Alkaline Soil, Saline Tube Well, and a Neighbour's Fertilizer Plan
Anju Devi farms 1.6 ha of sandy loam in Jhunjhunu district, Rajasthan. Her soil test reports pH 8.7, EC 0.72 dS/m, organic carbon 0.29%, available N 118 kg/ha (low), available P 19 kg/ha (medium) and available K 310 kg/ha (high). She irrigates wheat from a tube well whose water has EC 1.9 dS/m and residual sodium carbonate 3.1 meq/L. The wheat crop is at crown-root initiation. The stand is uneven; leaves on the sandiest patches are pale yellow, and a white crust forms along the furrows after each irrigation. She has 4 t of well-rotted FYM, 2 t of gypsum, urea, DAP and muriate of potash. Her plan, copied from a neighbour, is to broadcast 3 bags of DAP and 2 bags of MOP per hectare before the first irrigation. Each bag is 50 kg. She wants to know whether to spend her limited cash on more fertilizer or on something else.
1. Check Anju Devi's planned fertilizer dose against the soil test. What should she apply before the first irrigation, and what should she not buy? Show the nutrient calculation.
Read the worked answer
Three bags of DAP per hectare = 150 kg product. At 18% N and 46% P2O5, this supplies 27 kg N and 69 kg P2O5. Two bags of MOP = 100 kg product, supplying 60 kg K2O. The soil test shows available K at 310 kg/ha, which is high, so the MOP is an unnecessary purchase; wheat on this soil is unlikely to respond to it. Available P is medium, so 69 kg P2O5 is more than the crop needs. A more economical dose is 2 bags DAP/ha (100 kg, giving 18 kg N and 46 kg P2O5), which still covers the medium P status. To reach about 120 kg N/ha, she then needs roughly 102 kg N from urea, i.e. about 222 kg urea/ha. Apply half before the first irrigation and half at tillering. Do not buy more MOP. The money saved is better spent on zinc sulphate for the pale patches and on incorporating the FYM. The pale yellow patches on a high-pH sandy soil suggest possible zinc deficiency; a foliar spray of 0.5% ZnSO4 with 0.25% lime is a reasonable diagnostic-treatment step, but confirm with a DTPA-Zn test.
2. She has only 2 t of gypsum and 4 t of FYM. How should she allocate them over 1.6 ha, and how should she manage the poor-quality irrigation water?
Read the worked answer
Two tonnes of gypsum over 1.6 ha is about 1.25 t/ha. That is below a full reclamation dose for a soil with pH 8.7 and irrigation water of RSC 3.1 meq/L, but it is still useful as a maintenance application. Apply it uniformly before the first irrigation and incorporate it into the top 10 cm rather than leaving it on the surface. The 4 t of FYM works out to 2.5 t/ha; spread and incorporate it at the same time. Do not mix urea or DAP directly with the gypsum in one band. For the water, avoid light, frequent irrigations that leave salts in the root zone. Give heavier but less frequent irrigations, and use canal water to dilute or alternate with the tube well if any canal supply is available. Ensure the field drains freely; leaching sodium without drainage only moves the problem sideways. Mulch between furrows to reduce evaporation and crusting. Monitor EC and pH after the season, and do not judge the gypsum by one crop.
3. Explain why the white crust appears and its likely effect on wheat nutrition. Name two practical checks Anju Devi should make.
Read the worked answer
The white crust is salt accumulated at the evaporation front. The tube well water has EC 1.9 dS/m and RSC 3.1 meq/L, and the soil already has EC 0.72 dS/m. When water moves upward by capillary action and evaporates, dissolved salts are left behind along the furrows. The high pH and sodium status reduce the availability of phosphorus, zinc and iron, which explains the pale patches and uneven stand better than a simple nitrogen shortage. Two practical checks are: (1) measure soil EC in a 1:2 soil-water extract from the crusted and non-crusted zones, and (2) test DTPA-extractable zinc, since the pale sandy patches are consistent with zinc deficiency. A leaf tissue test would confirm whether the crop is actually taking up the nutrients. The crust itself is a warning to manage water and amendments, not a reason to add more DAP.