A soil report that claims deficiency in a field already over-supplied
A 1.5-hectare tomato field in Chittoor district has the following report: pH 8.7, EC 1.9 dS/m, organic carbon 0.31%, available N 165 kg/ha, available P2O5 58 kg/ha (high), available K2O 340 kg/ha (very high), free lime 12%, DTPA-Fe 2.0 ppm (deficient), DTPA-Zn 0.35 ppm (deficient). The borewell used for irrigation tests EC 2.4 dS/m and SAR 8.4. Plants at 45 days are stunted, with interveinal yellowing of the youngest leaves and a purple tinge on some older leaves; roots are sparse with brown tips. The farmer has already applied 250 kg DAP, 200 kg muriate of potash and 100 kg urea as basal, and now proposes another 100 kg DAP plus 25 kg zinc sulphate broadcast, arguing that the purple leaves prove phosphorus deficiency and 'more DAP will fix it'. The previous crop was paddy with 5 t/ha of FYM, and the borewell has been the only water source for six years.
1. Interpret the soil report and the field symptoms. Is the purple colouring evidence of phosphorus deficiency here?
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No. Phosphorus is already high at 58 kg/ha and the field has received 250 kg of DAP, so a fresh P response is implausible; adding more P would increase salinity, cost and the risk of induced zinc and iron deficiency through antagonism. The purple tinge on older leaves is a stress response - anthocyanin accumulation under root injury, salinity, or sharp temperature variation - not a P diagnosis in this field.
The real signal is the pairing of high pH (8.7) with 12% free lime. At that pH, iron and zinc are practically unavailable even when present, and both DTPA values confirm deficiency. Interveinal yellowing of young leaves is the classic iron-deficiency pattern; zinc deficiency adds rosetting and small leaves. Sparse, brown-tipped roots point to salinity injury (EC 1.9 dS/m in soil, irrigation water at 2.4 dS/m) and possibly poor aeration rather than disease alone. Available potassium is very high, so the 200 kg of muriate of potash was unnecessary and has added chloride to an already salt-loaded soil.
In short: this is an induced micronutrient deficiency plus salinity stress on a high-lime soil, made worse by the basal fertiliser programme. The purple leaf is being misread.
2. Rewrite the fertiliser and corrective plan for this crop. What should be stopped, what should be applied, and in what form and manner?
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Stop: further DAP, further muriate of potash, and broadcast zinc sulphate without checking placement. With P at 58 kg/ha and K at 340 kg/ha, the crop does not need a P or K build-up; the basal DAP already applied is more than a season's requirement.
Iron: at pH 8.7, ferrous sulphate applied to soil oxidises within days. Use a chelated iron (Fe-EDDHA) through drip near the root zone, and support it with 2–3 foliar sprays of 0.5% ferrous sulphate plus 0.1% citric acid at 10-day intervals on the young leaves. This gives quick greening while the soil treatment takes effect.
Zinc: foliar zinc sulphate at 0.5% with 0.25% lime to prevent leaf burn, two sprays a fortnight apart, is faster and cheaper than soil application on a calcareous soil; if soil application is preferred, band a modest dose near the root zone rather than broadcasting over the whole field.
Nitrogen and potassium: continue nitrogen in small split doses through drip, matched to crop stage, and replace muriate of potash with sulphate of potash where potassium is genuinely needed.
General: raise organic carbon - this field is at 0.31%, which is why micronutrient availability collapses. Add well-decomposed compost or green manure before the next crop, and consider acidifying strips or elemental sulphur in the bed to bring the rhizosphere pH down modestly. Track leaf tissue analysis, not just soil analysis, to verify the correction.
3. Is gypsum justified and will leaching work with this water? Give your recommendation with the arithmetic you would use.
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Gypsum should not be applied automatically here. Free lime of 12% means the soil already carries abundant calcium carbonate; gypsum earns its place only where the exchangeable sodium percentage is high. The report does not give ESP, so the first action is to get exchangeable sodium and the laboratory gypsum requirement, and to apply gypsum only if ESP is appreciably above about 10. A water SAR of 8.4 is a moderate sodicity hazard but not extreme.
Leaching with this water is the weak link. Tomato yield starts to decline above a soil saturation-extract EC of about 2.5 dS/m, and the irrigation water alone is 2.4 dS/m. A leaching requirement calculated as LR = ECw/(5×ECe − ECw) gives roughly 2.4/(12.5 − 2.4) ≈ 0.24, i.e. about a quarter extra water. But that calculation assumes the leached salt actually leaves the root zone - which requires free drainage and a water low in salt. With this borewell, leaching merely moves salt around and adds more.
Practical recommendation: blend or alternate with canal water wherever possible, shift to drip with mulching to keep the wetting front controlled, irrigate in small frequent applications rather than heavy drenches, install drainage at the lower end of the field, and re-test soil EC twice a season. Treat the water as the constraint and plan the crop around it, choosing a more salt-tolerant crop in the rotation.