The White Crust in Block Kharhar
Farmers in Block Kharhar report a white crust on the soil surface after the June irrigation, patchy germination in paddy nurseries, and standing water that does not infiltrate for two days. A composite soil test from 60 holdings returns pH 8.9, electrical conductivity 0.90 dS/m, exchangeable sodium percentage 28, cation exchange capacity 12 cmol(p+)/kg, and organic carbon 0.31 per cent. Wheat yield has fallen from 4.2 to 2.6 tonnes per hectare over three seasons.
Two suppliers visit in the same week. Supplier A sells gypsum at ₹2,400 per tonne and recommends 1.5 tonnes per acre, followed by green manuring with dhaincha and a flushing irrigation. Supplier B sells agricultural lime at ₹1,800 per tonne and recommends 2 tonnes per acre together with DAP. A farmer with two acres wants a decision before the nursery window closes. The block agriculture officer must also justify the choice to a district committee that has already procured 40 tonnes of urea for the block.
1. Diagnose the problem and decide between the two suppliers.
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The three symptoms together - surface crust, dispersion that blocks infiltration, and stunted germination - point to sodicity, not acidity. The numbers confirm it: pH 8.9 is strongly alkaline, ESP 28 is far above the 15 per cent sodicity threshold, EC 0.90 dS/m shows salinity is low, and organic carbon 0.31 per cent shows the soil is degraded. The problem is sodium on the exchange complex, so the remedy must supply calcium to displace sodium and be leached away. Gypsum (calcium sulphate) does exactly that, and because EC is low there is no risk of pushing salinity up. Lime is calcium carbonate: it neutralises acidity, so on a soil already at pH 8.9 it does nothing for sodium and can raise pH further. Supplier B is wrong; Supplier A's product is correct and the dhaincha and flushing steps are agronomically sensible.
2. Estimate the gypsum requirement and check Supplier A's rate for the two-acre holding.
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Use GR (t/ha) = (ESP initial − ESP target) ÷ 100 × CEC × 1.72, with a reclamation target of ESP 10.
- (28 − 10) ÷ 100 = 0.18
- 0.18 × 12 = 2.16
- 2.16 × 1.72 = 3.72 t/ha
Converting, 3.72 ÷ 2.471 ≈ 1.5 t per acre, so for two acres the farmer needs about 3 tonnes, costing roughly ₹7,200 at ₹2,400 per tonne. Supplier A's recommendation of 1.5 t/acre therefore matches the laboratory estimate almost exactly. Two cautions belong in the note: the formula gives a first approximation, so a re-test after one season should guide the second dose; and gypsum must be spread evenly, incorporated into the root zone and followed by irrigation with good-quality water, otherwise the displaced sodium simply stays in the plough layer.
3. Explain why paddy is a poor choice this season and why the 40 tonnes of urea will not solve the problem.
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Paddy is grown under puddled, flooded conditions. Puddling breaks aggregates and, in a sodic soil where clay is already dispersed, produces a dense pan that worsens the infiltration problem and traps sodium. A more rational sequence is: level the field, apply and incorporate gypsum, flush with canal or tubewell water of good quality, establish dhaincha as a green manure to rebuild organic carbon, and only then take a tolerant crop such as barley or a tolerant wheat variety; re-test before returning to paddy. Urea is not irrelevant, but it does not address sodium. Worse, surface-applied urea on a pH 8.9 soil loses nitrogen as ammonia by volatilisation, so money spent on 40 tonnes of urea without reclamation gives a poor return. The correct advice is to keep urea for after reclamation and incorporate it, not to treat it as a substitute for gypsum.