Soil Test, DAP Stock and Zinc-Deficient Patches
Gurmeet has 5 acres under wheat after potato. The soil test report shows organic carbon 0.34%, available nitrogen low at 180 kg/ha, Olsen phosphorus 9 kg/ha (low), available potassium 310 kg/ha (high), pH 8.4, EC 0.18 dS/m and DTPA-zinc 0.38 ppm. The block agriculture office recommends 50 kg N, 25 kg P2O5 and 12.5 kg K2O per acre, plus zinc sulphate where the soil is deficient. Gurmeet already has 4 bags of DAP (50 kg each), 6 bags of urea and 1 bag of MOP at home. He wants to buy nothing more because DAP is expensive this year, and plans to apply only urea in three splits. He also points to pale, stunted patches about 15 m across near the tube well, which are more obvious on the old potato ridges. Sowing is fixed for 12 November.
1. Calculate the fertilizer materials needed per acre and for 5 acres if DAP is used as the phosphorus source. Is his stock sufficient?
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For 1 acre: DAP 55 kg gives 25 kg P2O5 and 55 x 0.18, about 9.9 kg N. The nitrogen balance is 50 - 9.9 = 40.1 kg, so urea is 40.1/0.46, about 87 kg. MOP is 12.5/0.60, about 21 kg. For 5 acres: DAP 275 kg (5.5 bags), urea 435 kg (8.7 bags) and MOP 105 kg (2.1 bags). His stock of 200 kg DAP, 300 kg urea and 50 kg MOP is short by 75 kg DAP, 135 kg urea and 55 kg MOP. However, soil potassium is high at 310 kg/ha, so MOP can be dropped for this crop and that money used for the missing DAP; potash is not limiting here. Zinc sulphate is separate: 10 kg/acre for 5 acres is 50 kg. The minimum sensible purchase is therefore about 2 bags of DAP, 3 bags of urea and 1 bag of zinc sulphate, with no MOP.
2. Evaluate his plan to apply only urea.
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Applying only urea would supply nitrogen but no phosphorus. Wheat needs phosphorus early for root development, and in this field Olsen P is 9 kg/ha, which is low, while pH 8.4 reduces phosphorus availability. DAP placed near the seed at sowing is the efficient way to supply it; broadcasting phosphorus later cannot fully compensate. His plan also ignores the nitrogen already supplied by DAP, so he would over-apply urea. The saving on two bags of DAP is small compared with the likely yield loss; on a low-phosphorus soil a 2-4 quintal/acre difference is plausible, worth far more than the DAP cost. The correct plan is to buy the missing DAP, apply all phosphorus at sowing, and adjust urea to about 87 kg/acre. Split the urea between sowing and the first irrigation. Do not skip phosphorus merely because the previous potato crop received a heavy dose; the soil test measures what is actually available now.
3. What is the likely cause of the pale patches near the tube well, and how would you confirm and correct it?
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Pale, stunted patches near the tube well can be zinc deficiency, waterlogging or beginning root rot; do not assume nitrogen deficiency because the rest of the field is green. First check the soil at the patch: if it is saturated or the roots are brown and rotten, drainage and a root-rot check are needed, not zinc. If the soil is merely moist and roots are healthy, DTPA-zinc of 0.38 ppm, below the 0.6 ppm sufficiency mark, supports zinc deficiency, which often appears in patches in levelled or coarse spots. Confirm with a tissue test if possible. Correction: zinc sulphate at 10 kg/acre (25 kg/ha) mixed into the soil at sowing, or a 0.5% zinc sulphate foliar spray twice, 15 days apart. Do not mix zinc sulphate carelessly with DAP in the same drill box; apply separately for even placement. Re-test after harvest.