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Class evaluations. Soil Chemistry. Ion Exchange. Ions adsorbed to soil surfaces can be exchanged with ions in soil solution. Cations and anions. Ion exchange. Organic colloids and inorganic micelles (clays) are sites of ion exchange Where do ions in soil come from?

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Class evaluations

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Class evaluations

  • Class evaluations


Soil chemistry

Soil Chemistry


Ion exchange

Ion Exchange

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

  • Cations and anions


Ion exchange1

Ion exchange

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

  • Where do ions in soil come from?

    • Release from organic matter

    • Rain

    • Weathering of parent material


Ion exchange2

Ion exchange

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

  • Soluble cations (in solution)

    can be removed by leaching.


Class evaluations

  • When soil is dried…

    …exchangeable cations hold to adsorption sites on soil surfaces.

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


Examples of soluble cations precipitating

Examples of soluble cations precipitating


Ion exchange3

Ion exchange

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


On soil surfaces there are exchangeable and nonexchangeable ions

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

Exchangeable: weakly held, in contact with soil solution, ready for quick replacement.

“outer sphere complex”

Nonexchangeable:

“inner sphere complex”

  • adsorbed by strong bonds or held in inaccessible places

    • (e.g., the K+ between layers of illite)

  • not part of ion exchange !


Cation exchange capacity cec

Cation exchange capacity (CEC)

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


Class evaluations

CEC

Expressed in:

milliequivalents per 100 g (meq/100g)


Base saturation

Base saturation

% of exchange sites occupied by basic cations

Basic cations are cationsother than H+ and Al+3

Base saturation

+ H+ ion saturation

should equal 100%


For midwest us soils

For midwest US soils

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

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


Equilibrium

equilibrium

Strive for equivalent proportions of solution and exchangeable ions.

Upset equilibrium by:

removal by plants

leaching

fertilization

weathering

Initiate ion exchange


Ion exchange example add h ions to soil

Ion exchange example:Add H+ ions to soil :

H+

H+

Ca+

Ca+

Ca+

Ca+

H+

Ca+

Ca+

soil

H+

Ca+

+

+

H+

Ca+

Ca+

H+

H+

Ca+

H+

Ca+

Ca+

Ca+

Ca+

solution

Ca+

Ca+

exchangeable

solution

exchangeable

solution


Rules of ion exchange

Rules of ion exchange

  • Process is Reversible

  • Charge by charge basis

  • Ratio Law:

    • ratio of exchangeable cations will be same as ratio of solution cations


Add k fertilizer

Add K fertilizer…

K+

Ca+2

Ca+2

K+

Ca+2

+

K+

+

K+

K+

Ca+2

K+

K+

K+

1 Ca : 2 K

1 Ca : 2 K

Same ratio


Energy of adsorption

Energy of adsorption

Strong --------------------------------------Weak

Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+

(based on charge and hydrated radius)


Soil ph importance

Soil pH importance

  • Determines solubility of nutrients

    • Before plants can get nutrients, they must be dissolved in soil solution

  • Microbial activity also depends on pH


Class evaluations

pH

negative log of the hydrogen ion concentration

(also a measure of OH- concentration)

If H+concentration > OH- : acidic

If OH- > H+ : basic

Soil pH is pH of solution, NOT exchange complex


General soil ph conditions

“Slightly acid”

6.0 – 6.6

“Moderately acid”

5.0 – 6.0

“Strongly acid”

< 5.0

“Slightly basic”

7.4 – 8.0

“Moderately basic”

8.0 – 9.0

“Strongly basic”

> 9.0

General soil pH conditions:


Class evaluations

In soil, both H+ and Al+3 ions produce acidity

Al+3 produces H+ ions when it reacts with water.

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


Causes of soil basicity

Causes of soil basicity

  • Hydrolysis of basic cations

  • Hydrolysis of carbonates


1 hydrolysis of basic cations especially ca 2 mg 2 k nh 4 na

1. Hydrolysis of basic cations:(especially Ca+2, Mg+2, K+, NH4+, Na+)

(also called exchangeable bases)

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

Na

Na

Na

Na

+

+ OH-

H2O

Na

H

Na

+

Na

Na

Na

Na


Class evaluations

K+ and Na+ are weakly held compared to Ca+2 and Mg+2.

  • Recall energy of adsorption

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


2 hydrolysis of carbonates especially caco 3 mgco 3 na 2 co 3

2. Hydrolysis of carbonates(especially CaCO3, MgCO3, Na2CO3)

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

    • Calcareous soils remain alkaline because H+ ions combine with OH- to form H2O.

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

      • Basic cations replaced by Al+3 and H+

Ca+2 + HCO3- + OH-

CaCO3 + H2O

Na2CO3 + H2O

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


Causes of soil acidity

Causes of soil acidity

  • Accumulation of soluble acids

  • Exchangeable acids (Al+3, H+)


Accumulation of soluble acids at faster rate than they can be neutralized or removed

Accumulation of soluble acidsat faster rate than they can be neutralized or removed

  • Carbonic acid

    (respiration and atmospheric CO2)

    b. Mineralization of organic matter

    (produces organic, nitric, sulfuric acids)

    Precipitation increases both a and b


2 exchangeable acids

2. Exchangeable acids

Exch. H+ or Al+3 dissociate

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

Al+3 + H2O

AlOH+2 + H+


Cec and ph

CEC and pH

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

Others are pH-dependent:

1:1 kaolinite:

low pH: low CEC

high pH: high CEC

Oxidic clays


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