soil buffering and management of acid soils. ph ph = - log (h + ) if (h + ) = 1 x 10 -3 mol/l (h + )...

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