Transcript Soil Buffering and Management of Acid Soils
Soil Buffering and Management of Acid Soils
pH = - log (H + ) If (H + ) = 1 x 10 -3 mol/L (H + ) = 0.001 mol/L pH = - log (1 x 10 -3 ) pH = - (-3) pH = 3
pH
pH = - log (H + ) If (H + ) = 1 x 10 -5 mol/L (H + ) = 0.00001 mol/L pH = - log (1 x 10 -5 ) 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 + + OH -
Acid (=10)
H + H + H H + + H + H H + + H + H + H + H 2 O
Base(=10) OH OH OH OH OH OH OH OH OH OH -
Acidic basic
0 7 14
H + + OH -
Acid (=10)
H + H + H + H + H + H + H + H + H + H + H 2 O
Base (=6) OH OH OH OH OH OH -
Acidic basic
0 7 14
H + + OH -
Acid (=10)
H + H + H H + + H + H H + + H + H + H + H 2 O
Base(=15) OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH -
Acidic basic
0 7 14
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.
H + Al +3
Si Al
Si Si
Al
Si Al
Al
Si
Al Al Al Al Al Al
Types of Acidity
Exchangeable Active Acidity H + H + H + H + H + H + H + H + H + H + H H H + + + H + Soil Solution Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+ Clay minerals/Organic matter
CEC and Acidity CEC suggests the ability of a soil to store important plant nutrients (K, Mg, Ca, Fe) Is CEC always a good indicator of fertility?
Percent Acid Saturation
(charge basis) Acid Cations (cmol/kg) Cation exchange capacity (cmol/kg) Acid charge = 14 Exch. Cap. = 26 % A.S. = 53.8%
Na + Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + H + Ca 2+ H + Al +3 Ca 2+ Ca 2+ H +
Clay minerals/Organic matter Acid Cations: Al, H +
Percent Base Saturation
(charge basis) Exchangeable bases (cmol/kg) Cation exchange capacity (cmol/kg) Base charge = 12 Exch. Cap. = 26 % B.S. = 46.2%
Na + Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + H + Ca 2+ H + Al +3 Ca 2+ Ca 2+ H +
Clay minerals/Organic matter 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
Soil Buffering
Due to ultimate equilibrium between solution and colloids.
Na + K +
H + H + H + H +
Ca Na + +2
H +
Na +
H +
Ca
H H + +
K +2 +
pH = 6
Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+ Clay minerals/Organic matter
Soil Buffering Add acid: HCl => H
+
+ Cl
-
H +
Na +
H +
Na + H + K + H
H +
+ Na + H + H +
H
H +
H + +
Ca H H + + +2 Ca +2 H +
H + H +
K +
H +
Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+ Clay minerals/Organic matter
pH = 4
Soil solution pH initially declines due to acid addition
Soil Buffering Final equilibrium
Na
Na +
K + Na + + H + H + H + H +
H +
Na + H + Ca +2 H +
K + H + Ca 2+
Ca +2
Ca
H H + + K +
2+
pH = 5.5
Al +3 Na +
H + H + H + H + H + H + H + H + H + H + H + H +
Al +3 Ca 2+ Clay minerals/Organic matter 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
+ H + H 2 O
CO
3 2 + H + HCO (Neutralization of acid) 3 -
NaOH Common Bases water Na + + OH water CaCO 3 Ca 2+ + CO 3 2-
OH
+ H + H 2 O
CO
3 2 + H + HCO (Neutralization of acid) 3 -
A common base used to increase the pH of soil is CaCO 3 water CaCO 3 Ca 2+ + CO 3 2 CO 3 2 + H + HCO 3 CaCO 3 + H + Ca 2+ + HCO 3 2 Adding calcium carbonate to soils is called “liming”
Soil Buffering
Equilibrium between solution and colloids.
Na + K +
H + H + H + H +
Ca Na + +2
H +
Na +
H +
Ca
H H + +
K +2 +
pH = 6
Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+
Clay minerals/Organic matter
Soil Buffering
water CaCO 3 CO 3 2 + H + Ca 2+ + CO 3 2 HCO 3 Ca Ca
CO
+2
CO
+2
3 -2 3
Na
-2
+ Na +
H +
K +
H +
Ca +2 H + Na + Ca +2
H + H CO 3 -2 +
Ca +2
H H + + CO 3 -2
Ca +2
H + CO 3 -2
K +
pH = 7
Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+ Clay minerals/Organic matter Soil solution pH initially rises due to base addition
CaCO 3
Soil Buffering
water Ca 2+ + CO 3 2 Ca Ca
CO 3 -2
+2
CO
+2
3 -2
Na + Na + H + K + H + Ca +2 H + Na + Ca +2 H + H
CO 3 -2
+ H H + +
CO 3 -2
Ca +2 Ca +2 H +
CO 3 -2
K + Al +3 Na + H + Na + H + H + H + Ca 2+ K + H + Ca 2+ H + Al +3 Ca 2+ Clay minerals/Organic matter
pH = 6.5
H + removed from exchange sites returns to soil solution
Liming: raising soil pH
MgCO 3 CaCO 3 CaCO 3 Ca 2+ + CO 3 2 Displaces cations From exchange sites Combines with Hydrogen ions (neutralization) CO 3 2 + H + = HCO 3 -
Active Acidity
Plant Alfalfa Sweet Clover Beets Cauliflower Spinach Peas Carrots Cotton Wheat Tomatoes Potatoes Blueberries Azaleas pH Range 6.0 – 8.0
5.5 - 8.0
5.3 - 7.5
5.0 – 7.2
4.5 – 5.5
< 5 Adjusting soil pH requires a knowledge of exchangeable acidity
Buffering Capacity 1.
CEC
Kaolinite Smectite Organic Matter
2.
% base saturation
pH and Nutrient Availability
Florida Soils Tend to be Acidic
H +
(In Rainfall)
H + Ca Mg
Al 3+
Ca Na H + Na H +
Al 3+
Na Mg H + Mg K Na
Al 3+
Ca H +
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 Oxides of these metals tend to be dissolved at low pH Fe(OH) 3 + 3H + = Fe 3+ + 3H 2 0 Availability generally increases With increasing soil acidity (low pH) These are plant essential, but can be toxic in high amounts Acidity can be local: roots – acids - organisms