Soil Chemistry and Soil Reaction
Soil colloids and cation exchange
Clay minerals and humus carry negative charges on their surfaces and hold cations (Ca2+, Mg2+, K+, Na+, NH4+, H+, Al3+) in exchangeable form. This is cation exchange capacity (CEC), expressed in cmol(+)/kg (numerically equal to meq/100 g). Kaolinite (1:1) has low CEC; montmorillonite (2:1) and humus have high CEC. Base saturation = (sum of exchangeable bases ÷ CEC) × 100.
Soil reaction
Soil pH measures active acidity in the soil solution. Exchangeable acidity comes from H+ and Al3+ held on colloids; residual acidity is not exchangeable. Classes: below 5.5 strongly acidic, 5.5–6.5 moderately acidic, 6.5–7.5 neutral, 7.5–8.5 alkaline, above 8.5 usually sodic. Buffering is the resistance of soil to pH change, greatest in soils with high clay and organic matter. Acid soils are limed with CaCO3; sodic soils (ESP above 15, pH above 8.5) are reclaimed with gypsum, which replaces Na+ with Ca2+.
Worked example
A soil has CEC 24 cmol(+)/kg, exchangeable Ca 12, Mg 4, K 1.5 and Na 1.5 cmol(+)/kg. Sum of bases = 19; base saturation = 19/24 × 100 = 79%. ESP = 1.5/24 × 100 = 6.3%, well below 15%, so the soil is not sodic and gypsum is not justified. Writing each step prevents confusing base saturation with ESP.
Traps
Confusing active and exchangeable acidity; assuming pH above 7 always means fertile soil (sodic soils are alkaline but infertile); applying gypsum to acid soils; using lime and gypsum interchangeably; forgetting that nutrient availability peaks near pH 6.5, that Fe, Mn and Zn become unavailable at high pH, and that Mo becomes unavailable at low pH.