When more DAP makes the wheat worse
A farmer in western Uttar Pradesh has grown rice–wheat for 12 years on 1 ha of loam. Each season he applies 150 kg DAP and 130 kg urea per hectare before sowing, plus 25 kg muriate of potash, and he burns the rice straw in the field. The latest soil test (0–15 cm) reports pH 8.4, EC 0.32 dS/m, organic carbon 0.34%, Olsen P 48 kg/ha, available K 190 kg/ha and DTPA-Zn 0.38 ppm; sulphur was not tested. Wheat now yields 3.1 t/ha against 4.5 t/ha five years ago, and for the last two years the young wheat has shown pale green stripes on the lower half of the leaves, stunted growth and delayed maturity, worst on the higher, sandy part of the field. The farmer's response has been to raise DAP to 175 kg/ha and to spray a micronutrient mixture labelled Fe, Mn, B and Mo - but not Zn. His tube well water has RSC 2.8 meq/L and bicarbonate 4.2 meq/L.
1. Diagnose what is limiting the crop from the soil test values and the symptom pattern, and explain why both of the farmer's responses cannot work.
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Diagnosis: zinc deficiency, induced and aggravated by the phosphorus programme.
- DTPA-Zn 0.38 ppm is below the 0.6 ppm critical level, and the symptoms match: pale green to whitish bands on the lower half of young wheat leaves, stunting and delayed maturity, patchy and worst on the higher sandy spots where leaching is greatest and organic carbon is lowest.
- Olsen P 48 kg/ha is roughly four times the 10–12 kg/ha critical level after 12 years of heavy DAP. A wide P:Zn ratio in soil and plant further suppresses zinc uptake - this is classic phosphorus-induced zinc deficiency.
- pH 8.4 and irrigation water with 4.2 meq/L bicarbonate both reduce zinc solubility, and organic carbon at 0.34% provides little chelation. Potassium at 190 kg/ha is not limiting.
Why the responses fail:
- Raising DAP from 150 to 175 kg/ha adds about 11.5 kg/ha more P2O5 and no zinc at all; it worsens the P:Zn imbalance and adds cost.
- The micronutrient spray omits Zn, the one element that is actually deficient. Sprays of Fe, Mn, B and Mo at label rates cannot correct a Zn shortage, and foliar sprays in any case do not fix a soil-level deficiency for long.
Confirm before committing: a tissue test (flag-leaf Zn below about 20–25 ppm supports the diagnosis) and a small strip treated with zinc sulphate will show greening within two to three weeks. Also note that straw burning removes the zinc and potassium held in the residue, so the field loses both every year.
2. Give a corrected plan for the wheat season with quantities, timing and method, and state clearly what you would cut, keep, add or postpone, with the arithmetic.
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Add - zinc: ZnSO4·7H2O at 25 kg/ha drilled with the seed at sowing (or 12.5 kg/ha banded plus a 0.5% foliar spray at tillering and again at booting if stripes persist). This is the single highest-return input in the plan; do not substitute a multi-micronutrient mix.
Cut - phosphorus: with Olsen P at 48 kg/ha and wheat removing roughly 6 kg P2O5 per tonne of grain plus straw, a 4.5 t/ha crop removes about 50–55 kg P2O5/ha. The soil can supply this for two or three seasons. Apply only a starter dose of 30 kg P2O5/ha (about 65 kg SSP, which also supplies about 7 kg S and suits the untested sulphur and high pH), and re-test Olsen P after harvest before resuming P.
Keep/adjust - nitrogen: wheat needs about 120 kg N/ha in three splits (half at sowing/CRI, a quarter at tillering, a quarter at booting). Removing DAP removes 27 kg N/ha, so urea must rise to about 260 kg/ha. On a pH 8.4 soil with heavy dew, surface-broadcast urea loses ammonia, so apply just before irrigation or incorporate.
Keep - potassium: 190 kg/ha is medium; wheat removes about 20 kg K2O/t of grain, near 65 kg K2O/ha, so retain about 40 kg K2O/ha as MOP (about 67 kg MOP).
Add - organic matter and residue: 5 t/ha FYM or a dhaincha green manure in the rice season, and stop burning straw - return it or use it as mulch, which also returns K, S and Zn.
Watch - water quality: RSC 2.8 meq/L is above the 2.5 meq/L safe limit and bicarbonate is high, so monitor EC and ESP yearly and apply gypsum at about 2.5 t/ha if ESP rises above 15 or infiltration deteriorates. Re-test the soil after harvest for Olsen P, DTPA-Zn and organic carbon.
3. The farmer wants to keep the higher DAP dose for one more season and 'see'. Describe the on-farm trial and monitoring you would set up, and the evidence that would settle the question without risking his crop.
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Run a replicated strip trial in the farmer's own field so the comparison is credible and he carries no all-or-nothing risk.
Treatments (0.1 ha strips, three replications, randomised):
- Farmer's practice - 175 kg DAP, no zinc.
- No DAP, 25 kg/ha ZnSO4·7H2O, N balanced with urea.
- No DAP, ZnSO4 plus 5 t/ha FYM.
- 30 kg P2O5/ha as SSP plus ZnSO4.
Place at least one replication on the higher sandy patch, because that is where the deficiency is worst; leave the farmer free to manage the rest of the field as he wishes.
Measurements: plant stand and tiller count per square metre at about 45 days; leaf greenness (SPAD) at tillering; tissue Zn at tillering; biomass at 90 days; grain yield and 1,000-grain weight at harvest; and soil Olsen P, DTPA-Zn and organic carbon before sowing and after harvest. Note the cost of each treatment so a partial budget can be built at local prices.
What settles it: treatment 1 should show no advantage in P response (the soil is already four times above the critical Olsen P) and should stay zinc-deficient; treatments 2–4 should green up and out-yield it. If treatment 2 equals the farmer's practice in yield while cutting DAP cost and adding only 25 kg of zinc sulphate, the economics are decisive.
Safeguards: keep a P-free strip only for one season, since a high-P soil will not run short in a single crop; verify the soil test with a fresh sample if the high Olsen P looks anomalous; and never compare strips that differ in irrigation or sowing date, since those differences would be read as fertiliser effects.