Soil Buffering and
Management of Acid Soils
pH
pH = - log (H+)
pH = - log (H+)
If (H+) = 1 x 10-3 mol/L
If (H+) = 1 x 10-5 mol/L
(H+) = 0.001 mol/L
(H+) = 0.00001 mol/L
pH = - log (1 x 10-3)
pH = - log (1 x 10-5)
pH = - (-3)
pH = - (-5)
pH = 3
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)
+ OH-
H+
Base(=10)
Acid (=10)
H+
OH- OHOH- OHOH-OH-
H+
H+
+
H
H+ H+
+
H
H+ +
H
H+
OH
OHOHOH-
Acidic
0
H2O
basic
7
14
+ OH-
H+
Base (=6)
Acid (=10)
H+
H+
H+
OH- OHOH- OHOH-OH-
H+
H+ H+
H+ H+
H+ H+
Acidic
0
H2O
basic
7
14
+ OH-
H+
Base(=15)
Acid (=10)
H+
OH- OHOHOH- OHOH
OH
OH OHOH-OHOH- OH
OH
OH-
H+
H+
+
H
H+ H+
+
H
H+ +
H
H+
Acidic
0
H2O
basic
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+
Si
Al
Si
Al+3
Si
Al
Si Al
Al Al Al Al Al Al
Al
Si
Types of Acidity
Active Acidity
H + H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Exchangeable
Al+3 Na+
H+
Ca2+
H+
Na+ H+
H+ Ca2+ K+ H+
H+
Al+3 Ca2+
H+
Soil Solution
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)
Na+
Al+3 Na+
H+
Acid charge = 14
Exch. Cap. = 26
Ca2+
H+
Na+ H+
H+
H+ Ca2+ K+ H+
H+
Al+3 Ca2+
Ca2+
% A.S. = 53.8%
Clay minerals/Organic matter
Acid Cations: Al, H+
H+
Percent Base Saturation
(charge basis)
Exchangeable bases (cmol/kg)
Cation exchange capacity (cmol/kg)
Na+
Al+3 Na+
H+
Base charge = 12
Exch. Cap. = 26
Ca2+
H+
Na+ H+
H+
H+ Ca2+ K+ H+
H+
Al+3 Ca2+
Ca2+
% B.S. = 46.2%
Clay minerals/Organic matter
Base Cations: Na, K, Mg, Ca
H+
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.
H+
Na+
K+
Na+
Ca+2
H+
H+
H+
Ca+2
H+
Na+
H+
H+
+
H
K+
pH = 6
Al+3 Na+
H+
Ca2+
H+
Na+ H+
H+ Ca2+
K+ H+
H+
Al+3 Ca2+
Clay minerals/Organic matter
Soil Buffering
Add acid: HCl => H+ + ClH+
Na+
H+
H+
Ca+2
H + H+
H+
+
+
H+ H + H
H
+
Na
Na+
H+
H+
+
H
+2
Ca
H+
H+
K+
+
H
K+
Al+3 Na+
H+
Ca2+
H+
Na+ H+
H+ Ca2+
K+ H+
H+
Al+3 Ca2+
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+
Ca+2
Ca2+
H+
H+
H+
Na+
H+
+
H
Ca+2 H+
K+
+
H
Ca2+
Al+3 Na+
H+
H+
H+ H+
H+ H+ H+ H+ H+
H + H+ H+
Al+3 Ca2+
K+
pH = 5.5
Clay minerals/Organic matter
Soil pH does not decline as much as expected
Base
A substance which decreases the
Hydrogen ion concentration in solution
OHCO32SO42-
Bases react with hydrogen and
remove it from soil solution
OH- + H+
CO32- + H+
H 2O
HCO3-
(Neutralization of acid)
Common Bases
NaOH
CaCO3
water
water
OH- + H+
CO32- + H+
Na+ + OH-
Ca2+ + CO32H 2O
HCO3-
(Neutralization of acid)
A common base used to increase
the pH of soil is CaCO3
CaCO3
water
CO32- + H+
CaCO3 + H+
Ca2+ + CO32HCO3Ca2+ + HCO32-
Adding calcium carbonate to soils is called “liming”
Soil Buffering
Equilibrium between solution and colloids.
H+
Na+
K+
H+
Na+
H+
H+
Na+
Ca+2 H+
Ca+2
H+
H+
H+
pH = 6
Al+3
H+
Ca2+
Na+
H+
Na+ H+
H+ Ca2+ K+
H+
Al+3
H+
Ca2+
K+
Clay minerals/Organic matter
Soil Buffering
CaCO3
water
Ca2+ + CO32-
CO32- + H+
Ca+2
Ca+2
+
H
+
CO3-2 K+ H CO3-2
H+
H+
+2
Ca
-2
Ca+2
CO
3
Na+ Na+
-2
CO3
H+
+
H
+2
Ca
H+ CO3-2
K+
+
Ca+2 Na
H+
pH = 7
HCO3-
Al+3 Na+
H+
Ca2+
H+
Na+ H+
H+ Ca2+
K+ H+
H+
Al+3 Ca2+
Clay minerals/Organic matter
Soil solution pH initially rises due to base addition
Soil Buffering
CaCO3
+2
Ca
+
Ca+2 Na
H+
CO3-2 K+ H+ CO3-2
H+
H+
+2
Ca
-2
Ca+2
CO
3
Na+ Na+
-2
CO3
H+
+
+2
H
Ca
Ca+2 H+ CO -2
3
K+
H+
water
Ca2+ + CO32-
Al+3 Na+
H+
Ca2+
H+
Na+ H+
H+ Ca2+
K+ H+
H+
Al+3 Ca2+
Clay minerals/Organic matter
pH = 6.5
H+ removed from exchange sites returns to soil solution
Liming: raising soil pH
MgCO3 CaCO3
CaCO3
Ca2+ + CO32Displaces cations
From exchange sites
Combines with
Hydrogen ions
(neutralization)
CO32- + H+ = HCO3-
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
Kaolinite
1.
CEC
Smectite
Organic Matter
2.
% base saturation
pH and Nutrient Availability
Florida Soils Tend to be Acidic
H+
(In Rainfall)
H+
Ca
Mg
Al3+
Ca
H+ Na
Al3+
Na
H+
H+Mg
K
Na
Al3+
Ca
Mg
Na
H+
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 pH
Organism dependent.
Phosphorus: H2PO4- and HPO42Greatest 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+ = Fe3+ + 3H20
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