Liming and Fertiliser Planning on an Acid Sandy Loam
A farmer in Jamui, Bihar, has a 1 ha field with a red sandy loam soil. A soil test from the block laboratory reports pH 5.1, organic carbon 0.29%, available N 165 kg/ha, available P2O5 16 kg/ha, available K2O 215 kg/ha and CEC 8 cmol(p+)/kg. The previous rice crop was stunted and showed poor tillering; the farmer attributes this to 'weak seed'. He plans to sow lentil in the last week of October after rice and has already bought 200 kg DAP, 50 kg MOP and 2 t of well-rotted FYM. Agricultural lime is available at the block office, 15 km away, at ₹6/kg, but the farmer has only ₹5,000 for amendments and fertiliser this season. He wants to apply the full 200 kg DAP at sowing because 'DAP gives both N and P'.
1. Interpret the soil test. Which is the first limiting factor, and how should the farmer prioritise the ₹5,000?
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pH 5.1 is strongly acidic for a sandy loam; at this pH, phosphorus is fixed by aluminium and iron, molybdenum availability falls, and Rhizobium survival and nodulation in lentil are poor. Organic carbon 0.29% and available N 165 kg/ha are low, P2O5 16 kg/ha is medium and K2O 215 kg/ha is high. The first limiting factor is acidity, not seed quality or potassium. Lime is therefore the priority, but the budget cannot buy the full 2–3 t/ha needed to raise pH to about 6.5. A reasonable judgement is to buy 800 kg agricultural lime (about ₹4,800), broadcast it and incorporate it 2–3 weeks before sowing, and use the already-purchased FYM to improve CEC and moisture retention. If sowing cannot wait, apply 500 kg lime in the furrows and accept that pH correction will take two seasons; do not spend the limited cash on MOP, because K is already high. The 200 kg DAP should not be applied as a single dose.
2. How should the 200 kg DAP and 50 kg MOP be adjusted for lentil? Show the nutrient calculations.
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200 kg DAP contains 36 kg N and 92 kg P2O5. Lentil after rice generally needs about 20 kg N, 40–50 kg P2O5 and 20 kg K2O/ha, with FYM. Applying all 200 kg DAP would supply nearly twice the P requirement and unnecessary N, and DAP has an acidifying effect. Use 100 kg DAP at sowing, which gives 18 kg N and 46 kg P2O5, close to the recommendation. The 2 t FYM supplies some N, P and K and micronutrients. Do not apply the 50 kg MOP: 50 kg MOP is about 30 kg K2O, and the soil already tests high in K (215 kg/ha); extra K is an unnecessary cost and may imbalance calcium and magnesium. Keep the remaining 100 kg DAP for the next crop, and if a starter N is needed, a small dose of urea (10–15 kg/ha) is enough. Seed should be treated with Rhizobium and phosphate-solubilising bacteria.
3. What else would you advise about lime placement, seed inoculation and crop choice if the farmer cannot raise pH quickly?
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Lime must be mixed into the top 10–15 cm, not left on the surface, and ideally applied 2–3 weeks before sowing. If only a small quantity is available, band it in the furrows but keep it 2–3 cm away from the seed to avoid seedling injury. Lentil is more sensitive to acidity than wheat or mustard; if pH stays below about 5.5, the safer choice is to grow mustard or wheat this rabi and apply lime for the next crop. Inoculating lentil seed with Rhizobium leguminosarum and PSB can partly compensate for poor native nodulation, but it is not a substitute for lime. Also test the lime's neutralizing value and use agricultural lime, not cement. A split application over two seasons is better than a very small dose that raises pH only slightly.