Soil Chemistry and Soil Fertility
This domain tests whether you can link colloidal soil chemistry - exchange complexes, soil reaction, buffering, salinity and sodicity - to practical fertility diagnosis and amendment decisions.
Core rules
- CEC is reported in cmol(+)/kg, which is numerically identical to meq/100 g. Do not convert between them.
- CEC is contributed by humus (roughly 150–300 cmol/kg) and clay; 2:1 clays (montmorillonite > illite) far exceed 1:1 kaolinite (often 3–15 cmol/kg).
- Base saturation (%) = (Ca + Mg + K + Na)/CEC × 100. ESP (%) = exchangeable Na/CEC × 100; the denominator is always CEC, never the base sum.
- Soil pH is measured in 1:2.5 soil:water; below 5.5 expect Al/Mn toxicity and P fixation, 6.5–7.5 is the agronomic optimum, and pH above 8.5 suggests sodicity.
- pH in 0.01 M CaCl2 reads about 0.5–1.0 unit lower than in water, so always state the method used.
- Positive-charge sites on kaolinite and Fe/Al oxides give anion exchange, which governs sulphate and phosphate retention in acid soils.
Worked example
A soil has CEC 20 cmol/kg with exchangeable Ca 10, Mg 4, K 1 and Na 1 cmol/kg. Base sum = 16, so base saturation = 16/20 × 100 = 80%. ESP = 1/20 × 100 = 5%, well below the 15% sodicity threshold, so gypsum is not indicated even though sodium is present. If the same soil read pH 8.6, suspect free CaCO3 buffering the pH near 8.2 rather than a sodium problem.
Traps that cost marks
- Computing ESP against the base sum instead of CEC.
- Labelling a calcareous soil sodic on the basis of pH alone.
- Assuming a high CEC always means high fertility; Al-saturated acid clays have high CEC and poor fertility.