Reclaiming a Sodic Block Before the Wheat Season
A 2.5 ha block in Karnal yields 2.1 t/ha of wheat, while the adjacent field yields 4.8 t/ha. The surface crusts and ponds after rain, and infiltration is below 2 mm/h. A 1:2 soil:water test gives pH 8.9, EC 1.1 dS/m, organic carbon 0.31%, CEC 16 cmol(c)/kg and ESP 26%. The 0–15 cm furrow slice weighs about 2.25 × 10^6 kg/ha. Canal water is available at 6 cm per 12-day turn, with EC 0.4 dS/m. The farmer has 20 t of old FYM, can buy gypsum at Rs 4,500/t, and plans to puddle the field for rice in the coming kharif. He proposes to apply only 2 t/ha of gypsum, spread over the whole block, and asks whether that is enough, whether rice is the right reclaiming crop, and whether the FYM should go before or after the gypsum.
1. Estimate the gypsum needed to lower ESP to about 10% in the 0–15 cm layer. Is the farmer's 2 t/ha adequate? Show the calculation and state the assumptions behind your figure.
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Calculation
Exchangeable Na to be replaced = CEC × change in ESP = 16 cmol/kg × (0.26 − 0.10) = 2.56 cmol/kg.
Soil mass in the 0–15 cm layer = 2.25 × 10^6 kg/ha, so charge to replace = 2.56 × 2.25 × 10^6 = 5.76 × 10^6 cmol = 57.6 kmol of charge per ha.
Gypsum (CaSO4·2H2O) has a molecular mass of about 172 kg/kmol and supplies 2 mol of charge per mol, so required pure gypsum = 57.6/2 × 172 = 4.95 t/ha.
Adjustments
- Field-grade gypsum is rarely 100% pure; at 85% purity the figure becomes about 5.8 t/ha.
- The calculation assumes uniform incorporation to 15 cm and that the laboratory ESP is representative. A laboratory gypsum-requirement test should be used to confirm the field dose.
Verdict
2 t/ha is only about 35–40% of the estimated requirement. Spread thinly over 2.5 ha, it is unlikely to lower ESP enough to improve infiltration; the farmer would spend money without solving the problem. It is better to treat a smaller area at the full dose than to treat everything at a sub-threshold dose.
2. Sequence the reclamation operations for the next 12 months. Decide whether rice should be grown, and place the FYM and gypsum correctly. Justify each decision.
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Sequence
- Apply gypsum first, before the monsoon or before the kharif canal turns. Spread 5–6 t/ha uniformly on the worst patches and incorporate to about 10 cm. Do not leave it on the surface, where it will crust.
- Apply well-rotted FYM after gypsum incorporation. Old FYM improves aggregate stability and infiltration and supplies Ca and organic matter. It should not be used as a substitute for gypsum at this ESP.
- Leach with good-quality water. Canal water at EC 0.4 dS/m is suitable, but 6 cm per 12 days is only about 60 cm over a 120-day kharif; that is not enough for a full rice crop, which commonly needs 1,000–1,200 mm. Rain leaching should be used as much as possible.
- Do not puddle for rice unless supplemental tube-well water is assured. Rice tolerates standing water and helps reclamation, but puddling destroys the structure that gypsum and FYM are trying to build. If water is short, grow dhaincha (Sesbania) as a green-manure crop, incorporate it before wheat, and keep the field covered.
- Grow wheat with a balanced fertilizer dose after the kharif reclamation. Avoid heavy nitrogen in one dose; split it and monitor infiltration.
Justification
The binding constraint is water, not the choice of rice. A rice crop that fails or is puddled with insufficient water can leave the field worse. Dhaincha plus gypsum plus rain leaching is the safer low-water route, with rice only if the farmer can guarantee at least 100–120 cm of irrigation.
3. The farmer can spend only Rs 25,000 on amendment this year. Give a phased plan and the economic reasoning, including what you would do on the untreated area.
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Phased plan
- Year 1: Treat the worst 1 ha at 5.5–6 t/ha. At Rs 4,500/t, 5.5 t costs Rs 24,750, which is within the budget. Add the available FYM to this block and leach it.
- Year 2: Treat the next 1 ha from the next season's budget, using the same dose. Use dhaincha or a tolerant fodder crop on it in the meantime.
- Year 3: Treat the remaining 0.5 ha, or continue with FYM and green manure if the ESP has fallen.
Economic reasoning
A wheat yield gain of 1.2–1.5 t/ha is plausible on a reclaimed sodic patch. At about Rs 22,750/t, that is Rs 27,300–34,125 per ha, against an amendment cost near Rs 25,000/ha, before the cost of leaching water. The first-season benefit-cost ratio is therefore modest, but gypsum has a residual effect over several seasons, so the investment is still justified on the worst land.
Spreading 2 t/ha over 2.5 ha would cost about Rs 22,500 but would not cross the threshold needed to lower ESP and improve infiltration. Thin spreading is the least economic option because the farmer pays most of the cost without getting a yield response. On untreated areas, grow a tolerant crop, keep the soil covered, and avoid puddling; do not apply more nitrogen to compensate for sodicity.