Gypsum on a sodic patch: how much and where?
A 1.0 ha wheat field in a canal command has pH 9.2 (1:2.5 soil:water), EC 0.8 dS/m, ESP 24% and CEC 16 cmol/kg. Infiltration is only 4 mm/h and a 0.35 ha patch shows yellowing, crusting and poor tillering; the rest of the field is better but still patchy. The wheat is 35 days old. The farmer has 2.5 t of agricultural gypsum (about 70% purity) and wants to broadcast it over the whole hectare three days before the next irrigation, then top-dress urea. He also plans to mix the urea with the gypsum to save an operation. His tube-well water has EC 1.1 dS/m and SAR 8. He asks whether the field is saline or sodic, whether 2.5 t is enough, and whether the urea plan is safe.
1. Classify the soil problem using the pH, EC, ESP and infiltration data. What does the EC value rule out or confirm?
Read the worked answer
The pH of 9.2 and ESP of 24% place the soil in the sodic class: ESP above 15% is the key exchangeable-sodium criterion, and pH above 8.5 fits the same diagnosis. The EC of 0.8 dS/m is below 4 dS/m, so this is not a saline soil and not a saline-sodic soil. That matters because the management is not primarily drainage of excess salt; it is replacement of exchangeable sodium by calcium and then leaching of the displaced sodium. The very low infiltration of 4 mm/h supports the diagnosis: dispersed clay and sodium weaken aggregates and seal the surface. The patchy yellowing and crusting are consistent with poor aeration, poor root growth and sodium effects rather than a simple nitrogen shortage. Because the field is sodic but not saline, gypsum is the appropriate amendment, provided it is incorporated and followed by leaching with good-quality water. Broadcasting gypsum and expecting it to act from the surface would be a weak response.
2. Using the gypsum requirement formula GR = (ESP_i − ESP_t)/100 × CEC × 1.72, estimate the requirement for the 0.35 ha patch (target ESP 10) and adjust for 70% purity. How should the 2.5 t be allocated?
Read the worked answer
For the patch, the exchangeable-sodium reduction needed is 24 − 10 = 14 percentage points. GR = 14/100 × 16 × 1.72 = 3.85 t/ha of pure gypsum. For 0.35 ha this is 3.85 × 0.35 = 1.35 t of pure gypsum. At 70% purity, the product needed is 1.35/0.70 = 1.93 t. The farmer has 2.5 t, so the correct judgement is to treat the 0.35 ha patch properly with about 1.9 t, incorporate it into the top 10–15 cm, and leach with the best water available. Spreading 2.5 t over the whole hectare would give only 2.5 t/ha of product, or about 1.75 t/ha of pure gypsum, which is less than half the calculated requirement and likely to give disappointing reclamation. The remaining 0.57 t is too little for another 0.35 ha at the same rate; it should be held for spot treatment or used with organic matter on the next worst strip, not spread thinly over the entire field.
3. Critique the plan to mix urea with gypsum and top-dress before irrigation. What immediate and next-season steps would you recommend?
Read the worked answer
The main problem is not chemical incompatibility between urea and gypsum; it is the surface application of urea on a soil of pH 9.2. Urea hydrolyses to ammonium carbonate, and on an alkaline, calcareous surface much of the ammonium can be lost as ammonia before it is incorporated or moved into the soil. Mixing with gypsum does not remove that risk. Gypsum also needs to be incorporated, not merely broadcast, and it needs leaching water to remove displaced sodium. A better sequence is to spread and incorporate gypsum in the patch, irrigate carefully to leach sodium, and apply urea when the soil is moist or band it and incorporate it. Split the nitrogen rather than applying it all at once. For the next season, add well-decomposed FYM or compost, maintain the gypsum-treated patch, avoid irrigating with the saline tube-well water alone, and choose a crop and variety tolerant of sodicity. Do not expect full reclamation in one wheat season; monitor pH and ESP after the monsoon.