Soil Acidity and Review of Colloid Charge

Mineral Charge

Positive charge

Al3+ Si4+

Negative charge

OH- O2-

“Perfect” Mineral

=

Si

Al

Tetrahedral Substitution

Al3+ for Si4+

Positive charge

Al3+ Si4+

Negative charge

OH- O2-

=

Al

Aluminum contributes less (+) charge than silicon

Octahedral Substitution

Mg2+ for Al3+

Positive charge

Al3+ Si4+

Negative charge

OH- O2-

=

Magnesium contributes less (+) charge than Aluminum

Na+

Na+

Na+Na+

Na+Na+

Na+

Charge

K

HH

O

HH

O

HH

O

H H

O

+

Hydrated Cations

SmectitesSignificant substitution in the octahedra

(Mg2+ for Al3+)

Ca2+ Mg2+ Na+

Cations satisfying charge

CEC = 80-120 cmolc kg

•Layers weakly held together by cations•Highly expansible

Mg2+Mg2+

Source of negative charge Is very close to the adsorbedcations

Layers tightly boundModerately expansible

Vermiculite

Significant substitution in tetrahedra

CEC = 100-180 cmolc kg

Soil Organic Matter and Reactivity

The functional groups on organic colloids are weak acids.

Acids give up hydrogen ions (H+) to water.

HCl H+ + Cl-

Strong acids give up all their H+ ions

Weak acids give up some of their H+ ions

COOH COO- + H+

OH O- + H+

Soil Solution

COOH COO- + H+

Dissociation of weak acid functional groupsresults in the development of negative charge

The amount of negative charge dependson how much dissociation takes place

Low pH = lots of H+ = less dissociation = low chargeHigh pH = little H+ = more dissociation = high charge

The dissociation is inhibited by H+ in soil solution

COOH COO- + H+

COOH

O-

K+ K+COO-

COO-

COO-

COO-

OHO-

O-

O-

COO-

K+K+

Ca2+

Na+K+

Na+

Na+Na+

Mg2+

Mg2+Mg2+

Mg2+

Na+

Na+

Na+

Soil Solution

H+

H+

H+

H+

Mineral Colloids derive charge from substitution of lower-charged cations for higher charged cationsin the crystal matrix during mineral formation. The result is permanent negative charge.

Organic colloids derive their charge from dissociationof hydrogen ions from acidic functional groups on organic matter/humus. The result is pH-dependentcharge

Mineral Colloids – 0 – 180 cmol/kgOrganic Colloids – 100 – 500 cmol/kg

SiAlSi

Total CEC =

Mineral Organic

+

Soil Cation Exchange Capacity

pH-Dependent

You have 2 identical soils (same clay content, mineralogy, and O.M. content).

Soil A has a pH of 6.5Soil B has a pH of 4.0

Which soil has the higher CEC?

Reactivity of Soil Horizons

Contribution to fertility.

Silicate Clay

Organic Matter Organic Matter Organic Matter

Silicate Clay

Silicate Clay

Soil Acidity and pHSoil Acidity and pH

AcidAcid

Any substance which increases the Hydrogenion concentration in solution.

H+

Hydrogen Ions and Organic MatterHydrogen Ions and Organic Matter

0 7 14

Acidic basic

High H+concentration

Low H+concentration

Inhibits dissociationof acidic functional groups

Increases dissociationof acidic functional groups

Low charge onOrganic Colloids

High charge onOrganic Colloids

COOH COO- + H+

Sources of Hydrogen Ions

CO2 from microbial respiration/atmosphere

CO2 dissolved in water produces carbonic acid H2CO3

Acid functional groups on organic matter

COOH COO- + H+

Plant root exudation (release) of H+

Anthropogenic acids in rainfall

Gaseous nitrogen and sulfur oxides make acid rain

Hydrolysis of aluminum can produce acids

CO2 + H2O H2CO3

What are the acids in soils?

Two Ions considered Acidic in Soils

H+ Aluminum

Remember that aluminum is a common constituent of silicate clays

Aluminum participates in “hydrolysis”reactions with water.

Both are cationic

Hydrolysis of Water

(ionization of water)

H2O OH- + H+

H+ is the acid OH- is the base

In pure water, the amounts of H+ and OH- are equal

Bases are the opposite of acids

Aluminum has the ability to breakwater apart and react with the resulting OH-

Aluminum

H2O OH- + H+

Hydrogen and AluminumHydrogen and Aluminum

The only soil ions considered to be acidic

Hydolysis by Aluminum

Al3+ + H20 = Al(OH)2+ + H+

Al(OH)2+ + H20 = Al(OH)2+ + H+

Al(OH)2+ + H20 = Al(OH)o

3 + H+

H2O OH- + H+

Strongly Acid Soils (pH < 5)

Aluminum is soluble => Al3+ or Al(OH)2+

Both are exchangeable and strongly adsorbedThey are also in equilibrium with aluminum in solutionThe aluminum cations in solution can hydrolyze => H+

Moderately Acid Soils (pH 5 – 6.5)

Aluminum exists as Al(OH)2+ and Al(OH)2+

Both are exchangeable and can be strongly adsorbedThey are also in equilibrium with aluminum in solution.The solution aluminum cations hydrolyze => H+

Types of Soil Acidity

Active Acidity

Acidity associated with soil solution

H+ ions

Active Acidity

Acidity associated with the soil solution

Typically a 1:1 or 2:1 extract

10 g soil and 10 mL water

10 g soil and 20 mL water

0 7 14

Acidic basic

5.6

pH meter

0 7 14

Acidic basic

PlantAlfalfaSweet CloverBeetsCauliflowerSpinachPeasCarrotsCotton Wheat TomatoesPotatoesBlueberriesAzaleas

pH Range

6.0 – 8.0

5.5 - 8.0

5.3 - 7.5

5.0 – 7.2

4.5 – 5.5< 5

Sometimes soil pH must be adjusted to accommodate plants

Active Acidity

Exchangeable Acidity

Exchangeable Acidity

Acidity associated with cation exchange sites on mineral or organic colloids.

H+ Al+3

Types of AcidityTypes of Acidity

Active Acidity

H+ H+

H+ H+H+

H+

H+

H+

H+ H+

H+

H+

H+

H+

Soil Solution

Al+3 Na+ H+

H+ H+ K+

H+ Al+3

Na+

Ca2+ Ca2+

Ca2+

H+

H+

Clay minerals/Organic matter

Exchangeable

Measurement of Exchangeable Acidity

Initial EquilibriumInitial Equilibrium

Ca+2 Ba+2

Na+

Na+

Mg+2

K+

Ca+2

Ca+2Ca+2

Cu+2

Ca+2

Mg+2

Soil Solution

Al+3 Na+ H+

H+ H+ K+

H+ Al+3

Na+

Ca2+ Ca2+

Ca2+

H+

H+

Clay minerals/Organic matter

ExchangeableActive

H+

H+H+

H+

H+

H+

H+

Flood System with BaFlood System with Ba2+2+

Ba+2 Ba+2

Ba+2

Ba+2

Ba+2

Ba+2

Ba+2

Ba+2Ba+2

Ba+2

Ba+2

Ba+2

Soil Solution

Al+3 Na+ H+

H+ H+ K+

H+ Al+3

Na+

Ca2+ Ca2+

Ca2+

H+

H+

Clay minerals/Organic matter

Exchangeable

Total AcidityTotal Acidity

Ba+2

Soil Solution

Ba+2 Ba+2 Ba+2

Ba+2 Ba+2 B+2

Ba+2 Ba+2

Ba+2

Ba2+ Ba2+

Ba2+

Ba+

H+

Clay minerals/Organic matter

Active and Exchangeable

H+ H+

H+ H+H+

H+

H+

H+

H+ H+

H+

H+

H+

H+

Na+

Ca2+

K+

K+

K+

Ca2+

Exchangeable acidity can be many times greater than

active acidity.

Managing soil acidity must accountfor exchangeable acidity as well as

active acidity.

Next: Acidity and Fertility