Soil Acidity and Review of Colloid Charge

Mineral Charge

Positive charge Al 3+ Si 4+ Negative charge OH - O 2- “Perfect” Mineral = Si Al

Tetrahedral Substitution Al 3+ for Si 4+ Positive charge Al 3+ Si 4+ Negative charge OH - O 2- = Al Aluminum contributes less (+) charge than silicon

Octahedral Substitution Mg 2+ for Al 3+ Positive charge Al 3+ Si 4+ Negative charge OH - O 2- = Magnesium contributes less (+) charge than Aluminum

Na + Na + Na + Na + Na + Na + Na + Charge

K HH O HH O HH O HH O + Hydrated Cations

Smectites Significant substitution in the octahedra (Mg 2+ for Al 3+ ) Ca 2+ Mg 2+ Na + Cations satisfying charge CEC = cmol c kg Layers weakly held together by cations Highly expansible

Mg 2+ Source of negative charge Is very close to the adsorbed cations Layers tightly bound Moderately expansible Vermiculite Significant substitution in tetrahedra CEC = cmol c kg

Soil Organic Matter and Reactivity

The functional groups on organic colloids are weak acids. Acids give up hydrogen ions (H+) to water. HClH + + 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 groups results in the development of negative charge The amount of negative charge depends on how much dissociation takes place Low pH = lots of H + = less dissociation = low charge High pH = little H + = more dissociation = high charge The dissociation is inhibited by H + in soil solution

COOH COO - + H + COOH O-O- K+K+ K+K+ COO - OH O-O- O-O- O-O- COO - K+K+ K+K+ Ca 2+ Na + K+K+ Mg 2+ Na + Soil Solution H+H+ H+H+ H+H+ H+H+

Mineral Colloids derive charge from substitution of lower-charged cations for higher charged cations in the crystal matrix during mineral formation. The result is permanent negative charge. Organic colloids derive their charge from dissociation of hydrogen ions from acidic functional groups on organic matter/humus. The result is pH-dependent charge Mineral Colloids – 0 – 180 cmol/kg Organic Colloids – 100 – 500 cmol/kg

Si Al Si 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 6.5 Soil B has a pH of 4.0 Which soil has the higher CEC?

Reactivity of Soil Horizons Contribution to fertility. Silicate Clay Organic Matter Silicate Clay

Soil Acidity and pH

Acid Any substance which increases the Hydrogen ion concentration in solution. H+H+

Hydrogen Ions and Organic Matter Acidic basic High H+ concentration Low H+ concentration Inhibits dissociation of acidic functional groups Increases dissociation of acidic functional groups Low charge on Organic Colloids High charge on Organic Colloids COOHCOO - + H +

Sources of Hydrogen Ions CO 2 from microbial respiration/atmosphere CO 2 dissolved in water produces carbonic acid H 2 CO 3 Acid functional groups on organic matter COOHCOO - + 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 CO 2 + H 2 O H 2 CO 3

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) H 2 O 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 break water apart and react with the resulting OH - Aluminum H 2 O OH - + H +

Hydrogen and Aluminum The only soil ions considered to be acidic Hydolysis by Aluminum Al 3+ + H 2 0 = Al(OH) 2+ + H + Al(OH) 2+ + H 2 0 = Al(OH) H + Al(OH) H 2 0 = Al(OH) o 3 + H + H 2 O OH - + H +

Strongly Acid Soils (pH < 5) Aluminum is soluble => Al 3+ or Al(OH) 2+ Both are exchangeable and strongly adsorbed They are also in equilibrium with aluminum in solution The 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 adsorbed They 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 Acidic basic

5.6 pH meter Acidic basic

Plant Alfalfa Sweet Clover Beets Cauliflower Spinach Peas Carrots Cotton Wheat Tomatoes Potatoes Blueberries Azaleas pH Range 6.0 – – – 5.5 < 5 Sometimes soil pH must be adjusted to accommodate plants Active Acidity

Exchangeable Acidity

Acidity associated with cation exchange sites on mineral or organic colloids. H+H+ Al +3

Types of Acidity Active Acidity H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Soil Solution Al +3 Na + H+H+ H+H+ H+H+ K+K+ H+H+ Al +3 Na + Ca 2+ H+H+ H+H+ Clay minerals/Organic matter Exchangeable

Measurement of Exchangeable Acidity

Initial Equilibrium Ca +2 Ba +2 Na + Mg +2 K+K+ Ca +2 Cu +2 Ca +2 Mg +2 Soil Solution Al +3 Na + H+H+ H+H+ H+H+ K+K+ H+H+ Al +3 Na + Ca 2+ H+H+ H+H+ Clay minerals/Organic matter Exchangeable Active H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+

Flood System with Ba 2+ Ba +2 Soil Solution Al +3 Na + H+H+ H+H+ H+H+ K+K+ H+H+ Al +3 Na + Ca 2+ H+H+ H+H+ Clay minerals/Organic matter Exchangeable

Total Acidity Ba +2 Soil Solution Ba +2 B +2 Ba +2 Ba 2+ Ba + H+H+ Clay minerals/Organic matter Active and Exchangeable H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Na + Ca 2+ K+K+ K+K+ K+K+

Exchangeable acidity can be many times greater than active acidity. Managing soil acidity must account for exchangeable acidity as well as active acidity.

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