White crust, patchy wheat and a ₹15,000 reclamation budget
Ramesh farms 1.2 hectares in Kanpur Dehat in a rice–wheat rotation. His last wheat crop (sown 20 November at 120 kg/ha) germinated very unevenly: bare patches with a white powdery crust appeared where the soil dried after the first irrigation, and roughly 30 per cent of the seedlings died within three weeks. He blames bad seed and wants to buy a new variety. The block soil-testing laboratory reports pH 9.3, electrical conductivity 0.31 dS/m, organic carbon 0.27 per cent, available nitrogen 130 kg/ha, available phosphorus 19 kg/ha and available potassium 280 kg/ha. The field is levelled, canal water is available for kharif, and the tube well water has a residual sodium carbonate of 3.4 meq/L. Last year he applied 130 kg N/ha as urea in three splits, 60 kg P2O5, 40 kg K2O and 2 tonnes of farmyard manure. He has ₹15,000 set aside for soil correction this season and wants to know whether to spend it on gypsum, extra urea, or a different wheat variety.
1. What is the most probable cause of the patchy germination and the white crust? Use the test values to argue for one diagnosis and against the alternatives, and explain why extra urea will not help.
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Diagnosis: sodic (alkaline) soil, not salinity and not bad seed.
- pH 9.3 is strongly alkaline, and the low EC of 0.31 dS/m is far below the 4 dS/m used to classify a saline soil. So the problem is exchangeable sodium, not soluble salt.
- Sodium disperses clay particles, so the surface seals and crusts on drying; seedlings cannot emerge through the hard crust, and the emerging radicle is injured. The white deposit is carbonate/bicarbonate salt left behind as water evaporates.
- Organic carbon at 0.27 per cent is very low, so structure is already weak and the field crusts easily. Phosphorus (19 kg/ha) is medium and potassium (280 kg/ha) is high, so fertility is not the limiting factor.
Why urea will not fix it: at pH 9.3, surface-broadcast urea hydrolyses to ammonia and much is lost to the air before it can be used; the high pH also locks up zinc, iron and manganese, so plants yellow even when nitrogen is applied. Extra urea therefore costs money without removing sodium.
Alternatives to rule out quickly: bad seed would give gaps along drill rows, not patches that follow the crusted spots; dig up a few failed plants to check for termites or root rot. Salinity is ruled out by the low EC. Waterlogging is unlikely on a levelled field with canal water only in kharif. Ask the laboratory for exchangeable sodium percentage and a gypsum requirement value to size the amendment.
2. Where should the ₹15,000 go? Give a concrete plan with quantities, method and the reason for each item.
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Almost all of it goes to gypsum plus organic matter; none to a new variety this year.
- Gypsum (the main item). Take the laboratory's gypsum requirement (GR). As an illustration, at 4 t/ha for 1.2 ha he needs about 4.8 t; at roughly ₹2,000/t delivered this is about ₹9,600, plus ₹1,500 for spreading and incorporation. Gypsum supplies calcium, which displaces sodium from the clay; the sodium then has to be leached away with water.
- Method matters more than dose. Broadcast half the gypsum before kharif, incorporate it into the top 10 cm, then pond and leach with good water. Apply the second half before rabi. There must be a drainage outlet, otherwise the sodium simply stays in the field.
- Use good water for leaching. His tube well water has RSC 3.4 meq/L, which is itself sodic; using it to leach would add more sodium. Use canal water where possible, or blend the two, or add gypsum to the tube well water.
- Organic matter. Spend about ₹2,000–3,000 on carting and spreading farmyard manure (5 t/ha) or on dhaincha seed for green manuring before rice; organic matter rebuilds aggregates and keeps the surface from sealing.
- Zinc sulphate only if the soil test confirms deficiency (about 25 kg/ha, roughly ₹1,500 for this field).
Buying a new variety or more urea would spend money on symptoms, not the cause. Re-test after two or three seasons, because reclamation is gradual.
3. Which kharif and rabi crops should he grow now, and why? What single irrigation practice would you change immediately?
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Kharif: transplanted rice, followed by green manuring. Rice tolerates pH up to about 9.5 and, more importantly, the ponded water it needs leaches sodium downward out of the root zone. Grow dhaincha after the rice is harvested (or before transplanting where the calendar allows) and incorporate it before the next crop. Berseem and maize are poor choices here: both are sensitive to sodicity.
Rabi: barley first choice, wheat second, mustard next. Among the common rabi crops the tolerance order is roughly barley > wheat > oats > mustard, while berseem and lentil are sensitive. Wheat is acceptable if he chooses a variety known to perform on alkaline soils, and it is what the family eats. Band-place a little gypsum in the furrow near the seed rather than broadcasting all of it, so the seedling root zone is protected.
The one practice to change: stop surface-broadcasting urea and irrigating immediately afterwards in small amounts. At pH 9.3 that is the fastest way to lose nitrogen as ammonia. Incorporate urea into moist soil, or apply it just before an irrigation so it moves into the root zone, and split the dose so no single application is large.
Realistic expectation: the field will not be cured in one season. Expect improvement in the second or third crop, and keep a mulch or crop residue cover on the surface between crops to reduce the crust.