climate, biota, topography)

•You cannot modify parent material, climate, or timeYou cannot modify parent material, climate, or time

•You can modify biota (vegetation easily, microbes You can modify biota (vegetation easily, microbes less easily) and, with some effort, topographyless easily) and, with some effort, topography

SOIL ORDERS (12 major units of classification SOIL ORDERS (12 major units of classification according to the US 10th Approximation)according to the US 10th Approximation)

Alfisols: Alfisols: Clay migration, moderately high %BSClay migration, moderately high %BS

Andisols:Andisols: Volcanic parent material, high P fixationVolcanic parent material, high P fixation

Aridisols:Aridisols:Arid soils, high in salts and pHArid soils, high in salts and pH

Entisols:Entisols: Not well-developed even after long periods (can occur anywhere) Not well-developed even after long periods (can occur anywhere)

Gelisols:Gelisols: PermafrostPermafrost

Histosols:Histosols: Soils formed from organic matter (peats and mucks)Soils formed from organic matter (peats and mucks)

Inceptisols:Inceptisols:Still forming, water is available for soil formation Still forming, water is available for soil formation

Mollisols:Mollisols:Organic-rich A horizons, %BS usually > 50% Organic-rich A horizons, %BS usually > 50%

Oxisols:Oxisols: Highly-weathered (e.g., tropical rainforest)Highly-weathered (e.g., tropical rainforest)

SpodosolsSpodosols: : Fe, Al, and organic matter transport, whitish E Horizon (e.g., boreal Fe, Al, and organic matter transport, whitish E Horizon (e.g., boreal

forest) forest)

Ultisols:Ultisols:Clay transport like Alfisols, but much more acidic; higher temperature; Clay transport like Alfisols, but much more acidic; higher temperature;

often highly weathered (e.g., Southeastern U.S.)often highly weathered (e.g., Southeastern U.S.)

VertisolsVertisols: : Mixed soils; Swelling clays, frost, etc cause lower horizons to mix Mixed soils; Swelling clays, frost, etc cause lower horizons to mix

with upper horizons; Often characterized by crackswith upper horizons; Often characterized by cracks

The central concept of Alfisols is that of soils that have an argillic, a kandic, or a natric horizon and a base saturation of 35% or greater. They typically have an ochric epipedon, but may have an umbric epipedon. They may also have a petrocalcic horizon, a fragipan or a duripanhttp://soils.usda.gov/technical/classification/orders/alfisols.html.

Alfisols: Relatively high base saturation; not organic rich; evidence of clay transport

The central concept of Andisols is that of soils dominated by short-range-order minerals. They include weakly weathered soils with much volcanic glass as well as more strongly weathered soils. Hence the content of volcanic glass is one of the characteristics used in defining andic soil properties.Materials with andic soil properties comprise 60 percent or more of the thickness between the mineral soil surface or the top of an organic layer with andic soil properties and a depth of 60 cm or a root limiting layer if shallower.http://soils.usda.gov/technical/classification/orders/andisols.html.

Andisols: Soils derived major properties from volcanic parent material. High P fixation. Many soils locally derived from andesite are “andic”

The central concept of Aridisols is that of soils that are too dry for mesophytic plants to grow. They have either:(1) an aridic moisture regime and an ochric or anthropic epipedon and one or more of the following with an upper boundry within 100 cm of the soil surface: a calcic, cambic, gypsic, natric, petrocalcic petrogypsic, or a salic horizon or a duripan or an argillic horizon, or(2)A salic horizon and saturation with water within 100 cm of the soil surface for one month or more in normal years.An aridic moisture regime is one that in normal years has no water available for plants for more than half the cumulative time that the soil temperature at 50 cm below the surface is >5° C. and has no period as long as 90 consecutive days when there is water available for plants while the soil temperature at 50 cm is continuously >8° Chttp://soils.usda.gov/technical/classification/orders/aridisols.html.

Aridisols:Arid soils; Low in organic matter; high in salts and pH

The central concept of Entisols is that of soils that have little or no evidence of development of pedogenic horizons. Many Entisols have an ochric epipedon and a few have an anthropic epipedon. Many are sandy or very shallowhttp://soils.usda.gov/technical/classification/orders/aridisols.html.

Entisols: Leftovers; Not well-developed even

after long periods (can occur anywhere)

The central concept of Gelisols is that of soils that have permafrost within 100 cm of the soil surface and/or have gelic materials within 100 cm of the soil surface and have permafrost within 200 cm.Gelic materials are mineral or organic soil materials that have evidence of cryoturbation (frost churning) and/or ice segeration in the active layer (seasonal thaw layer) and/or the upper part of the permafrosthttp://soils.usda.gov/technical/classification/orders/gelisols.html.

Gelisols: permafrost

The central concept of Histosols is that of soils that are dominantly organic. They are mostly soils that are commonly called bogs, moors, or peats and mucks.A soil is classified as Histosols if it does not have permafrost and is dominated by organic soil materials.http://soils.usda.gov/technical/classification/orders/histosols.html.

Histosols: Soils formed from organic matter (peats and mucks)

The central concept of Inceptisols is that of soils of humid and subhumid regions that have altered horizons that have lost bases or iron and aluminum but retain some weatherable minerals. They do not have an illuvial horizon enriched with either silicate clay or with an amorphous mixture of aluminum and organic carbon.The Inceptisols may have many kinds of diagnostic horizons, but argillic, natric kandic, spodic and oxic horizons are excluded.http://soils.usda.gov/technical/classification/orders/histosols.html.

Inceptisols:Still forming; Water is available for soil formation (e.g., glaciated soils). Common in the Sierra Nevada

The central concept of Mollisols is that of soils that have a dark colored surface horizon and are base rich. Nearly all have a mollic epipedon. Many also have an argillic or natric horizon or a calcic horizon. A few have an albic horizon. Some also have a duripan or a petrocalic horizon. http://soils.usda.gov/technical/classification/orders/mollisols.html.

Mollisols:Brown-black surface horizons; High in organic matter, vermiculite or smectite clays; Base saturation usually > 50% (e.g., Iowa farm soils) Most extensive in the US (25%), present in the Great Basin at higher elevations

The central concept of Oxisols is that of soils of the tropical and subtropical regions. They have gentle slopes on surfaces of great age. They are mixtures of quartz, kaolin, free oxides, and organic matter. For the most part they are nearly featureless soils without clearly marked horizons. Differences in properties with depth are so gradual that horizon boundaries are generally arbitrary. . http://soils.usda.gov/technical/classification/orders/oxisols.html.

Oxisols: Highly-weathered; Only quartz, kaolinite, and Fe and Al oxides left (e.g., tropical rainforest)

The central concept of Spodosols is that of soils in which amorphous mixtures of organic matter and aluminum, with or without iron, have accumulated. In undisturbed soils there is normally an overlying eluvial horizon, generally gray to light gray in color, that has the color of more or less uncoated quartz.Most Spodosols have little silicate clay. The particle-size class is mostly sandy, sandy-skeletal, coarse-loamy, loamy, loamy- skeletal, or coarse-siltyhttp://soils.usda.gov/technical/classification/orders/spodosols.html.

Spodosols: Evidence of Fe, Al, and organic matter transport; Often a whitish E Horizon

(e.g., boreal forest)

The central concept of Ultisols is that of soils that have a horizon that contains an appreciable amount of translocated silicate clay (an argillic or kandic horizon) and few bases (base saturation less than 35 percent). Base saturation in most Ultisols decreases with depth.http://soils.usda.gov/technical/classification/orders/ultisols.html.

Ultisols:Clay transport like Alfisols, but much more acidic. Higher temperature; Often

highly weathered (e.g., Southeastern U.S.)

The central concept of Vertisols is that of soils that have a high content of expending clay and that have at some time of the year deep wide cracks. They shrink when drying and swell when they become wetter. http://soils.usda.gov/technical/classification/orders/vertisols.html.

Vertisols: Mixed soils; Swelling clays, frost, etc cause lower horizons to mix with upper horizons; Often characterized by cracks. Present locally, San Rafael park

Basic Soil Physical PropertiesBasic Soil Physical Properties• Texture: Texture: particle size distribution (sand, silt, clay)particle size distribution (sand, silt, clay)• Structure: Structure: arrangement of particles (blocky, arrangement of particles (blocky,

single grained, massive, platy…)single grained, massive, platy…)• Coarse fragments/rocks: Coarse fragments/rocks: > 2mm> 2mm• Bulk density: Bulk density: soil weight in g cmsoil weight in g cm-3-3

• Porosity: Porosity: inversely proportional to bulk densityinversely proportional to bulk density• Water properties: Water properties: field capacity, permanent field capacity, permanent

wilting percentage, available water capacity, wilting percentage, available water capacity, hydraulic conductivityhydraulic conductivity

•You cannot modify texture or coarse fragmentsYou cannot modify texture or coarse fragments

•You can modify structure, bulk density, porosity, You can modify structure, bulk density, porosity, water propertieswater properties

Textural TriangleTerms like “sandy loam” actually mean something specific relative to the fine earth fraction (that is, the fraction of soil that passes through a 2 mm sieve).

Image from: http://en.wikipedia.org/wiki/Soil_texture

Image: Courtesy of NASA, Soil Science Education Home Page http://ltpwww.gsfc.nasa.gov/globe/pvg/prop1.htm, accessed 14 Feb 2008 .

Soil structure

http://ltpwww.gsfc.nasa.gov/globe/pvg/prop1.htm

Soil Water

Increasing soil water content

-1x106 -1x105 -1500 kPa -33 kPa 0

Permanent WiltingPercentage

Available Water Available Water CapacityCapacity

Field Moisture Capacity

SaturationOvenDry

AirDry

Soil Water Potential

Plant AvailablePlant AvailableWaterWater

Current SoilMoisture

UnavailableUnavailableWaterWater

Soil moisture release curve: A plot of soil moisture vs the

tension at which it is held

• Notice that the sandy soil holds less moisture at a given tension than the loamy or clay soil do

• This is because the sandy soil has fewer fine pores

Soil water (Ө)

So

il m

ois

ture

te

ns

ion

(k

Pa

)

-1500(PWP)

-33 (FMC)

Sandy Loamy Clayey

• High clay soils hold more total

water than coarser textured soils

• However, less of the water in high clay soils is available to plants (lower Available Water Capacity)

• Thus, loamy soils have the best characteristics for holding water for plants.

Clay percentage

So

il w

ate

r (Ө

)Sandy Loamy Clay

Permanent wilting percentage (-1500 kPa)

Field moisture capacity (-33 kPa)

Available water capacity

CompactionCompaction

• Increases bulk density (by definition)Increases bulk density (by definition)

• Reduces total porosityReduces total porosity

• Reduces average pore sizeReduces average pore size

• Reduces infiltrationReduces infiltration

• Because of reduced pore size, compaction can Because of reduced pore size, compaction can

•Decrease plant available water in finer textured soilsDecrease plant available water in finer textured soils

•IIncreasencrease plant available water in coarse textured soils (Gomez plant available water in coarse textured soils (Gomez

et al., 2002)et al., 2002)

Basic Soil Basic Soil ChemicalChemical Properties Properties

Total C: Total C: organic matter + carbonatesorganic matter + carbonates

Total N: Total N: mostly organicmostly organic

C:N Ratio: C:N Ratio: major factor affecting N availabilitymajor factor affecting N availability

Cation exchange capacity (CEC): Cation exchange capacity (CEC): permanent charge permanent charge (clays) and pH-dependent (organic matter)(clays) and pH-dependent (organic matter)

Base saturation: Base saturation: [(Ca[(Ca2+2+ + Mg + Mg2+2+ + K + K++ + Na + Na++)/CEC] x 100)/CEC] x 100

Adsorbed ortho-P and SOAdsorbed ortho-P and SO442-2-: : related to sesquioxide related to sesquioxide

concentrations and organic coatingsconcentrations and organic coatings

Micronutrients: Micronutrients: varying factors affect themvarying factors affect them

•You cannot modify permanent charge CEC or You cannot modify permanent charge CEC or sesquioxidessesquioxides•You can modify organic matter, N, C:N ratio, base You can modify organic matter, N, C:N ratio, base saturation, adsorbed ortho-P and SOsaturation, adsorbed ortho-P and SO44

2-2-

Cation Exchange Capacity (CEC)Cation Exchange Capacity (CEC)

Sources:Sources:

1.1. Ionizeable HIonizeable H+;+;

• Organic matter, clay edges Organic matter, clay edges

• pH-dependent, just as in the case of a weak acid.pH-dependent, just as in the case of a weak acid.

2.2. Isomorphous substitution in clays:Isomorphous substitution in clays:

• Substitution of AlSubstitution of Al3+3+ for Si for Si4+4+ in the tetrahedral layer in the tetrahedral layer

of claysof clays

• Substitution of MgSubstitution of Mg2+2+ for Al for Al3+3+ in the octahedral layer in the octahedral layer

of clayof clay

• This type of CEC is often referred to as permanent This type of CEC is often referred to as permanent

charge CEC because it is not affected by pH.charge CEC because it is not affected by pH.

Kaolinite

K K K K K K K

1.0

nm

Mica (Primary mineral)

H+ K K K H+ H+

1.0

nm

Illite (Med. CEC)

0.93

nm

Chlorite (Low-Med CEC)

≈1.4

nmCa Mg H 2O Ca H 2O

Vermiculite (High CEC,

expands/contracts somewhat)

Ca Mg H 2O Ca H 2O

Smectite (or Montmorillonite

(High CEC, expands/contracts a lot)

SiO 4Al(OH) 3

≈1.8 to 4.0

nm

0.72

nm H+ bonding

Silicate clays: permanent charge CEC

------

..Na+

..Na+

A simple example: Ca2+ exchange displaces exchangeable Na+

[Ca2+]

------

..Ca2+ [Na+]

[Na+]

2XNa+ + Ca2+ XCa2+ + 2Na+

Negatively-charged clay

Dissolved in soil solution

X = exchangeable

Cation ExchangeCation Exchange

•Strength of cation adsorption (lyotropic series):Strength of cation adsorption (lyotropic series):

NaNa++ < K < K++ = NH = NH44++ < Mg < Mg2+2+ = Ca = Ca2+2+ < Al < Aln+n+ < H < H++

•Adsorption depends on charge density (charge/vol), so Adsorption depends on charge density (charge/vol), so

increases with valence and decreases with size. increases with valence and decreases with size.

•Not all exchangeable ions are AlNot all exchangeable ions are Aln+n+ and H and H++ because mass because mass

action allows the others to be present; but at equal soil action allows the others to be present; but at equal soil

solution conc's, this will be the order.solution conc's, this will be the order.

Mass action: Mass action:

•Displacement of one adsorbed/exchangeable cation by Displacement of one adsorbed/exchangeable cation by

another by competition for sites when the second has a another by competition for sites when the second has a

high number of ions in solution (high concentration)high number of ions in solution (high concentration)

•Works even when trying to drive off most strongly Works even when trying to drive off most strongly

absorbed cations like Habsorbed cations like H++ and Al and Al3+3+

•This is why fertilization with KThis is why fertilization with K++, Mg, Mg2+2+ and liming (Ca and liming (Ca2+2+) )

workwork

Al3+

Al3+

Al3+

Al3+

Ca2+Ca2+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Cat

ion

Ex c

h ang

e S

ite

Ca2+

Ca2+

Ca2+

Cat

ion

Ex c

h ang

e S

ite

Ca2+

Ca2+ Displaces Al3+ by Mass Action even though Al3+

is more strongly absorbed

pH increases, Al precipitates as gibbsite:Al3+ + 3OH- Al(OH)3

Soil organic matter as a source of CECSoil organic matter as a source of CEC

•Temporary (will ultimately decompose)Temporary (will ultimately decompose)•Nearly insoluble in water, but soluble in base (high pH)Nearly insoluble in water, but soluble in base (high pH)•Contains 30% each of proteins, lignin, complex sugarsContains 30% each of proteins, lignin, complex sugars•50% C and O, 5% N50% C and O, 5% N•Very high CEC on a weight basisVery high CEC on a weight basis•Develops a net negative charge due to the dissociation of HDevelops a net negative charge due to the dissociation of H++

from from •enolic (-OH), carboxyl (-COOH), and phenolic ( -OH) enolic (-OH), carboxyl (-COOH), and phenolic ( -OH)

groups as pH increases (solution Hgroups as pH increases (solution H++ concentration decreases): concentration decreases):

No chargeNo charge CEC and exch. KCEC and exch. K++ (could be any (could be any cation)cation)

R-OHR-OH00 + OH + OH- - --------> R-O --------> R-O-- ……KK++ + H + H22O O

(R stands for some organic molecule)(R stands for some organic molecule)

• This leaves a net negative charge on the organic colloid (R-OThis leaves a net negative charge on the organic colloid (R-O --) which ) which attracts cations just as the net negative charge on an attracts cations just as the net negative charge on an isomorphously-substituted clay does.isomorphously-substituted clay does.

• Organic matter is the most important source of pH-dependent CEC Organic matter is the most important source of pH-dependent CEC

in soils.in soils.

pH-dependent CEC on Organic MatterpH-dependent CEC on Organic Matter

OH

OH

O-

OH

K +

Low pH, sites protonatedno CEC

High pH (depronotated,

cation exchange site)

Organic matter : pH-dependent CEC

+ OH- + H2O

NRES 322 SoilsNRES 322 SoilsMeasurement of Cation Exchange Capacity (CEC) and Measurement of Cation Exchange Capacity (CEC) and

Base Saturation (%BS)Base Saturation (%BS)

•CEC is measured by applying concentrated ammonium chloride CEC is measured by applying concentrated ammonium chloride (NH(NH44Cl) or ammonium acetate (NHCl) or ammonium acetate (NH44OAc) to the sample to OAc) to the sample to

exchange all exchangeable cations with NHexchange all exchangeable cations with NH44++ by mass action by mass action

•The extractant solution is analyzed for CaThe extractant solution is analyzed for Ca2+2+, Mg, Mg2+2+, K, K++, Na, Na++, and in , and in some cases Al to determine what was on the exchanger. some cases Al to determine what was on the exchanger.

•At that point, one measure of CEC can be made. Then the NHAt that point, one measure of CEC can be made. Then the NH44++ is is

displaced by another cation (typically Nadisplaced by another cation (typically Na++ or K or K++ ) by mass action, ) by mass action, and NHand NH44

++ is then measured to obtain another estimate of CEC. is then measured to obtain another estimate of CEC.

NRES 322 SoilsNRES 322 SoilsMeasurement of CEC and %BSMeasurement of CEC and %BS

•The usual assumption is that NHThe usual assumption is that NH44++ constitutes a negligible constitutes a negligible

proportion of CEC. proportion of CEC. •Exchangeable NHExchangeable NH44

++ is often measured separately using is often measured separately using

concentrated KCl extractant. concentrated KCl extractant. •HH++ (pH) is not measured on this extractant, either; exchangeable (pH) is not measured on this extractant, either; exchangeable HH++ is measured another way. is measured another way.

•Some soil scientists argue that there is no exchangeable HSome soil scientists argue that there is no exchangeable H++ on on mineral soils; all Hmineral soils; all H++ that becomes absorbed onto clay minerals that becomes absorbed onto clay minerals quickly enters the lattice structure and causes clay decomposition quickly enters the lattice structure and causes clay decomposition to hydrous oxides. to hydrous oxides.

There are three ways to measure CEC (two from one There are three ways to measure CEC (two from one method and one from another method): method and one from another method):

1. Sum of cations Method:1. Sum of cations Method:• The sum of CaThe sum of Ca2+2+, Mg, Mg2+2+, K, K++, Na, Na++, and Al after extraction with , and Al after extraction with

11M M NHNH44Cl (a neutral salt which does not buffer pH). Cl (a neutral salt which does not buffer pH).

• CEC by sum of cations, CECCEC by sum of cations, CECsumsum, and is measured in the first , and is measured in the first

extractant in Figure 1. extractant in Figure 1. • In a pure clay system (no organic matter Fe, Al hydrous In a pure clay system (no organic matter Fe, Al hydrous

oxides, of allophane; i.e., no pH-dependent CEC) this oxides, of allophane; i.e., no pH-dependent CEC) this

represents CEC and cations on the clay minerals represents CEC and cations on the clay minerals

(permanent charge CEC). (permanent charge CEC).

Soil Sample

Extractant

1M NH Cl4

NH displaces exchangeablecations

4+

Analyze for Ca, K, Mg, Na, and Al ; this gives exchangeable cations. Sum of these cations =

CECsum

2+ 2+

3++

+

Soil Sample

Extractant

1M NaCl

Na or K displaces exchangeableNH

+ +

+4

Analyze for NH ; this gives CEC

+4

eff

Step 1. Displace exchangeable cations with NH4

+ Step 2. Displace exchangeable NH4

+ with Na or K+ +

Figure 1. Measurement of exchangeable cations and CEC using neutral salt. (KCl)

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4 + Na

+

--------

-- NH-- NH-- NH-- NH-- NH-- NH

+4+4+4+4+4+4

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4

--------

-- Na-- Na-- Na-- Na-- Na-- Na

++

+


++

+

Extractant(ExchangeableCations, CEC sum )

Extractant (CEC )eff

2. Effective CEC (CEC2. Effective CEC (CECeffeff) at existing soil pH. ) at existing soil pH.

• This includes the permanent charge CEC plus that portion of This includes the permanent charge CEC plus that portion of

pH-dependent CEC that is in effect at existing soil pH. pH-dependent CEC that is in effect at existing soil pH.

• It is determined from the second extractant in Figure 1, After It is determined from the second extractant in Figure 1, After

the 1the 1M M NHNH44Cl extraction, the soil is washed with ethanol to Cl extraction, the soil is washed with ethanol to

remove soluble NHremove soluble NH44++ , and then extracted with 1 , and then extracted with 1M M NaCl to NaCl to

displace the exchangeable NHdisplace the exchangeable NH44++. .

• The extractant is analyzed for NHThe extractant is analyzed for NH44++ . .

Three ways to measure (cont.) Three ways to measure (cont.)

Soil Sample

Extractant

1M NH Cl4


4+

Analyze for Ca, K, Mg, Na, and Al ; this gives exchangeable cations. Sum of these cations =

CECsum

2+ 2+

3++

+

Soil Sample

Extractant

1M NaCl

Na or K displaces exchangeableNH

+ +

+4


+4

eff


+ Step 2. Displace exchangeable NH4

+ with Na or K+ +

Figure 1. Measurement of exchangeable cations and CEC using neutral salt. (KCl)

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4 + Na

+

--------


+4+4+4+4+4+4

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4

--------


++

+


++

+

Extractant(ExchangeableCations, CEC sum )

Extractant (CEC )eff

3. Ammonium acetate CEC (CEC3. Ammonium acetate CEC (CECOAcOAc). ).

•This includes permanent charge CEC + all pH-dependent This includes permanent charge CEC + all pH-dependent

CEC. Is is measured by extracting the soil with either CEC. Is is measured by extracting the soil with either

ammonium acetate (NHammonium acetate (NH44OAc, buffers pH at 7.0). (Figure OAc, buffers pH at 7.0). (Figure

2). 2).

•Then the same produre is followed as for the neutral salt Then the same produre is followed as for the neutral salt

CEC. CEC.

•Note: exchangeable AlNote: exchangeable Al should be measured separately should be measured separately

because Al precipitates as Al(OH)because Al precipitates as Al(OH)33 at high pH at high pH

Three ways to measure (cont.) Three ways to measure (cont.)

Soil Sample

Extractant

1M NH OAcBuffers pH at 7

4


4+

Analyze for Ca, K, Mg, and Na ; this gives exchangeable cations except for Al.

2+ 2+

3+

++

Soil Sample

Extractant

1M NaCl

Na displaces exchangeableNH

+

+4


+4


+

Step 2. Displace exchangeable NH4+

with Na +

Figure 2. Measurement of exchangeable cations and CEC buffering pH at 7 using ammonium acetate.

pH 7

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4 + Na

+

--------


+4+4+4+4+4+4

--------

-- Ca-- Mg-- K-- Na-- Al-- H

2+2+

+

+

+3+

+ NH+4

--------


++

+


++

+

Extractant(Exchangeable

Bases only; Al precipitates)

Extractant (CEC)

Permanent Charge CEC pH-dependent CEC

CECeff

CECOAc

CECsum: Measured as the sum of Ca + Mg + K + Na + Al extracted with ammonium chloride in the first extraction in Figure 1

CECeff: Measured with ammonium chloride, neutral salt, after second extraction in Fig 1

CECOAc: Measured with ammonium acetate at pH 7 in Figure 2

Figure 3. Types of CEC depend on how it is measured

CECsum

Base SaturationBase Saturation

Base Cation Saturation Percentage (BCSP) (often Base Cation Saturation Percentage (BCSP) (often stated as simply base saturation) BCSPis defined stated as simply base saturation) BCSPis defined as the sum of exchangeable base cations (Caas the sum of exchangeable base cations (Ca2+2+, , MgMg2+2+, K, K++, and Na, and Na++) divided by CEC. It is usually ) divided by CEC. It is usually expressed as a percentage of CEC thus: expressed as a percentage of CEC thus:

BS (%) =BS (%) = Ca + Mg + K + NaCa + Mg + K + Na

CECCEC x100 x100

Base SaturationBase Saturation

• Since CEC can be measured in different ways, BCSP will vary Since CEC can be measured in different ways, BCSP will vary

with the method used, and must be specified. with the method used, and must be specified.

• For a soil with a given amount of exchangeable bases, % Base For a soil with a given amount of exchangeable bases, % Base

saturation calculated from CECsaturation calculated from CECsumsum will be greater than that will be greater than that

calculated from CECcalculated from CECeffeff which will be greater than that calculated which will be greater than that calculated

from CECfrom CECtottot because more of the potential acidity on the pH- because more of the potential acidity on the pH-

dependen CEC is counted as CEC (i.e., CECdependen CEC is counted as CEC (i.e., CECsumsum < CEC < CECeffeff < CEC < CECtottot). ).

• The example in Figure 4 shows how this might occur. In each The example in Figure 4 shows how this might occur. In each

case, the base cations are the same (6 cmolcase, the base cations are the same (6 cmolc c kgkg-1-1); only the ); only the

measure of CEC (the denominator) changes.measure of CEC (the denominator) changes.

Base CationsCa2+ + Mg2+ + K+ + Na+ = 6 cmolc kg-1

Acid cationsAln+ = 1 cmolc kg-1

H+ = 3 cmolc kg-1

CECeff = 8 cmolc kg-1

CECOAc= 10 cmolc kg-1

Figure 4. Base saturation value depends on which CEC measure is used

CECsum = 7 cmolc kg-1

%BSsum=

__________________________

Ca2+ + Mg2+ + K+ + Na +

CECsum

=Ca2+ + Mg2+ + K+ + Na +

Ca2+ + Mg2+ + K+ + Na++ Aln+

________________________

= X 100

X 100

X 100 67

= 85%

%BSeff=Ca2+ + Mg2+ + K+ + Na +

CECrff

________________________ X 100 X 10068

= 75%=

%BSOAc=Ca2+ + Mg2+ + K+ + Na +

CECOAc

________________________ X 100 X 100 610

= 60%=

Anion adsorption and retention on soils:Anion adsorption and retention on soils:

Negatively-charged ions adsorbed on positively-charges Negatively-charged ions adsorbed on positively-charges

sites. sites.

• In general, anion adsorption is associated with In general, anion adsorption is associated with

allophane and the hydrous oxides of Fe and Al in soils. allophane and the hydrous oxides of Fe and Al in soils.

• HH22POPO44-- >> SO >> SO44

-2--2- >> NO >> NO33- - > Cl> Cl- - (the latter being nil in all (the latter being nil in all

but the most sequoixide-rich soils)but the most sequoixide-rich soils)

• Anion adsorption on these surfaces is highly Anion adsorption on these surfaces is highly

dependent upon pH. dependent upon pH.

• Usually much lower than CEC in temperate, non-Usually much lower than CEC in temperate, non-

volcanic ash soils.volcanic ash soils.

Al

OH

+

2

OH

Cl -

Low pH (protonated,

anion exchange site)

Al

OH

OH

Al

O-

OH

K +

Zero Point of Charge High pH (depronotated,

cation exchange site)

Allophane, Fe and Al hydrous oxides are amphoteric: they take on different charges depending upon pH

:

Soil pHSoil pH

• pH is the negative log of the HpH is the negative log of the H++ activity = -log (H activity = -log (H++); therefore, ); therefore,

• 1010-pH-pH = (H = (H++) (in moles L) (in moles L-1-1))

• Soil reaction, or pH is taken in a paste of water or 0.01 CaClSoil reaction, or pH is taken in a paste of water or 0.01 CaCl22. .

• The latter gives a lower pH than the former, in most cases, The latter gives a lower pH than the former, in most cases,

because the Cabecause the Ca2+2+ displaces exchangeable H displaces exchangeable H++ and Al and Al3+3+ by mass by mass

action. action.

Soil pHSoil pH

• pH decreases as base saturation decreases (recall that you pH decreases as base saturation decreases (recall that you

must keep the methods constant, that is by sum, eff, or Oac; must keep the methods constant, that is by sum, eff, or Oac;

the soil in Figure 4 has only one pH although base saturation the soil in Figure 4 has only one pH although base saturation

value differs by method). value differs by method).

• pH has a strong effect on plant growth and nutrient availability pH has a strong effect on plant growth and nutrient availability

It not only changes the solubility of many nutrients, but may It not only changes the solubility of many nutrients, but may

also cause direct toxicity (Al, usually) to plant roots.also cause direct toxicity (Al, usually) to plant roots.

http://www.terragis.bees.unsw.edu.au/terraGIS_soil/sp_soil_reaction_ph.html

Buffering capacity: Buffering capacity:

•Ability of the soil to resist changes in pH. Ability of the soil to resist changes in pH.

• Related to exchangeable HRelated to exchangeable H+ + and Aland Al3+3+ in acid soils and carbonates in in acid soils and carbonates in

alkaline soils. alkaline soils.

•CEC always plays a major role in buffering.CEC always plays a major role in buffering.

Total acidity on solid phase > 10,000 x that in soil solutionTotal acidity on solid phase > 10,000 x that in soil solution

PotentialAcidity

ActiveAcidity

Buffering

Basic Soil Biological PropertiesBasic Soil Biological PropertiesDifficult to generalize what is a “good” soil relative to Difficult to generalize what is a “good” soil relative to

microorganismsmicroorganisms

• C:N Ratio: major factor affecting decomposition and N availabilityC:N Ratio: major factor affecting decomposition and N availability• pH: low pH disfavors bacteria and favors fungipH: low pH disfavors bacteria and favors fungi• Aeration/flooding: anaerobes vs aerobesAeration/flooding: anaerobes vs aerobes

Soil Micro-organismsSoil Micro-organisms

Many ways to classifyMany ways to classify

•Based on how they get energy: Based on how they get energy:

• Autotrophic: Use sunlight of inorganic chemical reactions Autotrophic: Use sunlight of inorganic chemical reactions

for energyfor energy

• Heterotrophic: Use organic compounds for energyHeterotrophic: Use organic compounds for energy

•Based on oxygen requirements:Based on oxygen requirements:

• AerobicAerobic

• AnaerobicAnaerobic

• FacultativeFacultative

Some Important HeterotrophsSome Important Heterotrophs

•FungiFungi

• Decomposers (tolerate low pH) Decomposers (tolerate low pH)

• Mycorrhizae (vital for growth of many plants)Mycorrhizae (vital for growth of many plants)

•Bacteria/ActinomycetesBacteria/Actinomycetes

• Decomposers (do not tolerate low pH)Decomposers (do not tolerate low pH)

• Denitrifying bacteria (anaerobic)Denitrifying bacteria (anaerobic)

• Sulfur reducing bacteria (anaerobic)Sulfur reducing bacteria (anaerobic)

• Nitrogen fixersNitrogen fixers

•Rhizobium: legumesRhizobium: legumes

•Frankia actinomycetes: alders, snowbrush…Frankia actinomycetes: alders, snowbrush…

Decomposition and N-Mineralization and Decomposition and N-Mineralization and Immobilization in Soils: Immobilization in Soils:

Critical Processes for Plant Nutrition!!!Critical Processes for Plant Nutrition!!!

• Most N taken up by plants in forest ecosystems is derived Most N taken up by plants in forest ecosystems is derived

from decomposed organic matter (recycled)from decomposed organic matter (recycled)

• Decomposition is microbially mediated, yet microbial Decomposition is microbially mediated, yet microbial

biomass is < 3% of soil OMbiomass is < 3% of soil OM

• Microbes have higher concentrations of nutrients than the Microbes have higher concentrations of nutrients than the

substrates they consume substrates they consume

• For example, bacteria and fungi have C:N ratios of around 6 For example, bacteria and fungi have C:N ratios of around 6

to 1 (4 to 8% N), whereas substrates they consume have to 1 (4 to 8% N), whereas substrates they consume have

C:N ratios of 25 to 200 (3.0 to 0.05% N)C:N ratios of 25 to 200 (3.0 to 0.05% N)

Nitrogen Cycling in Soil: Microbes concentrate nutrients in their bodies This is termed immobilization

OrganicSubstrateC:N = 200

Carbon

NitrogenCO2

MicrobesC:N = 12

Organicacids

Immobilization

Nitrogen Cycling in Soil: When microbes concentrate nutrients in their bodies This is termed immobilization When microbes release N from during decomposition This is termed mineralization


Carbon

NitrogenCO2

MicrobesC:N = 12

Immobilization

Mineralization

MaterialMaterial C/N ratio C/N ratioSoil MicrobesSoil MicrobesBacteriaBacteria 6:1 6:1ActinomycetesActinomycetes 6:1 6:1FungiFungi 12:1 12:1

Litter TypesLitter TypesAlfalfa Alfalfa 13:1 13:1CloverClover 20:1 20:1StrawStraw 80:1 80:1Deciduous litterDeciduous litter 40:1 to 80:1 40:1 to 80:1Coniferous litterConiferous litter 60:1 to 130:1 60:1 to 130:1Woody litter Woody litter 250:1 to 600:1 250:1 to 600:1Soil Organic Matter 10:1 to 50:1Soil Organic Matter 10:1 to 50:1

Nitrogen Cycling in Soils: Importance of the C:N RatioNitrogen Cycling in Soils: Importance of the C:N Ratio

C:N RatioC:N Ratio

• In order for soil microbes to decompose most litter types, In order for soil microbes to decompose most litter types,

they must initially incorporate N from the soilthey must initially incorporate N from the soil

• Thus, inputs of high C/N ratio organic matter, such as Thus, inputs of high C/N ratio organic matter, such as

sawdust or wood chips, can cause N deficiency to plants sawdust or wood chips, can cause N deficiency to plants

unless accompanied by fertilizationunless accompanied by fertilization

• As C is lost at COAs C is lost at CO22 gas, the C/N ratio of the litter decreases gas, the C/N ratio of the litter decreases

to a value ranging from 20:1 to 30:1, at which point N is to a value ranging from 20:1 to 30:1, at which point N is

released from decomposing litterreleased from decomposing litter

Nitrogen Cycling in Soil: Microbes concentrate nitrogen in their bodies This is termed immobilization When microbes release N from during decomposition This is termed mineralization


Carbon

NitrogenCO2

MicrobesC:N = 12

Organicacids

Immobilization

NH4+

During decomposition, C:N ratio declines as C is lost to CO2

When C:N ratio reaches about 20, N mineralization commences

Before that, N is immobilized

C:N

600

20

N immobilization N mineralization

Time of decomposition

Sawdust

Aspen leaves

Clover

NH4+ NH4

+

Other Important heterotrophsOther Important heterotrophs

Mycorrhizae: Mycorrhizae: Essential for nutrient and water uptake in most Essential for nutrient and water uptake in most plantsplants

Denitrifying bacteria:Denitrifying bacteria: N loss to gas in anaerobic conditions N loss to gas in anaerobic conditionsNitrogen fixers: Nitrogen fixers: • Convert NConvert N22 gas in the atmosphere to ammonium (NH gas in the atmosphere to ammonium (NH44

++) ) • Very important source of N for soils and vegetation, Very important source of N for soils and vegetation,

especially in unpolluted areas – soils have no mineral N especially in unpolluted areas – soils have no mineral N source!source!

• The atmosphere is 78% NThe atmosphere is 78% N22 gas but plants cannot utilize it gas but plants cannot utilize it

because of the strong triple bond:because of the strong triple bond:• NN==NN• Nitrogen fixers take energy from host plants (symbiotic) Nitrogen fixers take energy from host plants (symbiotic)

or associate (non-symbiotic) and convert this N to or associate (non-symbiotic) and convert this N to usable form using nitrogenase enzymeusable form using nitrogenase enzyme

Some Important ChemautotrophsSome Important Chemautotrophs

Nitrifying bacteriaNitrifying bacteria

One of the most important autorophic bacteria are nitrifying One of the most important autorophic bacteria are nitrifying bacteria, who convert ammonium (NHbacteria, who convert ammonium (NH44

++) to nitrite (NO) to nitrite (NO22--) and ) and

nitrate (NOnitrate (NO33--):):

2NH2NH44++ + 3O + 3O22 2NO 2NO22

-- + 4H + 4H++ + 2H + 2H22OO NitrosomonasNitrosomonas

2NO2NO22-- + O + O22 _ _ 2NO 2NO33

-- NitrobacterNitrobacter

2NH2NH44++ + 4O + 4O2 2 4H 4H++ + 2H + 2H22O O 2NO2NO33

--

Note that nitrification is acidifying, and therefore self-limiting Note that nitrification is acidifying, and therefore self-limiting (in theory) because bacteria do now tolerate low pH. However, (in theory) because bacteria do now tolerate low pH. However, nitrification has been observed many times in very acid soils.nitrification has been observed many times in very acid soils.

Sulfur oxidizing bacteriaSulfur oxidizing bacteria

Another important chemautotroph is Genus Another important chemautotroph is Genus ThiobacillusThiobacillus; most ; most important of chemautotroph mineral oxidizers (elemental important of chemautotroph mineral oxidizers (elemental sulfur and sulfide minerals). For elemental S:sulfur and sulfide minerals). For elemental S:

2S + 3O2S + 3O22 + 2H + 2H22O -------> 4HO -------> 4H+ + + 2SO + 2SO442-2- Thiobacillus thiooxidansThiobacillus thiooxidans

Some Important ChemautotrophsSome Important Chemautotrophs

Another important reaction carried out by these bacteria is the Another important reaction carried out by these bacteria is the oxidation of pyrite, FeSoxidation of pyrite, FeS22, which occurs commonly in mine , which occurs commonly in mine

spoils by spoils by Thiobacillus thiooxidansThiobacillus thiooxidans and and Thiobaccillus Thiobaccillus ferroxidans:ferroxidans:

4FeS4FeS22 + 150 + 15022 + 2H + 2H22O O 2Fe 2Fe22(SO(SO44))33 + 4H + 4H++ + 2SO + 2SO442-2-

Both reactions produce strong acidBoth reactions produce strong acid

So given that brief background, what is a “good” So given that brief background, what is a “good” quality soil?quality soil?

I am guessing:I am guessing:

•Relatively rich in organic matterRelatively rich in organic matter

•Good N status (meaning: low C:N ratio)Good N status (meaning: low C:N ratio)

•Circumneutral pHCircumneutral pH

•Good supplies of P, K, Ca, Mg, S and micronutrientsGood supplies of P, K, Ca, Mg, S and micronutrients

•Good texture or structure (water holding capacity)Good texture or structure (water holding capacity)

•Bulk density near 1.0 (good infiltration and aeration, not Bulk density near 1.0 (good infiltration and aeration, not

compacted)compacted)

But what does all this imply for:But what does all this imply for:

•Plants with varying nutritional needsPlants with varying nutritional needs

• Invasive speciesInvasive species

•Water qualityWater quality

•Soil water characteristicsSoil water characteristics

??

Case Study 1: CompactionCase Study 1: Compaction

• Bob Powers LTSP sites included experimental compactionBob Powers LTSP sites included experimental compaction

• Gomez et al (2002)* reported some resultsGomez et al (2002)* reported some results

• Many thanks to Bob for providing the following slides for my Many thanks to Bob for providing the following slides for my

class!class!

*Gomez, A., R.F. Powers, M.J. Singer, and W.R. Horwath. 2002. Soil compaction *Gomez, A., R.F. Powers, M.J. Singer, and W.R. Horwath. 2002. Soil compaction effects on growth of young ponderosa pine following litter removal in effects on growth of young ponderosa pine following litter removal in California’s Sierra Nevada. Soil S ci. Soc. Amer. J. 66: 1334-1343. California’s Sierra Nevada. Soil S ci. Soc. Amer. J. 66: 1334-1343.

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0.60 0.80 1.00 1.20 1.40 1.60 1.80

At treatment

After 10 yrs

SE

VE

RE

CO

MP

AC

TIO

N B

UL

K D

EN

SIT

Y

SOIL BULK DENSITY (Mg m2 at 10-20 cm)

BULK DENSITY BEFORE COMPACTION

SANDY LOAM

0.00

0.10

0.20

0.30

0.40

0.50

0.60

No Compaction Severe Compaction

TREATMENT

PO

RE

VO

LU

ME

(cm

3 cm

-3

>30 m

30-0.2 m

<0.2 m

)

Soil Pore Diam.

INFLUENCE OF COMPACTION ON SOIL PORES BY FUNCTIONAL GROUPINGS

Soil Texture

Total Soil Porosity (cm3/cm3)

Total Porosity Decrease (cm3/cm3)

Non-Compacted Treatments

Compacted Treatments

Abs Rel %

Loam 0.64 0.60 0.04 6

Sandy loam 0.55 0.51 0.04 7

Clay loam 0.55 0.56 0.01 2Loam (volcanic

ash) 0.63 0.62 0.01 2

Measured Total Soil Porosity

Decrease in Soil Pores >30µm as a Result of Soil Compaction

Soil Texture

Soil Macro Pores (cm3/cm3)Decrease in Soil Macro

Pores

Non-Compacted Treatments

Compacted Treatments

Abs Rel %

Loam 0.29 0.18 0.11 38

Sandy loam 0.29 0.19 0.10 34

Clay loam 0.23 0.18 0.05 22Loam (volcanic

ash) 0.29 0.22 0.07 24

WHAT DOES COMPACTION DO TO SITE PRODUCTIVITY?

Clay

0

20

40

60

80

100

1 2

Trees

TO

TA

L B

IOM

AS

S (

Mg

ha

-1)


PRODUCTIVITY SEVERELY

REDUCED ON CLAY TEXTURES

Understory

PRODUCTIVITY SLIGHTLY REDUCED ON ASH AND LOAM

TEXTURES

Clay

0

20

40

60

80

100

1 2

Trees

Understory

TO

TA

L B

IOM

AS

S (

Mg

ha

-1)


Ashy

0

20

40

60

80

100

1 2

Trees

Understory

TO

TA

L B

IOM

AS

S (

Mg

ha

-1)



PRODUCTIVITY INCREASED ON

SANDY TEXTURES


Clay

0

20

40

60

80

100

1 2

Trees

Understory

TOTA

L B

IOM

ASS

(Mg

ha-1

)


Ashy

0

20

40

60

80

100

1 2

Trees

UnderstoryTO

TAL

BIO

MA

SS (M

g ha

-1)


Sandy Loam

0

20

40

60

80

100

1 2

Trees

UnderstoryTO

TAL

BIO

MA

SS (M

g ha

-1)


CLAYEY

0.0

1.5

2.0

1.0

0.5

RE

LA

TIV

E B

IOM

AS

S

SOIL TEXTURAL CLASS

4

LOAMY

16

SANDY

6

Uncompacted Control

EFFECT OF SEVERE SOIL COMPACTION ON PRODUCTIVITY VARIES BY SOIL TEXTURE

(26 LTSP INSTALLATIONS)

Gomez, A., R.F. Powers, M.J. Singer, and W.R. Horwath. 2002. Soil compaction Gomez, A., R.F. Powers, M.J. Singer, and W.R. Horwath. 2002. Soil compaction effects on growth of young ponderosa pine following litter removal in effects on growth of young ponderosa pine following litter removal in California’s Sierra Nevada. Soil S ci. Soc. Amer. J. 66: 1334-1343. California’s Sierra Nevada. Soil S ci. Soc. Amer. J. 66: 1334-1343.

Lesson from Case Study 1:Lesson from Case Study 1:

Compaction can either increase or decrease soil quality as defined by:Compaction can either increase or decrease soil quality as defined by:

•Soil water characteristicsSoil water characteristics•Tree growthTree growth

So given that brief background, what is a “good” So given that brief background, what is a “good” quality soil?quality soil?

Good N status (low C:N ratio) and circumneutral pH is fertile Good N status (low C:N ratio) and circumneutral pH is fertile ground for nitrifying bacteria. What does this imply for nitrate ground for nitrifying bacteria. What does this imply for nitrate pollution of ground and surface waters?pollution of ground and surface waters?

What does good N status imply for N-loving invasive species like What does good N status imply for N-loving invasive species like cheatgrass? What does it imply for native N-fixers like alder and cheatgrass? What does it imply for native N-fixers like alder and snowbrush?snowbrush?

What does circumneutral pH imply for the competitive advantage What does circumneutral pH imply for the competitive advantage of native acid-tolerant species on naturally acidic soils?of native acid-tolerant species on naturally acidic soils?

Case Study 2Case Study 2

Here are some soil data from two forested ecosystems with Here are some soil data from two forested ecosystems with different vegetation cover for the last 50 years. Which soil is of different vegetation cover for the last 50 years. Which soil is of better quality?better quality?

Depth Depth (cm)(cm)

SoilSoil pHpH %BS%BS C C (mg g(mg g-1-1))

N N (mg g(mg g-1-1))

C:NC:N Bray P Bray P (mg kg(mg kg-1-1))

DbDb

(g cm(g cm-3-3))

0-150-15 11 5.3±0.25.3±0.2 13±713±7 43±1143±11 1.6±0.41.6±0.4 2727 82±3982±39 0.96±0.110.96±0.11

0-150-15 22 4.5±0.24.5±0.2 8±58±5 95±2695±26 4.8±1.54.8±1.5 2929 21±1421±14 0.85±0.180.85±0.18

15-3015-30 11 5.3±0.25.3±0.2 12±812±8 29±1029±10 1.2±0.31.2±0.3 2424 43±2943±29 1.03±0.121.03±0.12

15-3015-30 22 4.8±0.24.8±0.2 7±47±4 59±1759±17 3.2±0.73.2±0.7 1818 10±610±6 0.86±0.150.86±0.15

30-4530-45 11 5.3±0.15.3±0.1 9±59±5 26±826±8 1.2±0.21.2±0.2 2222 30±1930±19 1.13±0.131.13±0.13

30-4530-45 22 4.9±0.24.9±0.2 9±89±8 58±2458±24 3.1±1.13.1±1.1 1919 8±38±3 1.01±0.211.01±0.21

DepthDepth pHpH %BS%BS C C (mg g(mg g-1-1)) N N (mg g(mg g-1-1))

cmcm Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2

0-150-15 5.3±0.25.3±0.2 4.5±0.24.5±0.2 13±713±7 8±58±5 43±1143±11 95±2695±26 1.6±0.41.6±0.4 4.8±1.54.8±1.5

15-3015-30 5.3±0.25.3±0.2 4.8±0.24.8±0.2 12±812±8 7±47±4 29±1029±10 59±1759±17 1.2±0.31.2±0.3 3.2±0.73.2±0.7

30-4530-45 5.3±0.15.3±0.1 4.9±0.24.9±0.2 9±59±5 9±89±8 26±826±8 58±2458±24 1.2±0.21.2±0.2 3.1±1.13.1±1.1

DepthDepth C:NC:N Bray P (mg kgBray P (mg kg-1-1)) Db (g cmDb (g cm-3-3))

cmcm Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2

0-150-15 2727 2929 82±3982±39 21±1421±14 0.96±0.110.96±0.11 0.85±0.180.85±0.18

15-3015-30 2424 1818 43±2943±29 10±610±6 1.03±0.121.03±0.12 0.86±0.150.86±0.15

30-4530-45 2222 1919 30±1930±19 8±38±3 1.13±0.131.13±0.13 1.01±0.211.01±0.21


Here are some soil data from two forested ecosystems with Here are some soil data from two forested ecosystems with different vegetation cover for the last 50 years. Which soil is of different vegetation cover for the last 50 years. Which soil is of better quality?better quality?


Soil 1 is from a Douglas-fir stand, soil 2 is from an adjacent red alder Soil 1 is from a Douglas-fir stand, soil 2 is from an adjacent red alder stand. Same soils before vegetation changed (Van Miegroet and Cole, stand. Same soils before vegetation changed (Van Miegroet and Cole, 1984). 1984). So how do these soil properties affect tree growth? So how do these soil properties affect tree growth?

DepthDepth pHpH %BS%BS C C (mg g(mg g-1-1)) N N (mg g(mg g-1-1))

cmcm Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2

0-150-15 5.3±0.25.3±0.2 4.5±0.24.5±0.2 13±713±7 8±58±5 43±1143±11 95±2695±26 1.6±0.41.6±0.4 4.8±1.54.8±1.5

15-3015-30 5.3±0.25.3±0.2 4.8±0.24.8±0.2 12±812±8 7±47±4 29±1029±10 59±1759±17 1.2±0.31.2±0.3 3.2±0.73.2±0.7

30-4530-45 5.3±0.15.3±0.1 4.9±0.24.9±0.2 9±59±5 9±89±8 26±826±8 58±2458±24 1.2±0.21.2±0.2 3.1±1.13.1±1.1

DepthDepth C:NC:N Bray P (mg kgBray P (mg kg-1-1)) Db (g cmDb (g cm-3-3))

cmcm Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2 Soil 1Soil 1 Soil 2Soil 2

0-150-15 2727 2929 82±3982±39 21±1421±14 0.96±0.110.96±0.11 0.85±0.180.85±0.18

15-3015-30 2424 1818 43±2943±29 10±610±6 1.03±0.121.03±0.12 0.86±0.150.86±0.15

30-4530-45 2222 1919 30±1930±19 8±38±3 1.13±0.131.13±0.13 1.01±0.211.01±0.21

Van Miegroet et al., 1989Van Miegroet et al., 1989

•Planting red alder on former red alder soil resulted in slower Planting red alder on former red alder soil resulted in slower growth than planting red alder on former Douglas-fir soil. growth than planting red alder on former Douglas-fir soil.

•At this stage, no response in Douglas-fir, but past experience has At this stage, no response in Douglas-fir, but past experience has shown that it will grow much better on former red alder soil. shown that it will grow much better on former red alder soil.

• (The stand was destroyed in a wind storm after these (The stand was destroyed in a wind storm after these measurements were taken). measurements were taken).

Interplantings of red alder and conifers (Binkley, 2003)Interplantings of red alder and conifers (Binkley, 2003)

•Initially, red alder inhibits D. fir growth in the poorer (Wind River) siteInitially, red alder inhibits D. fir growth in the poorer (Wind River) site•Over time, D. fir takes the site, grows faster because of more N in soil Over time, D. fir takes the site, grows faster because of more N in soil at Wind Riverat Wind River•At N-rich Cascade Head site, alder continues to inhibit D. FirAt N-rich Cascade Head site, alder continues to inhibit D. Fir

Van Miegroet et al., 1984Van Miegroet et al., 1984

What about water quality considerations?What about water quality considerations?

The red alder soil produced high rates of nitrate leaching and no The red alder soil produced high rates of nitrate leaching and no doubt contributed to soil acidification doubt contributed to soil acidification


Subsequent studies by Compton et al (2003) showed that nitrate in Subsequent studies by Compton et al (2003) showed that nitrate in streamwater from Oregon Coast Watersheds was related to the streamwater from Oregon Coast Watersheds was related to the presence of red alder. presence of red alder.


Harvesting in red alder caused large reductions in nitrate leachingHarvesting in red alder caused large reductions in nitrate leachingWater quality problems were related to excessive N-fixation, not Water quality problems were related to excessive N-fixation, not directly to soil properties in this casedirectly to soil properties in this case

Summary of Case Study 2Summary of Case Study 2

•Red alder improves C and N status of soils, which is good for Red alder improves C and N status of soils, which is good for Douglas-fir. But this comes at a price in water quality.Douglas-fir. But this comes at a price in water quality.

•Red alder acidifies soils which is (apparently) not good for red Red alder acidifies soils which is (apparently) not good for red alder but does not bother Douglas-firalder but does not bother Douglas-fir

•Therefore, what is good soil quality for Douglas-fir is not good Therefore, what is good soil quality for Douglas-fir is not good quality for red alder. quality for red alder.

Case Study 3, Site 1Case Study 3, Site 1

Here are some soil data from adjacent sites which have had Here are some soil data from adjacent sites which have had different vegetation cover for different vegetation cover for 100 years100 years. Which soil is of better . Which soil is of better quality?quality?

Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2 Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2

Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2 Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2

Case Study 3, Site 1 (cont)Case Study 3, Site 1 (cont)

Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2Vegetation 1 Vegetation 2

Soil data from adjacent sites which have had different vegetation Soil data from adjacent sites which have had different vegetation cover for 100 years.cover for 100 years.

Veg 1

Veg 2

Case Study 3, Site 2Case Study 3, Site 2

Here are some soil data from adjacent sites which have had the Here are some soil data from adjacent sites which have had the same two vegetation covers but for same two vegetation covers but for only two decadesonly two decades. Which soil . Which soil is of better quality?is of better quality?

Vegetation 1

Bulk Density (g cmBulk Density (g cm-3-3))

Vegetation 2

Veg 1

Veg 2

Veg 1

Veg 2

DepthDepth

cmcm

Soil 1Soil 1 Soil 2Soil 2

0-70-7 1.34±0.041.34±0.04 1.15±0.041.15±0.04

7-207-20 1.43±0.051.43±0.05 1.29±0.031.29±0.03

20-4020-40 1.42±0.051.42±0.05 1.32±0.021.32±0.02

Veg 2:Veg 2: Pinus jeffreyii Pinus jeffreyii

Comparisons of soils in beneath 5 paired adjacent, mature stands of Comparisons of soils in beneath 5 paired adjacent, mature stands of Ceanothus velutinusCeanothus velutinus and and Pinus jeffreyiiPinus jeffreyii in Little Valley, Nevada in Little Valley, Nevada (Johnson, 1995). (Johnson, 1995).

Veg 1:Veg 1: Ceanothus velutinus Ceanothus velutinus

Site 1: Upper Little Valley, Nevada. Mature, adjacent snowbrush and Site 1: Upper Little Valley, Nevada. Mature, adjacent snowbrush and 100 year old jeffrey pine stands. 100 year old jeffrey pine stands.

Immediate post-fireImmediate post-fire•Foliage and forest floor are totally Foliage and forest floor are totally combustedcombusted•Soil organic matter losses unknownSoil organic matter losses unknown 20 years post-fire20 years post-fire

•80% N-fixing 80% N-fixing Ceanothus velutinusCeanothus velutinus•20%20% non-fixing shrubsnon-fixing shrubs

Site 2: In and near 1981 wildfire in lower Little Valley, Nevada. 20-year-Site 2: In and near 1981 wildfire in lower Little Valley, Nevada. 20-year-old snowbrush and nearby mature jeffrey pine stands. old snowbrush and nearby mature jeffrey pine stands.

Soil solutions from beneath snowbrush in upper Little Valley Soil solutions from beneath snowbrush in upper Little Valley had extremely low (< 3 umolhad extremely low (< 3 umolcc L L-1-1) nitrate concentrations ) nitrate concentrations

(Johnson, 1995)(Johnson, 1995)

Soil solutionsSoil solutions from from snowbrush-dominated snowbrush-dominated former fire site in Little former fire site in Little Valley had only slightly Valley had only slightly elevated nitrate elevated nitrate concentrationsconcentrations(Stein, 2006)(Stein, 2006)

• Both studies showed that snowbrush improves soil quality. Both studies showed that snowbrush improves soil quality. • Furthermore, soil solution data from both sites showed that Furthermore, soil solution data from both sites showed that

snowbrush, unlike red alder, produce only very slight increases in snowbrush, unlike red alder, produce only very slight increases in nitrate leaching and does not acidify soils, but in fact, increases nitrate leaching and does not acidify soils, but in fact, increases base saturation.base saturation.

• So why not manage for snowbrush if we want to improve soil So why not manage for snowbrush if we want to improve soil quality?quality?

• Because snowbrush competes with regenerating forest Because snowbrush competes with regenerating forest vegetation for water; if measures are not taken to vegetation for water; if measures are not taken to control it, former forests will revert to chaparral for 50-control it, former forests will revert to chaparral for 50-100 years after wildfire!100 years after wildfire!

Summary of Case Study 3Summary of Case Study 3Snowbrush studies in Little ValleySnowbrush studies in Little Valley

Nitrogen has some features that are unique among major nutrient that Nitrogen has some features that are unique among major nutrient that

make it most frequently limiting and problematic: make it most frequently limiting and problematic:

• No significant primary mineral sourceNo significant primary mineral source

• Will not accumulate in ionic form in soils in any substantial Will not accumulate in ionic form in soils in any substantial

amounts for longamounts for long

• N present in excess of biological demand nearly always nitrifies N present in excess of biological demand nearly always nitrifies

(if not already in NO(if not already in NO33-- form) and leaches away as NO form) and leaches away as NO33

--, causing , causing

water pollution and soil acidificationwater pollution and soil acidification

The Nitrogen ProblemThe Nitrogen Problem

Nitrogen has a very narrow “sufficiency or optimum plateau” after Nitrogen has a very narrow “sufficiency or optimum plateau” after

which bad things start to happen and before which N is deficient which bad things start to happen and before which N is deficient

(soil quality is low)(soil quality is low)


DeficiencyDeficiencyToxicityToxicity

SufficiencySufficiency

Growth-limitingGrowth-limiting

Enough but, possibly moreEnough but, possibly moreThen enough, but not too muchThen enough, but not too much

Too much – growth Too much – growth inhibited, negative effects inhibited, negative effects

on soil and wateron soil and water

NitrogenNitrogen

P, K, Ca, Mg, SP, K, Ca, Mg, S

Nitrogen is the most frequently limiting nutrient and a high quality soil Nitrogen is the most frequently limiting nutrient and a high quality soil

must have adequate N. must have adequate N.

However, it is very difficult to manage nitrogen at an optimal level for However, it is very difficult to manage nitrogen at an optimal level for

plant growth while at the same time maintaining water quality and plant growth while at the same time maintaining water quality and

not causing negative effects on other soil nutrients (and causing not causing negative effects on other soil nutrients (and causing

deteriorating soil quality)deteriorating soil quality)


A single soil quality standard does not make any sense:A single soil quality standard does not make any sense:

• Plants vary in nutritional needsPlants vary in nutritional needs

• Invasive species tend to love high quality soilInvasive species tend to love high quality soil

• We tend to like like oligotrophic (nutrient poor) surface waterWe tend to like like oligotrophic (nutrient poor) surface water

Soil quality, or soil fertility as it was previously termed, should be Soil quality, or soil fertility as it was previously termed, should be

viewed from the perspective of management objectives and priorities:viewed from the perspective of management objectives and priorities:

• Increase productionIncrease production

• Grow specific species (some love acid, some cannot tolerate it)Grow specific species (some love acid, some cannot tolerate it)

• Control invasive speciesControl invasive species

• Preserve water qualityPreserve water quality

Summary and ConclusionsSummary and Conclusions

It is probable that not all of these objectives can be met at once It is probable that not all of these objectives can be met at once

with the same soil quality standard!with the same soil quality standard!

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Soil Quality: The view through the prism of Soils 101 D.W. Johnson Natural Resources and...

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