Groundnut on an Acid Red Soil: Lime, Gypsum or More Urea?
Sunita has 1 ha of rainfed upland in Dhenkanal district, Odisha, with a red lateritic soil. Soil test: pH 5.3, organic carbon 0.35%, available N low, available P2O5 8 kg/ha, available K2O 120 kg/ha, exchangeable Ca 1.2 cmol/kg, exchangeable Mg 0.8 cmol/kg, available S 6 mg/kg, hot-water-soluble B 0.28 mg/kg, CEC 6 cmol/kg. She sowed groundnut (110-day bunch type) in the last week of June after 150 kg DAP/ha and 50 kg urea/ha at sowing. No lime, gypsum or micronutrients were applied. At 45 days, plants are pale yellow, nodulation is sparse, and flowering is uneven. Last season, pods were poorly filled with many 'pops' and dark patches. Rainfall was erratic, and the field has a slope of 2-3%. She plans to repeat the same fertilizer dose and add 25 kg urea/ha at flowering.
1. Diagnose the main nutritional and soil constraints. Why is the current DAP-plus-urea program not addressing them? Include calculations of the N and P2O5 already applied.
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What the soil test shows. pH 5.3 is strongly acidic for groundnut; exchangeable Ca 1.2 cmol/kg, S 6 mg/kg and B 0.28 mg/kg are all low. The 'pops' and dark patches are consistent with Ca/B deficiency and poor pod filling, not just N shortage. Sparse nodulation is likely from acidity, low Ca and drought.
What she applied. 150 kg DAP/ha supplies 27 kg N and 69 kg P2O5; 50 kg urea supplies 23 kg N. Total = 50 kg N and 69 kg P2O5, with no K, no lime, no gypsum and no micronutrients. The N is already more than groundnut needs at sowing and can suppress nodulation. The P is high on paper, but at pH 5.3 much of it is fixed by Fe/Al and not available. Repeating this mix and adding 25 kg urea at flowering will not correct Ca, S, B or acidity; extra N may delay maturity and increase vegetative growth.
2. Design a corrected package for the next groundnut crop on 1 ha. Give quantities, timing and placement for lime, gypsum, basal fertilizer, S and B. Explain why lime and gypsum are not interchangeable.
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Corrected package for 1 ha. Before sowing: apply 2 t agricultural lime/ha (confirm with lime requirement; target pH 6.0-6.5), broadcast and incorporate 2-3 weeks before sowing. Do not mix lime with DAP or urea. Basal at sowing: DAP 100 kg/ha (18-46-0) + MOP 70 kg/ha (60% K2O) + urea 15 kg/ha. This gives about 25 kg N, 46 kg P2O5 and 42 kg K2O/ha. Apply 10 kg borax/ha in a band with the basal fertilizer, not broadcast over the plants. Add 5-10 t FYM/ha. At 40-45 DAS, at pegging, apply 400 kg gypsum/ha in a band near the pod zone; this supplies Ca and S without raising pH. Lime corrects acidity and supplies Ca before sowing; gypsum is a neutral salt for Ca/S at pegging and cannot replace lime on an acid soil.
3. How would you monitor the crop and adjust if drought occurs? What would you not do?
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Monitor young leaves for S (pale, uniform), B (hollow stem, cracked pods), and Ca (pops). At 45 DAS check nodule number and color; if nodules are few and white, do not add N-check moisture and pH. If drought occurs at flowering/pegging, mulch with crop residue, keep the pegging zone undisturbed, and give light irrigation if available; do not apply lime at flowering or disturb the pegging zone with intercultivation. Do not apply more urea as a rescue; it will not fix Ca/B/S and can worsen lodging. If B deficiency persists in the next test, use a small foliar correction only after confirming with a leaf test.