Agricultural Chemistry Practical and Laboratory Techniques
Sampling and processing
Collect soil from 0–15 cm depth in cultivated fields at 10–15 random spots in a zigzag pattern, mix, quarter to about 500 g, air-dry in shade, grind and sieve through 2 mm; use 0.2 mm (60-mesh) for organic carbon. The sample must represent the field, so avoid bunds, marshy spots and recently fertilized patches.
Standard determinations
- Moisture: oven-dry at 105 °C to constant weight; moisture % = (loss ÷ oven-dry weight) × 100.
- pH: soil:water 1:2.5 with a glass electrode, calibrated against pH 4.0, 7.0 and 9.2 buffers.
- Electrical conductivity: same suspension, reported in dS/m, an index of salinity.
- Organic carbon: Walkley–Black wet oxidation with K2Cr2O7 and H2SO4, back-titrated with ferrous sulphate; OC% = (B − T) × N × 0.003 × 100 ÷ weight of soil, where B and T are blank and sample titres.
- Available N: alkaline permanganate method; available P: Olsen (0.5 M NaHCO3, pH 8.5) for neutral and alkaline soils, Bray-1 for acid soils; available K: 1 N ammonium acetate with flame photometer.
Worked example
One gram of soil gives a blank of 12.5 mL and a sample titre of 9.8 mL of 0.5 N ferrous sulphate. Difference = 2.7 mL. OC% = 2.7 × 0.5 × 0.003 × 100 ÷ 1 = 0.405%. This is below 0.5% and is rated low, so organic manure is recommended. Reasoning: subtract the blank first, include the 0.003 g carbon equivalent of 1 mL 1 N dichromate, and divide by the exact soil weight.
Traps
Forgetting the blank; using fresh instead of air-dry weight; using Bray-1 on calcareous soils; reporting organic carbon as organic matter (organic matter = OC × 1.724).