Lecture 14 Soil Buffering and Management of Acid Soils

Lecture 14 Soil Buffering and Management of Acid Soils -...

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Unformatted text preview: Soil Buffering and Management of Acid Soils Management pH pH = - log (H+) pH = - log (H+) If (H ) = 1 x 10 mol/L If (H ) = 1 x 10 mol/L (H+) = 0.001 mol/L (H+) = 0.00001 mol/L pH = - log (1 x 10 ) pH = - log (1 x 10 ) pH = - (-3) pH = - (-5) + -3 pH = 3 -3 + -5 -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) + OH- H+ Base(=10) 2 Acid (=10) H+ OH- OHOH- OHOH-OH- H+ H+ H H+ H+ H+ H+ H+ + H + OH- OHOHOH- Acidic 0 HO basic 7 14 + OH- H+ Base 2(=6) Acid (=10) H+ H+ H+ H OH- OHOH- OHOH-OH- + H+ H+ HH HH + + + 0 HO + Acidic basic 7 14 + OH- H+ Base(=15) 2 Acid (=10) H+ OH- OHOH OH- OHOH OH OH-OHOH OH OHOHOHOH- H+ H+ H H+ H+ H+ H+ H+ + H + Acidic 0 HO 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 A l Al Al Al Al Al Al Al Si Types of Acidity Types Active Acidity H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Exchangeable Al+3 Na+ H+ Ca2+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ 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 Percent (charge basis) Acid Cations (cmol/kg) Cation exchange capacity (cmol/kg) Na+ Al+3 Na+ H+ Acid charge = 14 Exch. Cap. = 26 % A.S. = 53.8% Ca2+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ H+ Ca2+ Ca2+ Clay minerals/Organic matter Acid Cations: Al, H+ H+ Percent Base Saturation Percent (charge basis) Exchangeable bases (cmol/kg) Cation exchange capacity (cmol/kg) Na+ Al+3 Na+ H+ Base charge = 12 Exch. Cap. = 26 % B.S. = 46.2% Ca2+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ H+ Ca2+ Ca2+ 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 H+ H+ H+ Ca H+ +2 Na H+ H+ H+ + + K + +2 pH = 6 + Al+3 H+ Ca2+ Na+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ Ca2+ Clay minerals/Organic matter Soil Buffering Add acid: HCl => H+ + ClNa+ H H + Ca+2 H+ H+ H+ H K+ H+ H+ H+ H++ Na+ Na+ H+ + H+ H+ Ca+2 H+ H K+ H + + pH = 4 Al+3 H+ Ca2+ Na+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ Ca2+ Clay minerals/Organic matter 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+ K+ pH = 5.5 Al+3 Na+ H H H HH + H + H+ H + + + Al+3 + H H H+ H+ + Ca2+ + + 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+ H2O HCO3- (Neutralization of acid) Common Bases NaOH CaCO3 water water O H- + H + CO32- + H+ Na+ + OHCa2+ + CO32H 2O HCO3- (Neutralization of acid) A common base used to increase the pH of soil is CaCO3 CaCO3 water CO32- + H+ CaCO + H Ca2+ + CO32HCO3Ca + HCO Adding calcium carbonate to soils is called “liming” 3 + 2+ 32- Soil Buffering Equilibrium between solution and colloids. H Na+ K+ Ca+2 H H + H + H+ Na+ H+ H+ Ca2+ Na+ H+ K+ H+ + Na+ Na+ + Ca+2 H Al+3 H + H H Ca2+ H+ Al+3 Ca2+ K ++ + + pH = 6 Clay minerals/Organic matter Soil Buffering water CaCO3 Ca2+ + CO32- CO32- + H+ Ca+2 CO3-2 Ca+2 CO3-2 Ca+2 H H CO3-2 + H+ H Ca+2 CO3-2 + H+ + H H CO3-2 H + pH+ = 7 HCO3- Al+3 H+ Ca2+ Na+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ Ca2+ + Clay minerals/Organic matter Soil solution pH initially rises due to base addition Soil Buffering CaCO3 Ca+2 CO3-2 Ca+2 CO3-2 Ca+2 H H H CO3-2 + H+ +2 Ca CO3-2 + H + H Ca+2 H CO3-2 + + water Ca2+ + CO32- Al+3 H+ Ca2+ Na+ H+ H+ Ca2+ H+ Al+3 Na+ H+ K+ H+ 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 Buffering 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 Na + Al3+ H Na Ca + Na H Al3+ Mg + H K Mg Al3+ Ca + 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 ...
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