Nitrogen top-dressing in flooded boro rice after unseasonal rain
A farmer in Nagaon district has 1.2 ha of transplanted boro rice (cv. Ranjit) at 45 days after transplanting. At transplanting he incorporated 30 kg N, 20 kg P2O5 and 20 kg K2O per hectare. The crop is now pale green, with yellowing starting from the tips of older leaves; tiller count is 210/m2 against a target of 300/m2. For the past four days the field has been under 12-15 cm of standing water after continuous rain, and the forecast is for isolated showers for three more days, then clear weather. Soil test before transplanting: organic carbon 0.62%, available N low (185 kg/ha), available P medium, available K high. The state recommendation is 60 kg N/ha for boro rice, in three splits. The farmer has 45 kg of urea in his store and asks whether to broadcast it immediately into the standing water.
1. Diagnose the most likely cause of the pale crop. Explain why broadcasting the entire 45 kg urea into 12-15 cm of standing water now would be a poor decision, naming the main nitrogen loss pathways involved.
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Diagnosis. The symptoms fit nitrogen deficiency rather than a disease: general pale green colour, yellowing beginning on older leaf tips, and tillering well below the target. The soil test also shows low available N. Waterlogging and rain do not create a new pathogen here; they create conditions for N loss.
Why immediate broadcasting is poor practice. Urea hydrolyses to ammonium in the floodwater and surface soil. In a flooded field, the surface layer can remain oxidised while the bulk soil is reduced. Ammonium in that thin oxidised layer can be converted to nitrate, which then moves into the reduced layer and is lost as nitrogen gas and nitrous oxide through denitrification. Broadcasting into deep standing water also allows runoff with further rain and some ammonia volatilisation, especially if the water is warm and the field is not drained. Much of the applied N may never reach the root zone.
Quantities. The crop has already received 30 kg N/ha, so 30 kg N/ha remains. For 1.2 ha that is 36 kg N, equivalent to about 78 kg urea at 46% N. The farmer's 45 kg urea contains only about 20.7 kg N, not even enough for the full remaining dose over 1.2 ha. Applying it all now into floodwater would waste a scarce input.
2. Using the 45 kg urea he has, give a practical allocation and application plan. State the timing, field water condition, method, and the calculation behind your allocation.
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Allocation. 45 kg urea × 0.46 = 20.7 kg N. If the priority dose is 25 kg N/ha, that quantity covers 20.7 ÷ 25 = 0.83 ha. He should therefore treat the most deficient 0.8 ha first, not spread a thin, ineffective dose over 1.2 ha.
Timing and water management. Wait until the heavy showers stop. Drain or open the field so that the water falls to a saturated, muddy condition rather than 12-15 cm of standing water. Apply urea when rain is not expected for at least 24 hours, preferably in the cooler part of the day.
Method. Broadcast uniformly on the muddy surface and incorporate lightly, or band it between rows, so the urea reaches the reduced soil zone rather than sitting in floodwater. Do not apply to dry, cracked soil and then flood immediately, and do not apply just before another heavy shower.
Follow-up. Use a leaf colour chart on the untreated and treated areas. Apply the next split when readings fall to 3 or below on at least the critical number of leaves. The remaining 0.4 ha and the rest of the requirement should receive 15-20 kg N/ha after drainage, followed by a final dose at panicle initiation if the crop is still pale. This converts one risky application into a staged, need-based programme.
3. If continuous rain prevents draining for another week and the crop reaches panicle initiation, what alternative would you consider, and what precautions would you take?
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Alternative. A foliar urea spray can be used as a temporary supplement when soil application is impossible. A 2% urea solution means 20 g urea per litre; at 500 litres per hectare this supplies about 10 kg urea, or 4.6 kg N/ha per spray. Two sprays 7-10 days apart supply only a small part of the 30 kg N/ha requirement, so this is a holding measure, not a replacement for soil-applied N.
Precautions. Spray in the early morning or evening to avoid leaf scorch. Do not exceed about 2% urea, because higher concentrations can burn rice leaves. Ensure the spray is applied to the leaf surface, not into the floodwater. Do not mix with incompatible pesticides without checking. Once the field can be drained, return to soil application and incorporate the urea; the foliar sprays should be credited when calculating the remaining soil N dose, so the crop is not over-fertilised later.