A Sodic Patch in a Paddy Field
Kuppusamy cultivates 0.8 ha of paddy in Villupuram district. A soil test reports pH 8.9, electrical conductivity 0.4 dS/m, exchangeable sodium percentage 28, organic carbon 0.28%, available nitrogen 180 kg/ha, available phosphorus 12 kg/ha and available potassium 310 kg/ha. His yield is 3.2 t/ha while a neighbouring field with similar management yields 5.5 t/ha. The borewell water used for irrigation has a residual sodium carbonate of 4.2 meq/L. He has been applying 150 kg urea per ha in three split doses and 100 kg DAP per ha at transplanting, and plans to increase urea further because the crop is pale. He can arrange 10 t of farmyard manure and has a budget for one amendment. He asks what is limiting the crop and how to correct it without wasting money on more urea.
1. Interpret the soil test and explain why more urea is unlikely to close the yield gap.
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The field is sodic, not simply nitrogen deficient. A pH of 8.9 with ESP 28 and irrigation water RSC 4.2 meq/L indicates sodium accumulation that disperses clay, destroys soil structure, reduces water infiltration and restricts root growth. Organic carbon is very low at 0.28%, so the soil has little buffering capacity and poor biological activity. Phosphorus is medium and potassium is high, so more DAP is not the priority. The pale crop is being read as nitrogen deficiency, but at high pH urea hydrolysis raises local pH and increases ammonia volatilisation, so recovery is poor. Adding more urea without correcting sodicity wastes money and may worsen nutrient imbalance. The yield gap is mainly due to poor soil physical condition, sodium toxicity and low organic matter, not a shortage of urea alone.
2. Which amendment should he buy, at what dose, and how should it be applied?
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Buy gypsum, not more urea. For an ESP of 28 and a target near 10, a reasonable field dose is about 2.5–3 t/ha of gypsum, which is about 2–2.4 t for 0.8 ha; a laboratory gypsum requirement test can refine this. Spread gypsum evenly and incorporate it into the top 10–15 cm before transplanting. Apply the 10 t of farmyard manure separately and incorporate it as well; organic matter improves structure and helps leaching. Then impound and leach with water, but avoid excessive leaching if drainage is poor. The high-RSC borewell water can still be used if gypsum is applied, but watch for salt build-up. Do not expect a one-season cure; sodic soils need repeated amendment and organic matter, and drainage must be maintained.
3. How should his fertiliser plan change for the current paddy crop?
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Stop increasing urea and rebalance the programme. Apply nitrogen in three or four splits, but use part of it as ammonium sulphate rather than all urea, because ammonium sulphate has an acidifying effect that suits a high-pH soil. Keep phosphorus at the recommended dose based on soil test; DAP is acceptable if placed near the root zone, but do not raise it blindly because P is already medium and high pH fixes phosphorus. Potassium is high, so omit or reduce potash. Add zinc sulphate at about 25 kg/ha if leaves show bronzing or khaira symptoms, since zinc is less available at high pH. The most important change is to correct sodicity and add organic matter first; fertiliser response will remain poor until soil structure improves.