Soil Buffering and Management of Acid Soils

pH pH = - log (H + ) If (H + ) = 1 x mol/L (H + ) = mol/L pH = - log (1 x ) pH = - (-3) pH = 3 pH = - log (H + ) If (H + ) = 1 x mol/L (H + ) = mol/L pH = - log (1 x ) pH = - (-5) pH = 5 Low pH = high hydrogen ion concentration

1. Acids increase the H + ion concentration in solution 2. Bases are the opposite of acids 3. Bases neutralize acids. 4. When acids and bases are in equal amounts in a solution, the pH is 7. Neutral pH. 5. When the number of acids exceeds the number of bases the pH is lowered. (acid conditions) 6. When the number of bases exceeds the number of acids, the pH is raised. (basic/alkaline conditions)

H+H+ OH - H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Acid (=10) Base(=10) Acidic basic H+H+ OH - H2OH2O +

H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Acid (=10) Base (=6) Acidic basic H+H+ OH - H2OH2O +

H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Acid (=10) Base(=15) Acidic basic H+H+ OH - H2OH2O +

Two types of acidity in soils: Active Acidity Exchangeable Acidity

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

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

CEC and Acidity Is CEC always a good indicator of fertility? CEC suggests the ability of a soil to store important plant nutrients (K, Mg, Ca, Fe)

Percent Acid Saturation 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 Na + Ca 2+ H+H+ H+H+ Acid Cations (cmol/kg) Cation exchange capacity (cmol/kg) (charge basis) Acid charge = 14 Exch. Cap. = 26 % A.S. = 53.8% Acid Cations: Al, H +

Percent Base Saturation 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 Na + Ca 2+ H+H+ H+H+ Exchangeable bases (cmol/kg) Cation exchange capacity (cmol/kg) (charge basis) Base charge = 12 Exch. Cap. = 26 % B.S. = 46.2% Base Cations: Na, K, Mg, Ca

You have two soils with the same CEC Soil A has a % B.S. = 35% Soil B has a % B.S = 65% Which soil is more fertile? Which soil is more acidic?

Soil Buffering The ability of soils to resist changes in pH

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 Soil Buffering Due to ultimate equilibrium between solution and colloids. Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 pH = 6

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 Soil Buffering Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 Add acid: HCl => H + + Cl - H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ pH = 4 Soil solution pH initially declines due to acid addition

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 Soil Buffering Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 Final equilibrium H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ pH = 5.5 Soil pH does not decline as much as expected

Base A substance which decreases the Hydrogen ion concentration in solution OH - CO 3 2- SO 4 2-

Bases react with hydrogen and remove it from soil solution OH - CO H + H 2 O + H + HCO 3 - (Neutralization of acid)

OH - CO H + H 2 O + H + HCO 3 - NaOHNa + + OH - CaCO 3 Ca 2+ + CO 3 2- (Neutralization of acid) water Common Bases

A common base used to increase the pH of soil is CaCO 3 CaCO 3 Ca 2+ + CO 3 2- water CO H + HCO 3 - CaCO 3 + H + Ca 2+ + HCO 3 2- Adding calcium carbonate to soils is called “liming”

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 Soil Buffering Equilibrium between solution and colloids. Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 pH = 6

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 Soil Buffering Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 pH = 7 CO 3 -2 Ca +2 CaCO 3 Ca 2+ + CO 3 2- water CO 3 -2 CO H + HCO 3 - Soil solution pH initially rises due to base addition

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 Soil Buffering Na + Ca +2 K+K+ H+H+ H+H+ H+H+ Na + H+H+ H+H+ H+H+ K+K+ H+H+ H+H+ Ca +2 pH = 6.5 CO 3 -2 Ca +2 CaCO 3 Ca 2+ + CO 3 2- water CO 3 -2 H + removed from exchange sites returns to soil solution

CaCO 3 CaCO 3 Ca 2+ + CO 3 2- Displaces cations From exchange sites Combines with Hydrogen ions (neutralization) Liming: raising soil pH MgCO 3 CO H + = HCO 3 -

Plant Alfalfa Sweet Clover Beets Cauliflower Spinach Peas Carrots Cotton Wheat Tomatoes Potatoes Blueberries Azaleas pH Range 6.0 – – – 5.5 < 5 Adjusting soil pH requires a knowledge of exchangeable acidity Active Acidity

Buffering Capacity CEC Kaolinite Smectite Organic Matter % base saturation 1. 2.

pH and Nutrient Availability

H+H+ Ca K Na Mg (In Rainfall) H+H+ H+H+ H+H+ H+H+ H+H+ Florida Soils Tend to be Acidic Al 3+ Low %B.S.

Aluminum Toxicity Aluminum most available at low pH Damages cell walls, binds to phosphorus

Macro-Nutrients Generalizations: Nitrogen: NH 4 + users below pH 5.5 NH 4 + NO 3 - Ammonium may accumulate at low pH Organism dependent. Phosphorus: H 2 PO 4 - and HPO 4 2- Greatest availability at pH 6-7 Potassium: K+ Liming tends to increase availability (Increased pH increases CEC)

Micro-Nutrients manganese, iron, cobalt copper, zinc Availability generally increases With increasing soil acidity (low pH) Acidity can be local: roots – acids - organisms Oxides of these metals tend to be dissolved at low pH These are plant essential, but can be toxic in high amounts Fe(OH) 3 + 3H + = Fe H 2 0