Bare Patches in a Rice–Wheat Field: Reading a Sodic Soil Test
A farmer in Hardoi district, Uttar Pradesh, grows rice–wheat on 1.2 ha served by a canal distributary. The field is levelled, but water stands for two to three days after heavy rain. A May soil test of the 0–15 cm layer reports pH 9.3, EC 0.9 dS/m, organic carbon 0.31%, CEC 22 cmol(p+)/kg, ESP 34, available N 180 kg/ha, Olsen P 9 kg/ha and available K 145 kg/ha. Wheat sown on 18 November emerges unevenly: circular bare spots 2–4 m across stay empty, and surviving plants tiller poorly with bluish-green leaves. The farmer has applied 120:60:40 kg N:P2O5:K2O per hectare to wheat and 100 kg DAP per hectare at paddy transplanting, and adds 5 t FYM per hectare every third year. He now plans to spread 10 t FYM per hectare in June and asks whether gypsum is also needed, at what rate, and whether his rice–wheat sequence should be changed.
1. Interpret the soil test result and explain why wheat fails in patches even though the field has been fertilised every season.
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Separate salinity from sodicity first. The EC is 0.9 dS/m, well below the 4 dS/m saline threshold, so salts are not the problem. With pH 9.3 and ESP 34, the soil is sodic (alkali), not saline.
Check the exchangeable sodium load. Exchangeable Na = 34% of CEC 22 = 7.5 cmol(p+)/kg. At this level sodium disperses clay into a surface seal, so infiltration falls and the soil stays ponded for 2–3 days. Poor air supply then limits root growth and tillering.
Link symptoms to chemistry, not to a fertilizer gap. At pH above 9, zinc and iron become largely unavailable - that is the source of the bluish-green, poorly tillering plants - while applied phosphate is fixed and surface-applied urea loses ammonia. The circular bare spots are pockets with still higher ESP, not missed fertilizer strips.
The 10 t FYM is worth applying for organic carbon and structure, but at ESP 34 organic matter alone cannot displace enough sodium; a chemical amendment is required. Adding more N and P now would raise cost without curing the physical constraint. Follow with ZnSO4 25 kg/ha, split nitrogen, and re-test ESP (not only pH) after two reclamation seasons to judge whether the amendment rate is enough.
2. Calculate the gypsum requirement for this field from the given data, state the dose you would actually recommend for the first year, and describe how it should be applied and leached.
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Step 1 - sodium to be replaced. Target ESP for a rice–wheat soil is about 10. Exchangeable Na at 34% ESP = 0.34 × 22 = 7.48 cmol(p+)/kg; at 10% ESP it would be 2.20 cmol(p+)/kg. Sodium to displace = 5.28 cmol(p+)/kg.
Step 2 - convert to gypsum mass. One cmol of CaSO4·2H2O is 1.72 g. Mass of 0–15 cm soil at bulk density 1.45 = 0.15 × 1.45 × 10,000 = 2,175 t/ha = 2.175 × 10^6 kg. So 1 cmol/kg needs 1.72 × 2.175 × 10^6 g ≈ 3.74 t/ha.
Step 3 - laboratory requirement. 5.28 × 3.74 ≈ 19.8, i.e. about 20 t/ha of pure gypsum.
Judgement on the field dose. Field trials rarely recover the full laboratory figure in one season, and commercial gypsum is only about 70–80% pure. I would apply 10–12 t/ha of good-quality, finely ground gypsum in the first year (half before kharif puddling, half before wheat), and adjust after re-testing. Apply to a dry surface, incorporate shallow, then leach with 2–3 irrigations of good canal water; do not mix with urea in the same operation, and keep drainage open so the displaced sodium actually leaves the root zone.
3. Should the rice–wheat sequence be changed, and what supporting practices would you add during reclamation?
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Keep rice in the kharif, but use it as part of the cure. Rice is the most sodicity-tolerant of the common field crops, and puddled, ponded culture is the cheapest way to leach displaced sodium downward. Removing rice would both lose a tolerant crop and lose the leaching opportunity, so the sequence itself need not be abandoned.
What does need changing is the supporting package.
- Insert a dhaincha (Sesbania) green manure in the summer window between wheat harvest and rice transplanting; its root channels and organic acids improve infiltration, which is the real bottleneck.
- Apply gypsum in the kharif season, before puddling, so leaching coincides with monsoon and canal supply.
- Use ZnSO4 25 kg/ha and split nitrogen into three doses; surface broadcasting of urea on a pH 9.3 soil wastes ammonia.
- Choose wheat varieties with better sodicity tolerance and sow slightly heavier; avoid the worst patches for wheat and put barley or berseem there until ESP falls below 15.
- Avoid irrigating with high-RSC tube well water; if it must be used, add gypsum to the water or blend it with canal water.
The decision rule is simple: the sequence stays, the amendment and water management change, and the farm re-tests ESP rather than pH to judge progress.