Soil test report, fertiliser arithmetic and a sulphur-hungry wheat crop
A 1.0 ha field at Waraseoni, Balaghat, is under a soybean-wheat rotation. The soil test report for the wheat season reads: pH 6.1, EC 0.14 dS/m, organic carbon 0.41 per cent, available nitrogen 172 kg/ha, available phosphorus 16 kg P2O5/ha, available potassium 285 kg K2O/ha, sulphur 7.5 mg/kg, zinc 0.52 mg/kg and boron 0.28 mg/kg. The recommended dose for wheat is 120 kg N, 60 kg P2O5 and 40 kg K2O per hectare, and the extension officer advises 20 kg S and 25 kg ZnSO4 per hectare. In the shed are 100 kg of DAP and 150 kg of urea. Local prices are urea Rs 300 per 45 kg, DAP Rs 1,350 per 50 kg, SSP Rs 500 per 50 kg, MOP Rs 900 per 50 kg, gypsum Rs 5 per kg and ZnSO4 Rs 40 per kg. The farmer wants to spend as little as possible this season, but he saw poor tillering in this field last year.
1. Work out, showing each step, what he must buy to meet 120 kg N, 60 kg P2O5, 20 kg S and 25 kg ZnSO4 per hectare, using the stock in the shed first.
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Work outward from the stock, because fertiliser already bought is a sunk cost.
- DAP in hand: 100 kg gives 46 kg P2O5 and 18 kg N.
- Balance P2O5: 60 - 46 = 14 kg. Use SSP: 14 / 0.16 = 87.5, say 88 kg SSP, which also gives about 9.6 kg S and 17 kg Ca.
- Balance S: 20 - 9.6 = 10.4 kg. Gypsum is 18 per cent S, so 10.4 / 0.18 = 58, say 60 kg gypsum for 10.8 kg S.
- Balance N: 120 - 18 = 102 kg. Urea = 102 / 0.46 = 221.7, say 222 kg urea. He has 150 kg, so buy 72 kg (about Rs 480).
- Zinc: 0.52 mg/kg is just below the 0.6 critical level, so 25 kg ZnSO4 (about Rs 1,000), applied once every two to three years.
- Purchase list: urea 72 kg, SSP 88 kg, gypsum 60 kg, ZnSO4 25 kg - roughly Rs 2,660. MOP 67 kg (Rs 1,200) is optional, as argued separately.
- Organic carbon: at 0.41 per cent, add 5 t FYM or compost. That, more than any bag of urea, is what limited tillering last year.
2. Should the balance phosphorus come from DAP or SSP? Compare on nutrient content, cost per kg of P2O5 and the sulphur need.
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Straight cost per unit of P2O5
- DAP: Rs 27 per kg product / 0.46 = Rs 58.7 per kg P2O5.
- SSP: Rs 10 per kg product / 0.16 = Rs 62.5 per kg P2O5. So DAP looks about Rs 4 cheaper per unit - but that comparison is incomplete here.
Cost of the actual balance lot
- 14 kg P2O5 needs 88 kg SSP costing Rs 880, against 30.4 kg DAP costing Rs 821. The premium for SSP is only about Rs 59.
- For that Rs 59, SSP brings 9.6 kg S and about 17 kg Ca. Bought as gypsum, 9.6 kg S would cost roughly Rs 270.
Verdict SSP is clearly the better buy for the balance. The soil has only 7.5 mg/kg sulphur against a critical level of about 10, and sulphur deficiency is a recognised cause of poor wheat tillering; the DAP-only route would leave that gap unfilled or fill it at four times the cost.
Use the DAP already in the shed, but do not buy fresh DAP for this field while sulphur is deficient. If a fresh purchase is unavoidable, 375 kg SSP alone would meet 60 kg P2O5 and about 41 kg S with no DAP at all. Place all P and S in the seed furrow, 2-3 cm below and away from the seed - banded, not broadcast - because phosphorus is immobile and sulphur leaches.
3. Is lime needed? Should potassium be applied? How should the nitrogen be placed and timed?
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Lime: not this season. pH 6.1 is comfortable for wheat, which tolerates roughly 6.0-7.5. It is not comfortable for soybean, which prefers about 6.5. So monitor: if the next test falls below 6.0, apply agricultural lime at 2-4 q/ha before the soybean crop, based on a lime requirement test, and incorporate it 3-4 weeks before sowing - do not lime and band DAP in the same furrow at the same time.
Potassium: skip it this season. 285 kg K2O/ha is in the high class. Wheat removes about 25-30 kg K2O per 50 quintals of grain. If straw is removed from the field, apply about 50 kg MOP (30 kg K2O); if straw is returned or burnt in situ, skip K and save Rs 1,200 without yield loss.
Nitrogen timing: split the 222 kg urea.
- Half at sowing.
- One quarter at crown root initiation, 20-25 days after sowing.
- One quarter at the first node or late tillering stage. All-basal nitrogen is leached by winter rain, lost as ammonia from a moist surface, and invites lodging and yellow rust.
Placement Drill all phosphorus, sulphur and the ZnSO4 with the seed drill at sowing; do not broadcast, and do not plant the seed in direct contact with fertiliser.