soil buffering and management of acid soils. ph ph = - log (h + ) if (h + ) = 1 x 10 -3 mol/l (h + )...
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Soil Buffering and Soil Buffering and Management of Acid SoilsManagement of Acid Soils
pH
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 = - 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-
H+H+
H+
H+
H+
H+
H+
H+
H+
OH-
OH-OH-OH-
OH-
OH-
OH-
OH-OH-
Acid (=10) Base(=10)
0 7 14
Acidic basic
H+ OH- H2O+
H+OH-
H+H+
H+
H+
H+
H+H+
H+
H+
OH-
OH-OH-OH-
OH-
Acid (=10) Base (=6)
0 7 14
Acidic basic
H+ OH- H2O+
H+OH-
H+H+
H+
H+
H+
H+
H+
H+
H+
OH-
OH-OH-OH-
OH-
OH-
OH-
OH-OH-
Acid (=10) Base(=15)
0 7 14
Acidic basic
H+ OH- H2O+
OH-
OH-OH-
OH-
OH-
Two types of acidity in soils:
Active AcidityExchangeable 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+ 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
CEC and Acidity
Is CEC always a good indicator of fertility?
CEC suggests the ability of a soil to storeimportant plant nutrients (K, Mg, Ca, Fe)
Percent Acid SaturationPercent Acid Saturation
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Na+
Ca2+
H+
H+
Acid Cations (cmol/kg)Cation exchange capacity (cmol/kg)
(charge basis)
Acid charge = 14Exch. Cap. = 26
% A.S. = 53.8%
Acid Cations: Al, H+
Percent Base SaturationPercent Base Saturation
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Na+
Ca2+
H+
H+
Exchangeable bases (cmol/kg)Cation exchange capacity (cmol/kg)
(charge basis)
Base charge = 12Exch. 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 BufferingSoil Buffering
The ability of soils to resist changes in pH
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Due to ultimate equilibrium between solution and colloids.
Na+ Ca+2
K+ H+H+
H+
Na+
H+
H+H+
K+
Na+
H+
H+
Ca+2
pH = 6
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Na+ Ca+2
K+ H+H+
H+
Na+
H+
H+ H+
K+
Na+
H+
H+
Ca+2
Add acid: HCl => H+ + Cl-
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+
K+
H+ Al+3
Na+
Ca2+
Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Na+ Ca+2
K+ H+H+
H+
Na+
H+
H+ H+
K+
Na+
H+
H+
Ca+2
Final equilibrium
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 theHydrogen ion concentration in solution
OH-
CO32-
SO42-
Bases react with hydrogen and remove it from soil solution
OH-
CO32-
+ H+ H2O
+ H+ HCO3-
(Neutralization of acid)
OH-
CO32-
+ H+ H2O
+ H+ HCO3-
NaOH Na+ + OH-
CaCO3 Ca2+ + CO32-
(Neutralization of acid)
water
water
Common Bases
A common base used to increase the pH of soil is CaCO3
CaCO3 Ca2+ + CO32-water
CO32- + H+ HCO3
-
CaCO3 + H+ Ca2+ + HCO32-
Adding calcium carbonate to soils is called “liming”
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Equilibrium between solution and colloids.
Na+ Ca+2
K+ H+H+
H+
Na+
H+
H+
H+
K+
Na+
H+
H+
Ca+2
pH = 6
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Na+ Ca+2
K+ H+H+
H+
Na+
H+
H+ H+
K+
Na+
H+
H+
Ca+2
pH = 7
CO3-2
Ca+2
Ca+2
Ca+2
Ca+2
CaCO3 Ca2+ + CO32-
water
CO3-2
CO3-2
CO3-2
CO3-2
CO32- + H+ HCO3
-
Soil solution pH initially rises due to base addition
Al+3 Na+ H+
H+ H+ K+
H+ Al+3
Na+
Ca2+ Ca2+
Ca2+
H+
H+
Clay minerals/Organic matter
Soil BufferingSoil Buffering
Na+
Ca+2
K+ H+H+
H+
Na+
H+
H+ H+
K+
Na+
H+
H+
Ca+2
pH = 6.5
CO3-2
Ca+2
Ca+2
Ca+2
Ca+2
CaCO3 Ca2+ + CO32-
water
CO3-2
CO3-2
CO3-2
CO3-2
H+ removed from exchange sites returns to soil solution
CaCO3
CaCO3 Ca2+ + CO32-
Displaces cationsFrom exchange sites
Combines withHydrogen ions(neutralization)
Liming: raising soil pH
MgCO3
CO32- + H+ = HCO3
-
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
Adjusting soil pH requires a knowledge of exchangeable acidity
Active Acidity
Buffering CapacityBuffering Capacity
CEC
Kaolinite
Smectite
Organic Matter
% base saturation
1.
2.
pH and Nutrient Availability
H+
CaK
CaCa
NaNa
Na
Na
Mg
Mg
Mg
(In Rainfall)
H+
H+
H+
H+
H+
Florida Soils Tend to be Acidic
Al3+
Al3+
Al3+ Low %B.S.
Aluminum Toxicity
Aluminum most available at low pH
Damages cell walls, binds to phosphorus
Macro-Nutrients
Generalizations: Nitrogen: NH4+ users below pH 5.5
NH4+ NO3
-
Ammonium may accumulate at low pHOrganism dependent.
Phosphorus: H2PO4- and HPO4
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+ = Fe3+ + 3H20
