Analytical Techniques and Laboratory Practices in Agricultural Chemistry
This domain tests sampling protocol, the standard method used for each soil parameter, and the calculations behind reported values.
Core rules
- Sample with a zig-zag pattern from 0–15 cm, composite 10–15 cores, quarter to about 500 g, air-dry, sieve through 2 mm; use oven-dry (105 °C) weight as the reporting basis.
- pH in 1:2.5 soil:water; EC in 1:5 extract, reported in dS/m. EC 1:5 can be multiplied by about 5 to approximate a saturated paste extract.
- Organic carbon: Walkley–Black wet oxidation with potassium dichromate and back-titration by ferrous ammonium sulphate; the customary 1.3 correction covers incomplete oxidation. Organic matter = OC × 1.724.
- Available N: alkaline permanganate method (Subbiah and Asija). Available P: Olsen (0.5 M NaHCO3, pH 8.5) for neutral and calcareous soils; Bray-1 for acid soils. Available K: neutral ammonium acetate. Micronutrients: DTPA at pH 7.3.
- Instrumentation: flame photometer for K and Na, atomic absorption for metals, spectrophotometer for P, Kjeldahl for total N.
Worked example
Walkley–Black: 0.5 g soil, blank 10.0 mL and sample 6.6 mL of 0.5 N ferrous ammonium sulphate used. OC% = (10.0 − 6.6) × 0.5 × 0.003 × 100/0.5 = 1.02%, then × 1.3 = 1.33%. Organic matter = 1.33 × 1.724 = 2.3%. Note that the 0.003 g is the carbon equivalent of one milliequivalent.
Traps that cost marks
- Omitting the 1.3 correction, or applying it twice.
- Reporting on air-dry rather than oven-dry weight.
- Using Bray-1 on calcareous soils, where it over-extracts P, or Olsen on strongly acid soils.