Sodic patch in a canal-command wheat field: is 25 t/ha of gypsum enough?
A farmer in Raisinghnagar, in the Indira Gandhi Canal command, holds 1.6 ha and sowed wheat on 15 November after a cotton harvest. The crop is patchy: plants in strips are 10–12 cm tall while bare patches carry a white crust. A 0–30 cm soil test reports pH 9.3, EC 1.6 dS/m, organic carbon 0.31%, available N 118 kg/ha, P 14 kg/ha, K 310 kg/ha, CEC 18 cmol(p+)/kg and ESP 28%. Infiltration is 0.4 cm/h; the last irrigation took five hours to cover a 0.2 ha plot and standing water persisted for a day. A neighbour's field on the same outlet, with pH 8.1, has a normal stand. The farmer has 500 kg single superphosphate and 200 kg urea in store, and can buy gypsum of 65% purity at ₹1,800 per tonne delivered. He plans to spread 25 t/ha of gypsum after the wheat harvest in April and to give heavy irrigations in June before sowing kharif.
1. Characterise the soil problem and test whether the planned 25 t/ha of 65% gypsum is enough to bring ESP down to about 10. Show the calculation.
Read the worked answer
pH 9.3 with ESP 28% and infiltration of 0.4 cm/h is a sodic (alkali) soil; EC 1.6 dS/m is well below the 4 dS/m salinity threshold, so salinity is not the main limit. The white crust is carbonate/bicarbonate of sodium, not chloride. Sodicity is patchy even on one outlet, which is why the neighbour at pH 8.1 has a normal stand.
Gypsum need to take ESP from 28 to 10:
- Exchangeable Na to replace = (28 − 10)/100 × 18 = 3.24 cmol/kg = 32.4 mol Ca per tonne of soil.
- Soil mass, 0–30 cm at bulk density 1.35 = 10,000 × 0.3 × 1.35 = 4,050 t/ha.
- Ca needed = 32.4 × 4,050 = 1,31,220 mol/ha × 172 g/mol ≈ 22.6 t pure gypsum/ha.
- At 65% purity: 22.6/0.65 ≈ 35 t/ha of field gypsum.
25 t of the material supplies only 16.25 t pure, about 72% of the need, and would bring ESP to roughly 15 - still above the 10–13 target. So the plan is short; buy 35 t, or apply 25 t now, retest after leaching and top up.
2. He asks whether the standing wheat can be saved with extra urea, and what to do with his SSP before the next crop. Advise, including a kharif crop choice.
Read the worked answer
Extra urea will not rescue this wheat. At ESP 28 the root system is already restricted and water uptake is poor, so nitrogen is not the limiting factor; the field will stay patchy whatever is applied now. A 0.2% zinc sulphate foliar spray (25 kg/ha ZnSO4 in soil for the next crop) is worth more, because zinc is fixed at high pH and the 0.31% organic carbon points to widespread micro-nutrient shortage.
Fertiliser logic for the next crop: gypsum must be incorporated 8–10 cm deep first, because calcium has to reach the exchange complex; then leach. SSP is a good choice on sodic soil because it supplies P, S and calcium sulphate - place 250 kg/ha about 5 cm below the seed rather than broadcasting. Urea is acidifying and acceptable, but split it; avoid nitrate fertilisers and sodium-containing products.
For kharif, grow dhaincha and incorporate it at 45 days, or a tolerant crop such as rice, cotton or barley. Keep chickpea, lentil, berseem and peas off the affected patches, and treat mustard as only moderately tolerant.
3. He intends to give 'heavy irrigations' in June to push the sodium down. What could go wrong, and how should leaching be done?
Read the worked answer
Leaching works only if the displaced sodium can leave the root zone. Before June, check the depth to the water table and whether a clay pan or impermeable layer exists. Where the water table is within about 1.5–2 m, a heavy irrigation will waterlog the field, raise the water table and bring the salt back by capillary rise - gypsum alone will not fix that land, which needs subsurface drainage or raised beds.
Where drainage is adequate, leach in 3–4 doses of 10–12 cm each with a few days between them, so sodium moves downward progressively. One heavy flooding is less efficient - much is lost to evaporation and it can seal the surface. Level the field, apply 10 t/ha FYM, incorporate the gypsum thoroughly with a disc harrow, and mulch the surface to slow crusting. Retest pH and ESP after the monsoon and add the balance of the gypsum if ESP is still above 13.