ASRB NET: Soil Science and Agricultural Chemistry

ASRB NET Soil Science and Agricultural Chemistry tests soil fertility, soil physics, soil microbiology and environmental chemistry. Expect calculations involving exchange capacity, nutrient availability and pollutant behaviour.

Core concepts

• Cation exchange capacity (CEC) is the sum of exchangeable cations in cmol(c)/kg; base saturation = exchangeable bases divided by CEC × 100.
• Soil pH controls nutrient availability; liming raises pH, while gypsum ameliorates sodic soils without necessarily raising pH.
• Soil texture determines water retention and aeration; bulk density and porosity are inversely related.
• Microbiology drives ammonification, nitrification, denitrification and biological nitrogen fixation; nitrification is aerobic and acidifying.
• Environmental chemistry: heavy-metal mobility depends on pH, redox, organic matter and clay; total concentration is not equal to the bioavailable fraction.

Worked example

A soil has CEC 20 cmol(c)/kg and exchangeable Ca, Mg, K and Na summing to 16 cmol(c)/kg. Base saturation = 16 divided by 20 × 100 = 80%. Reasoning: the remaining 4 cmol(c)/kg is exchangeable Al and H. A base saturation of 80% indicates a moderately fertile soil, but nutrient balance matters because high Mg can induce K deficiency.

Traps that cost marks

• Mixing cmol(c)/kg with meq/100 g; numerically they are equivalent, but units must be stated correctly.
• Assuming total soil nitrogen is plant-available; most is organic and requires mineralisation.
• Treating pH as the only fertility indicator; salinity and sodicity are separate constraints.
• Confusing nitrification, NH4+ to NO3-, with denitrification, NO3- to N2 or N2O.