Liming and Fertilizer Planning on an Acid Sandy Clay Loam
A soil test for a 1 ha field (0-15 cm, bulk density 1.45 g/cm3) going into maize-wheat reads: pH 5.3 (1:2.5 soil:water), organic carbon 0.36%, available N 145 kg/ha (low), available P 9 kg P2O5/ha (low), available K 165 kg K2O/ha (medium), CEC 7.8 cmol(p+)/kg, EC 0.11 dS/m, exchangeable Al 1.9 cmol(p+)/kg, texture sandy clay loam. The recommended dose for maize is 120:60:40 kg N:P2O5:K2O per ha. The farmer holds DAP (18-46-0), urea (46-0-0), muriate of potash (0-0-60) and single superphosphate (0-16-0, with 11% S); agricultural lime of 85% CaCO3 equivalence costs Rs 4,000/t. His only lime application, four years ago, was 1 t/ha. The field is rainfed kharif and irrigated rabi; district rainfall is 700 mm.
1. Calculate the lime requirement to neutralise exchangeable aluminium, and explain why liming should precede the phosphorus application rather than being mixed with it.
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Soil mass in the 0-15 cm layer per ha = 10,000 m2 x 0.15 m x 1.45 t/m3 = 2,175 t/ha = 2,175,000 kg/ha.
Exchangeable Al = 1.9 cmolc/kg, so the total to be neutralised = 1.9 x 2,175,000 = 4,132,500 cmolc = 41,325 molc per ha. One mole of CaCO3 equivalent is 50 g, so pure CaCO3 needed = 41,325 x 50 g = 2,066,250 g = 2.07 t/ha. Adjusting for 85% purity: 2.07 / 0.85 = 2.43 t/ha, i.e. about 2.4 t/ha of the product.
Judgement: aim to remove Al toxicity and bring pH to about 5.5-5.8, not to 7.0. Over-liming an already low-CEC (7.8 cmol/kg) soil with 0.36% organic carbon risks inducing zinc and boron deficiency. Apply lime 2-3 weeks before sowing and incorporate it into the top 10-15 cm; it needs moisture to react, so time it before expected rain. Do not mix lime with ammonium or phosphate fertilizers in the same band: lime plus ammonium fertilizer liberates ammonia, and lime applied together with phosphate rock or SSP reduces the immediate availability of the phosphate.
Why liming comes first: at pH 5.3 with 1.9 cmol/kg exchangeable Al, applied phosphate is rapidly fixed as insoluble aluminium and iron phosphates, and the Al also restricts root growth, so roots cannot reach the fertilizer band. Raising pH first dissolves the Al and frees both native and freshly applied P. Phosphorus banded after liming will therefore give a far larger response than the same P applied into the acid soil.
2. Prepare the fertilizer plan for maize (120:60:40) using the materials in the store, choosing between DAP and SSP for the phosphorus, and justify the choice. Give carrier quantities.
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Phosphorus choice: 60 kg P2O5/ha as DAP = 60/0.46 = 130 kg DAP/ha, which supplies 23.5 kg N/ha. As SSP: 60/0.16 = 375 kg SSP/ha, which supplies about 41 kg S/ha and calcium.
Recommendation: use SSP for at least part of the P. Reasons: (a) the soil is low in organic carbon (0.36%) and the S status is likely low, so 41 kg S/ha is a real benefit, particularly for the wheat in the rotation; (b) the soil is acid and Al-toxic, and the calcium in SSP helps root growth and offsets Al; (c) DAP placed with the seed delivers ammonia near the germinating seed. Against this, SSP is bulky (375 kg/ha) and costs more to handle. A reasonable compromise is 40 kg P2O5 as SSP (250 kg SSP/ha, 27 kg S) plus 20 kg P2O5 as DAP (43 kg DAP, 7.8 kg N), all banded 5 cm below and 5 cm to the side of the seed.
Nitrogen: total 120 kg N. From 43 kg DAP: 7.8 kg N. Balance 112 kg N as urea = 243 kg urea/ha. Split as one third basal (81 kg urea), one third at V6 (81 kg urea), one third at tasselling (81 kg urea), the last split skipped if the rain fails.
Potassium: 40 kg K2O/ha as MOP = 40/0.60 = 67 kg MOP/ha, banded at sowing. With a CEC of 7.8 and sandy clay loam texture, do not exceed this as a single basal dose; if the rate were raised, split it.
Total carriers: 250 kg SSP + 43 kg DAP + 243 kg urea + 67 kg MOP = 603 kg/ha. Bag counts and store space should be planned accordingly.
3. The farmer says he limed four years ago at 1 t/ha and 'it did not work'. Explain why, and set out the sequence of field operations and the monitoring plan for the coming two years.
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Why it did not work: 1 t/ha of lime is far below the 2.4 t/ha calculated to neutralise 1.9 cmol/kg of exchangeable Al. One tonne would have neutralised less than half of the exchangeable Al even if it was fully reactive, and the 85% purity and any surface application without incorporation would have cut the effective dose further. In addition, sandy clay loam with 0.36% organic carbon has little buffering, so pH falls back once the exchangeable Al is re-exposed. Also, if the lime was spread after the P was drilled or applied to the surface without incorporation, the P response would have been negligible regardless of the lime.
Sequence for year 1:
- Apply 2.4 t/ha agricultural lime (about 9.6 t for a 1 ha field at Rs 4,000/t = Rs 9,600) to the dry field, spread evenly by hand or by a lime spreader, and incorporate it into the top 10-15 cm 2-3 weeks before sowing, ideally just before a forecast rain. Do not mix it with fertilizer.
- At sowing, band SSP/DAP 5 cm below and 5 cm away from the seed; apply the basal N and the MOP.
- Top-dress the remaining N in two splits; do not apply urea together with lime.
Monitoring year 1 and 2:
- Re-sample the soil 0-15 cm at the same spots after harvest to track pH and exchangeable Al; lime should raise pH by about 0.3-0.6 units over two seasons and reduce Al saturation.
- Sample again at the start of the second season. Lime has a residual effect of 3-4 years, so do not reapply on the basis of a single season's data.
- Watch for Zn deficiency induced by the pH change; if it appears between the fourth and sixth leaf, apply 25 kg ZnSO4/ha to the soil or 0.5% foliar spray.
- Keep a record of the S applied with SSP, and reduce any future S application accordingly.