Sodic Soil, a Booked Soybean Seed Lot and a ₹25,000 Budget
Assume you are advising a farmer in Jalna district (Marathwada) who holds 1.2 ha of medium black soil. A soil test reports pH 9.2, EC 0.6 dS/m, organic carbon 0.35%, available N 165 kg/ha, available P2O5 24 kg/ha, available K2O 340 kg/ha and exchangeable sodium percentage 24. The borewell water used for supplemental irrigation has an EC of 1.1 dS/m. The field is flat with no outlet and a hardpan at 25-30 cm; water stands for two days after heavy rain. Last year the same plot grew soybean that stayed yellow throughout and yielded only 6 q/ha. He has already bought 200 kg DAP, 100 kg MOP and 300 kg urea and has booked seed of a popular soybean variety for kharif. He has ₹25,000 for soil amendment and will not spend more. Gypsum costs ₹4,500 per tonne, FYM ₹2,000 per tonne and dhaincha seed ₹300 per kg.
1. Should the farmer sow the booked soybean on this field this kharif? Justify your answer using the soil data, and state what you would do with the seed.
Read the worked answer
No - not on the whole 1.2 ha as planned.
- Soybean does best at pH 6.0-7.5. At pH 9.2 with ESP 24 the crop faces iron and zinc inactivation and poor nodulation, because rhizobia are sensitive to high pH and sodicity. The yellow crop of last year is consistent with this, not with a bad variety.
- EC 0.6 dS/m is not the limiting factor; sodicity (ESP 24), not salinity, is the problem.
- The hardpan at 25-30 cm plus two days of standing water is worse than the pH alone: soybean nodules stop working within 3-4 days of saturation, and the sodium-affected surface crusts and sheds water.
- Realistic expectation is 40-60% of potential, so the seed and fertilizer would be spent on a crop that cannot repay them.
Practical decision: divert the booked seed to 0.3-0.4 ha of the better-drained corner only after gypsum and an outlet are in place, or exchange it. Put the remaining area under reclamation this kharif - dhaincha for green manure, or a tolerant crop such as pearl millet for fodder - and bring soybean back in the second or third season once pH is near 8.5 and ESP below 15.
2. Prepare an amendment and water-management plan within ₹25,000 for the 1.2 ha, showing the arithmetic and explaining why lime, sand or extra urea cannot substitute.
Read the worked answer
Priority 1 - gypsum (calcium sulphate), not lime.
- Dose of about 2.5 t/ha for ESP 24 on medium black soil = 3 t for 1.2 ha × ₹4,500 = ₹13,500. The laboratory gypsum requirement test should confirm the figure.
- Gypsum supplies Ca²⁺, which displaces Na⁺ from the exchange complex; the sodium then has to be leached below the root zone. It also improves infiltration.
- Lime is the wrong choice: it adds calcium as carbonate and pushes pH above 9.2, making the sodium problem worse. Sand does not remove exchangeable sodium. Urea adds no calcium and will not touch the ESP.
Priority 2 - organic matter at a reduced dose.
- 4 t/ha FYM = 4.8 t × ₹2,000 = ₹9,600. Total so far ₹23,100, leaving ₹1,900 - so 30 kg dhaincha (₹9,000) does not fit. Either drop dhaincha this year, or cut FYM to 2 t/ha (₹4,800) and buy 15 kg dhaincha (₹4,500).
Water management, which costs labour rather than cash: break the hardpan with a chisel/subsoiler pass along the slope, cut a 30 cm outlet channel at the lowest corner, and lay ridges and furrows so rainwater passes through instead of ponding. Spread gypsum 2-3 weeks before sowing, incorporate to 10 cm, and follow with 1-2 leachings of about 5 cm. Do not band gypsum in the same furrow as DAP.
3. He has already bought 200 kg DAP, 100 kg MOP and 300 kg urea. Recalculate what should actually be applied and say what to do with the surplus.
Read the worked answer
Work per hectare first. 200 kg DAP over 1.2 ha is 167 kg/ha, supplying 30 kg N and 77 kg P2O5/ha.
- DAP: soil P is medium (24 kg/ha), so 40-50 kg P2O5/ha is enough for a legume - about 100-110 kg DAP/ha, i.e. 120-130 kg for the field. That leaves 70-80 kg DAP surplus.
- MOP: K2O at 340 kg/ha is high. No MOP is needed for soybean, and extra potassium will widen the K:Mg ratio and worsen the chlorosis seen last year. Keep the 100 kg for an onion or banana plot, or return it.
- Urea: a well-nodulated soybean needs only a 20-25 kg N/ha starter, but on this sodic soil nodulation will be poor, so 30-40 kg N/ha (35-50 kg urea/ha, 40-60 kg for the field) is defensible. The other ~240 kg should not be dumped on the field; store it sealed for the rabi crop.
Spend the saved money better: Rhizobium + PSB culture for the seed (₹200-300) and 25 kg ZnSO4/ha, because sodic soils are commonly zinc-deficient. Keep fertilizer and gypsum in separate bands away from the seed.