class evaluations class evaluations. soil chemistry

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Page 1: Class evaluations Class evaluations. Soil Chemistry

Class evaluationsClass evaluations

Page 2: Class evaluations Class evaluations. Soil Chemistry

Soil ChemistrySoil Chemistry

Page 3: Class evaluations Class evaluations. Soil Chemistry

Ion ExchangeIon Exchange

Ions adsorbed to soil surfaces can be Ions adsorbed to soil surfaces can be exchanged with ions in soil solution.exchanged with ions in soil solution.

Cations and anionsCations and anions

Page 4: Class evaluations Class evaluations. Soil Chemistry

Ion exchangeIon exchange

Organic colloids and Organic colloids and inorganic micelles inorganic micelles (clays) (clays) are sites of are sites of ion exchangeion exchange

Where do ions in soil come from?Where do ions in soil come from?Release from organic matterRelease from organic matterRainRainWeathering of parent materialWeathering of parent material

Page 5: Class evaluations Class evaluations. Soil Chemistry

Ion exchangeIon exchange

Exchangeable cationsExchangeable cations (on soil surfaces) (on soil surfaces) cannot be removedcannot be removed by leaching. by leaching.

Soluble cationsSoluble cations (in solution) (in solution) can be removed can be removed by by leaching.leaching.

Page 6: Class evaluations Class evaluations. Soil Chemistry

When soil is dried…When soil is dried…

……exchangeable cations hold to adsorption sites exchangeable cations hold to adsorption sites on soil surfaces.on soil surfaces.

… …soluble cations (and anions) precipitate or soluble cations (and anions) precipitate or crystallize as salts.crystallize as salts.

Page 7: Class evaluations Class evaluations. Soil Chemistry

Examples of soluble cations precipitatingExamples of soluble cations precipitating

Page 8: Class evaluations Class evaluations. Soil Chemistry

Ion exchangeIon exchange

Exchangeable Exchangeable ions on soil surface ions on soil surface trading trading places with places with ions in solutionions in solution..

Page 9: Class evaluations Class evaluations. Soil Chemistry

On soil surfaces, there are:On soil surfaces, there are:Exchangeable and Nonexchangeable IonsExchangeable and Nonexchangeable Ions

::

ExchangeableExchangeable: weakly held, in contact with soil : weakly held, in contact with soil solution, ready for quick replacement.solution, ready for quick replacement.““outer sphere complexouter sphere complex””

NonexchangeableNonexchangeable::““inner sphere complex”inner sphere complex”

adsorbed by strong bonds or held in inaccessible adsorbed by strong bonds or held in inaccessible places places (e.g., the K(e.g., the K++ between layers of illite) between layers of illite)

not part of ion exchange !not part of ion exchange !

Page 10: Class evaluations Class evaluations. Soil Chemistry

Cation exchange capacity (CEC)Cation exchange capacity (CEC)

Sum total of exchangeable cations that a Sum total of exchangeable cations that a soil can adsorbsoil can adsorb. ( prevents nutrients from . ( prevents nutrients from leaching away from roots)leaching away from roots)

Page 11: Class evaluations Class evaluations. Soil Chemistry

CECCEC

Expressed in:Expressed in:

milliequivalents per 100 g (meq/100g)milliequivalents per 100 g (meq/100g)

Page 12: Class evaluations Class evaluations. Soil Chemistry

Base saturationBase saturation

% of exchange sites occupied by % of exchange sites occupied by basic basic cationscations

Basic cations are cations Basic cations are cations other than Hother than H++ and and AlAl+3+3

Base saturation Base saturation

+ H+ ion saturation+ H+ ion saturation

should equal 100%should equal 100%

Page 13: Class evaluations Class evaluations. Soil Chemistry

For midwest US soilsFor midwest US soils

Notice neutral pH (7.0) Notice neutral pH (7.0) requires a base sat requires a base sat of 80%.of 80%.

(neutral pH is not 50% (neutral pH is not 50% because most base because most base cations have a + charge cations have a + charge of 2)of 2)

Page 14: Class evaluations Class evaluations. Soil Chemistry

equilibriumequilibrium

Strive for equivalent proportions of Strive for equivalent proportions of solutionsolution and and exchangeableexchangeable ions. ions.

Upset equilibrium by:Upset equilibrium by:

removal by plantsremoval by plants

leachingleaching

fertilizationfertilization

weatheringweathering

Initiate ion exchange

Page 15: Class evaluations Class evaluations. Soil Chemistry

Ion exchange example:Ion exchange example:Add HAdd H++ ions to soil : ions to soil :

soil

Ca+

Ca+

Ca+Ca+

Ca+

Ca+Ca+

Ca+

Ca+Ca+

Ca+Ca+

Ca+

Ca+

+

H+

H+

H+H+

H+H+

solution

exchangeable solution

+

H+

H+Ca+

Ca+

exchangeable solution

Page 16: Class evaluations Class evaluations. Soil Chemistry

Rules of ion exchangeRules of ion exchange

Process is Process is ReversibleReversible

Charge by chargeCharge by charge basis basis

Ratio LawRatio Law: : ratio of exchangeable cations will be ratio of exchangeable cations will be

same as ratio of solution cations same as ratio of solution cations

Page 17: Class evaluations Class evaluations. Soil Chemistry

Add K fertilizer…Add K fertilizer…

Ca+2

K+

Ca+2

+Ca+2 Ca+2K+

K+

K+ K+K+

K+

K++

1 Ca : 2 K 1 Ca : 2 K

Same ratio

Page 18: Class evaluations Class evaluations. Soil Chemistry

Energy of adsorptionEnergy of adsorption

Strong --------------------------------------WeakStrong --------------------------------------Weak

AlAl+3 +3 > Ca> Ca+2+2 > Mg > Mg+2+2 > [K > [K++ = NH = NH44+ + ] > Na] > Na+ + > H> H++

(based on charge and hydrated radius)(based on charge and hydrated radius)

Page 19: Class evaluations Class evaluations. Soil Chemistry

Soil pH importanceSoil pH importance

Determines solubility Determines solubility of nutrientsof nutrients Before plants can get Before plants can get

nutrients, they must be nutrients, they must be dissolved in soil dissolved in soil solutionsolution

Microbial activity also Microbial activity also depends on pHdepends on pH

Page 20: Class evaluations Class evaluations. Soil Chemistry

pHpH

negative log of the hydrogen ion concentrationnegative log of the hydrogen ion concentration

(also a measure of OH(also a measure of OH-- concentration) concentration)

If If HH++ concentration > OHconcentration > OH-- : : acidicacidic

If If OHOH-- > H > H++ : : basicbasic

Soil pH is pH of solution, NOT exchange complex

Page 21: Class evaluations Class evaluations. Soil Chemistry

General soil pH conditions:General soil pH conditions:

““Slightly acid” Slightly acid”

6.0 – 6.66.0 – 6.6

““Moderately acid”Moderately acid”

5.0 – 6.05.0 – 6.0

““Strongly acid”Strongly acid”

< 5.0< 5.0

““Slightly basic”Slightly basic”

7.4 – 8.07.4 – 8.0

““Moderately basic”Moderately basic”

8.0 – 9.08.0 – 9.0

““Strongly basic”Strongly basic”

> 9.0> 9.0

Page 22: Class evaluations Class evaluations. Soil Chemistry

In soil, both In soil, both HH++ and and AlAl+3+3 ions produce acidity ions produce acidity

AlAl+3+3 produces produces HH++ ions when it reacts with ions when it reacts with water.water.

((when pH below 6: when pH below 6: AlAl+3+3 is the cause of acidity is the cause of acidity))

Page 23: Class evaluations Class evaluations. Soil Chemistry

Causes of soil basicityCauses of soil basicity

1.1. Hydrolysis of basic cationsHydrolysis of basic cations

2.2. Hydrolysis of carbonatesHydrolysis of carbonates

Page 24: Class evaluations Class evaluations. Soil Chemistry

1. Hydrolysis of basic cations:1. Hydrolysis of basic cations:(especially (especially CaCa+2+2, Mg, Mg+2+2, K, K++, NH, NH44

++, Na, Na++))

((also called also called exchangeable basesexchangeable bases))

Extent to which exchangeable bases will hydrolyze Extent to which exchangeable bases will hydrolyze depends on ability to compete with H+ ions for depends on ability to compete with H+ ions for exchange sites.exchange sites.

NaNa

Na NaNa

Na

Na Na

NaNa

+H2O H

+ + OH-

Page 25: Class evaluations Class evaluations. Soil Chemistry

KK++ and Na and Na++ are weakly held compared to are weakly held compared to CaCa+2+2 and Mg and Mg+2+2..

Recall energy of adsorptionRecall energy of adsorption

So, KSo, K++ and Na and Na++ are hydrolyzed easily and are hydrolyzed easily and yield higher pHs .yield higher pHs .

Page 26: Class evaluations Class evaluations. Soil Chemistry

2. Hydrolysis of carbonates2. Hydrolysis of carbonates(especially (especially CaCOCaCO33, MgCO, MgCO33, Na, Na22COCO33))

• As long as there are carbonates in the soil, As long as there are carbonates in the soil, carbonate hydrolysis controls pHcarbonate hydrolysis controls pH..

• Calcareous soils remain alkaline because H+ ions Calcareous soils remain alkaline because H+ ions combine with OHcombine with OH-- to form H to form H22O.O.

• For those soils to become acid, all carbonates For those soils to become acid, all carbonates must be leached.must be leached.

• Basic cations replaced by AlBasic cations replaced by Al+3+3 and H and H++

CaCO3 + H2O Ca+2 + HCO3- + OH-

Na2CO3 + H2O Na + HCO3- + OH- (higher pH because Na more soluble)

Page 27: Class evaluations Class evaluations. Soil Chemistry

Causes of soil acidityCauses of soil acidity

1.1. Accumulation of soluble acidsAccumulation of soluble acids

2.2. Exchangeable acids (AlExchangeable acids (Al+3+3, H, H++))

Page 28: Class evaluations Class evaluations. Soil Chemistry

1.1. Accumulation of soluble acidsAccumulation of soluble acidsat faster rate than they can be neutralized or at faster rate than they can be neutralized or

removedremoved

a.a. Carbonic acidCarbonic acid(respiration and atmospheric CO(respiration and atmospheric CO22))

b. b. Mineralization of organic matterMineralization of organic matter

(produces organic, nitric, sulfuric acids)(produces organic, nitric, sulfuric acids)

Precipitation increases both a and bPrecipitation increases both a and b

Page 29: Class evaluations Class evaluations. Soil Chemistry

2. Exchangeable acids2. Exchangeable acids

Exch. HExch. H++ or Al or Al+3+3 dissociate dissociate

AlAl+3 +3 ties up OHties up OH- - from water, releases an from water, releases an equivalent amount of H+ ions.equivalent amount of H+ ions.

Al+3 + H2O AlOH+2 + H+

Page 30: Class evaluations Class evaluations. Soil Chemistry

CEC and pHCEC and pH

Only 2:1 silicate clays do not have pH-dependent CECs.Only 2:1 silicate clays do not have pH-dependent CECs.

Others are pH-dependent:Others are pH-dependent:

1:1 kaolinite:1:1 kaolinite:low pH: low CEClow pH: low CEChigh pH: high CEChigh pH: high CEC

Oxidic claysOxidic clays