Reclaiming a sodic patch before wheat
Sunita has 1.6 ha in Rohtak district with pH 9.1, EC 0.38 dS/m, organic carbon 0.29%, CEC 18 cmol/kg, ESP 26% and available zinc 0.35 ppm; the soil-test laboratory has recommended 8 t/ha gypsum. She grows rice-wheat. Last year rice yielded 4.0 t/ha and wheat only 2.2 t/ha, with patches where seedlings yellowed and died. Her canal turn gives good water (EC 0.3 dS/m) but only once in 10 days; the tube well water has EC 2.4 dS/m and RSC 4.5 meq/L. She has ₹40,000 for the whole season and a neighbour's tractor. Gypsum costs ₹2,200/t including transport, FYM ₹1,500/trolley, and ZnSO4 ₹55/kg. She wants to apply the entire 8 t/ha of gypsum to the wheat crop in December, skip FYM and zinc because 'the pH is already high', and use only tube well water.
1. Is her plan to put all 8 t/ha of gypsum on wheat sound? Recommend rate, timing, method and water source for leaching, with reasons.
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The plan is unsound in timing and in water choice. Gypsum works only when it dissolves and the displaced sodium is leached out; her high-RSC tube well water adds sodium carbonate and will re-sodify the soil.
Rate and timing. Apply the recommended 8 t/ha in two splits: 4 t/ha before rice, incorporated in the puddled field, and 4 t/ha before wheat; where drainage is assured, the whole amount before rice is also acceptable. Do not dump it all on wheat in December.
Method. Spread evenly, incorporate into the top 10-15 cm, and apply 2-3 weeks before sowing. Do not mix gypsum with DAP, because calcium reacts with phosphate and reduces phosphorus availability.
Water. Leach with canal water, which has low RSC, and check that surplus water has an outlet. Two to three heavy irrigations after gypsum application are needed. If canal turns fail, delay reclamation rather than leaching with the tube well.
2. Plan the crop sequence and the zinc and fertilizer strategy for the next two years, and show why her zinc reasoning is wrong.
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Sequence. Rice is the best first crop: it tolerates sodicity, and ponded water with gypsum leaches sodium. Follow with wheat once ESP falls; if pH stays above 9, barley is a more tolerant rabi choice than wheat. Retest the soil every two years and stop gypsum when pH falls below about 8.5 and ESP below 15.
Zinc. Her reasoning is reversed. High pH reduces zinc availability, and 0.35 ppm is below the critical level (about 0.6 ppm), so deficiency is expected, especially in rice and wheat. Apply ZnSO4·7H2O at 25 kg/ha at sowing, not mixed with DAP, and spray 0.5% zinc sulphate if symptoms appear.
Fertilizer. Add FYM at 10 t/ha to improve structure, apply nitrogen in three splits, and place phosphorus near the root zone. On sodic soil, ammonium sulphate is preferable to urea where cost allows, because it acidifies the root zone.
3. She has only ₹40,000. Rank her spending if she cannot buy everything, and state the condition under which gypsum should wait.
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Ranking by yield response per rupee and by irreversibility:
- Gypsum, 8 t/ha, about ₹17,600 - without it nothing else works.
- Zinc sulphate, 40 kg, about ₹2,200 - cheap insurance against a hidden loss.
- FYM, 4 trolleys, about ₹6,000 - improves structure and water movement.
- Seed, fertilizer and the rest.
The condition. Gypsum without assured leaching water is money wasted and can even raise salinity in the root zone. If the canal turn cannot be guaranteed, postpone gypsum, spend on FYM and a tolerant crop, and reclaim in a year when water is assured. Reclamation is a two- to three-year investment, not a one-season purchase.