Sodic patch after paddy transplanting: how much gypsum can ₹18,000 actually buy?
Ramesh cultivates 1.6 ha in Nilokheri block, Karnal. His soil test reports pH 9.6, EC 1.1 dS/m, organic carbon 0.31%, CEC 18 cmol(+)/kg and ESP 34. The surface crusts and sets hard, rainwater stands for hours, and the paddy he transplanted three weeks ago is yellow, stunted and patchy, with no earthworm casts to be seen. His own tube well is the only source he can use freely and it tests at RSC 4.2 meq/L. A neighbour has offered loose, coarsely ground gypsum at ₹1,800 per tonne. Ramesh has ₹18,000 set aside for amendment, about 20 tonnes of well-rotted FYM, and roughly one month before kharif transplanting begins. He is inclined to spread extra urea and keep the field flooded, arguing that "paddy will loosen the soil anyway". His rotation is paddy in kharif followed by wheat in rabi.
1. Estimate a gypsum dose for the 0–15 cm layer, test it against his ₹18,000 budget, and state what he should do if the full dose is unaffordable.
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Step 1 - exchangeable sodium to be replaced. Exchangeable Na = ESP × CEC ÷ 100 = 34 × 18 ÷ 100 = 6.12 cmol(+)/kg.
Step 2 - how much must come off. Rice will tolerate roughly ESP 10, so the reduction needed is 34 − 10 = 24 ESP points, i.e. 24 × 18 ÷ 100 = 4.32 cmol(+)/kg.
Step 3 - convert to gypsum. A standard field thumb rule is that 1 cmol(+)/kg of exchangeable Na in the top 15 cm needs about 1.9 t/ha of pure gypsum. So 4.32 × 1.9 ≈ 8.2 t/ha, and for 1.6 ha about 13 t.
Step 4 - budget check. 13 t × ₹1,800 = ₹23,400, well above ₹18,000. He can afford about 8 t.
What to do instead of a token dose everywhere: spreading 5 t/ha over the whole farm usually fails, because the dose is too small to displace Na and the field stays sodic. Better to treat about 1 ha fully this season (about 8 t), put that hectare under paddy, and leave the remaining 0.6 ha for FYM plus a dhaincha green manure crop, reclaiming it next year. He should also ask the cooperative/HSAMB about subsidised gypsum, and get a sieve test - at least 60% should pass a 60-mesh sieve - because coarse, half-ground gypsum reacts slowly and the 1.9 t figure assumes a reactive, finely ground material with bulk density near 1.5. A laboratory gypsum requirement test on his own soil should override the thumb rule.
2. Explain why urea and flooding alone will not reclaim this soil, and set out the correct order of operations from amendment to the first paddy crop.
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Why urea and flooding fail. The problem is not nitrogen, it is sodium held on the clay exchange complex. High ESP makes clay disperse, so pores seal, infiltration stops and the surface crusts; adding urea feeds the crop but leaves Na in place. Flooding alone simply keeps the dispersed clay saturated and can raise the water table, concentrating salts at the surface as it dries. Reclamation needs a calcium source to displace sodium, followed by water to leach the displaced sodium below the root zone, plus organic matter to rebuild structure.
Order of operations:
- Get a proper soil test (pH, EC, ESP, gypsum requirement) rather than working from pH alone.
- Level the field - laser levelling if possible - so leaching water is not wasted in high spots.
- Spread the full gypsum dose evenly and incorporate to about 10 cm; a surface scatter without mixing is much less effective.
- Pond with the best-quality water available and give two or three leaching irrigations, keeping field drains open so sodium leaves the field rather than moving sideways.
- Apply FYM (10 t/ha is realistic) and incorporate.
- Grow rice as the first crop - it tolerates standing water and speeds reclamation - then wheat after another leaching.
- Re-test pH and ESP after two seasons; do not expect one season to finish the job, and do not put a sensitive crop such as mustard or berseem on the untreated patch.
3. His tube well water has RSC 4.2 meq/L. Should it be used for the leaching irrigations? What water and crop management do you advise for the first two seasons?
Read the worked answer
No - not as the main leaching water. Residual sodium carbonate of 4.2 meq/L is high; applied in quantity it adds carbonate/bicarbonate that precipitates calcium and pushes sodium back onto the exchange complex, so it partly undoes the gypsum. Leaching with this water is close to recycling the problem.
Practical advice:
- Use canal water for the leaching irrigations wherever a turn is available, even if it means waiting for the warabandi and storing water in a small tank.
- If tube well water must be used, blend it with canal water, and add gypsum to the water or increase the soil gypsum dose to counteract the RSC; never use it as the sole source for ponding.
- Keep a drainage outlet; leaching without drainage only moves salts around the field.
- Allocate roughly 20–25% more water than a normal paddy season for the leaching programme.
Crop choice: rice in kharif is the right reclaiming crop - choose a variety suited to the area and tolerant of the conditions rather than a highly sensitive one; follow with wheat, which is moderately tolerant. Keep mustard, berseem and vegetables off the untreated portion until ESP falls.
Longer term: get the tube well water tested each year, keep adding organic matter, avoid irrigating wheat heavily with high-RSC water in later seasons, and re-test the soil before deciding the third-season crop.