Sodic patch, high-RSC water, and a gypsum dose that does not add up
Ramesh farms 2.0 ha of flat, poorly drained land. A composite 0–15 cm sample gives pH 9.4 (1:2.5 soil:water), EC 0.38 dS/m, ESP 28%, CEC 20 cmol/kg, and bulk density 1.5 g/cm3. Patches of white crust appear after every irrigation, water infiltration is less than 5 mm/h, and wheat yielded 1.6 t/ha against 4.2 t/ha in a neighbouring field. The tube-well water has EC 1.9 dS/m and RSC 4.2 meq/L. Last season he broadcast 2 t/ha gypsum four weeks before transplanting 40-day-old rice seedlings, applied 60 kg N, 30 kg P and 20 kg K/ha, and added no farmyard manure for three years. He now plans another 2 t/ha gypsum, more urea, and the same rice variety. His target is to bring ESP down to 10 in the plough layer and to grow wheat after rice without a yield collapse.
1. Classify the soil constraint and explain why extra urea will not close the wheat yield gap. Use the diagnostic values in your answer.
Read the worked answer
The soil is sodic, not saline. EC 0.38 dS/m is below the usual 4 dS/m salinity threshold, but pH 9.4, ESP 28% and a white crust point to sodium carbonate/bicarbonate dominance. The high pH disperses clay, so infiltration falls below 5 mm/h and roots meet poor aeration. Urea can acidify soil slightly during nitrification, but it supplies no calcium to displace exchangeable sodium and cannot restore structure. In fact, surface-applied urea hydrolyses to ammonium carbonate and can raise pH temporarily, increasing ammonia volatilisation; phosphorus and zinc also become less available at pH above 9. Adding urea would therefore increase leaf nitrogen concentration but not root exploration or water intake. The irrigation water makes the problem worse: RSC 4.2 meq/L adds carbonate/bicarbonate and sodium with every irrigation. The correct first intervention is a calcium source (gypsum), incorporated and followed by leaching and drainage, plus organic matter to stabilise aggregates. Without that, the field remains sodic and wheat cannot express its yield potential.
2. Calculate the gypsum requirement to lower ESP from 28% to 10% in the 0–15 cm layer. State assumptions, and comment on whether 2 t/ha is adequate.
Read the worked answer
Use the exchangeable-sodium excess in the plough layer.
- Desired fall in ESP = 28 − 10 = 18 percentage points = 0.18 as a fraction.
- Sodium to replace = 0.18 × CEC 20 cmol/kg = 3.6 cmol charge/kg soil.
- Soil mass per ha, 0–15 cm: 0.15 m × 10,000 m2 = 1,500 m3; at 1.5 t/m3 = 2,250 t/ha = 2.25 × 10^6 kg/ha.
- 3.6 cmol charge/kg = 0.036 mol charge/kg. One mole of CaSO4·2H2O supplies 2 mol charge, so Ca needed = 0.018 mol/kg.
- Gypsum mass = 0.018 × 172 g/mol = 3.10 g/kg. Over 2.25 × 10^6 kg/ha = 6.97 t/ha of pure gypsum.
- At 85% purity, commercial product ≈ 8.2 t/ha.
The farmer's 2 t/ha is only about 29% of the calculated requirement, so it cannot lower ESP from 28% to 10%. The calculation assumes uniform incorporation and adequate leaching; without drainage, even the full dose will not work. It also excludes the extra gypsum needed to neutralise the water's RSC. Split the dose rather than applying it all at once.
3. Draft a three-season reclamation and cropping plan. Include amendment sequencing, leaching, crop choice and nutrient management.
Read the worked answer
Season 1 (rice): Apply 4 t/ha commercial gypsum evenly, incorporate to 10–12 cm, and pond the field. Use a tolerant rice variety and transplant rather than direct-seed. Leach with the tube-well water only after the exchange reaction has had time to act, and provide a surface drain so salts leave the field. Add 10 t/ha well-rotted FYM or green manure. Monitor pH and ESP after harvest; do not expect the target in one season.
Season 2 (wheat): Apply the remaining 4 t/ha gypsum before sowing, incorporate it, and sow a moderately tolerant wheat variety after a pre-sowing irrigation and leaching. Add 25 kg/ha zinc sulphate and place phosphorus in the root zone. Avoid high-RSC water for germination; mix it with better water if possible.
Season 3: Re-test 0–15 cm. If ESP is below 15%, introduce a more sensitive crop; if not, repeat a smaller gypsum dose. Throughout, keep the field level, maintain drainage, add organic matter every year, and split nitrogen. Do not grow a sensitive crop immediately after the first gypsum application, and do not assume that rice alone reclaims the soil-rice is a tool, not a substitute for calcium and drainage.