Late Rice Harvest and a Short Window for Blackgram
Murugan farms 1.2 ha of clay loam paddy land in the Thanjavur delta. His ADT 43 rice crop was harvested on 12 December, but the field is still wet and the canal water release ends on 20 January; the next rice season starts only in June. He wants to raise a pulse crop on residual moisture. He has 20 kg of VBN 8 blackgram seed (65-day duration) and 15 kg of a local blackgram variety (85 days). Labour is scarce and expensive, so he can afford only one rotavator pass and one life-saving irrigation. The previous rice crop received 100:50:50 kg N:P2O5:K2O per ha, and no gypsum was applied for several years. Night temperature is around 22°C and day temperature 30°C. He asks whether he should sow at all, which seed lot to use, and how to place the seed in the wet field.
1. Should Murugan sow the pulse crop, and which seed lot should he use? Show the timing calculation.
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Sow, but only with VBN 8. Sowing around 18–20 December gives VBN 8 (65 days) a harvest window around 21–23 February. Flowering will begin near 30–35 days after sowing, i.e. about 17–24 January, just before the canal closes on 20 January. That means the crop can reach flowering and early pod fill on residual moisture plus one irrigation. The local 85-day variety would flower in late January to early February and fill pods through February, when no canal water is available; that is a high risk of terminal moisture stress and poor grain filling. Seed rate also matters: VBN 8 at about 16–17 kg/ha uses the 20 kg lot for the full 1.2 ha. The local seed lot should not be used simply because it is available. If the field is still too wet to traffic, wait a few days rather than puddle it further, but do not delay beyond the last week of December.
2. How should he place the seed and treat it, given one rotavator pass and scarce labour?
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Treat the seed with Rhizobium and Trichoderma before sowing. In a wet clay field, deep placement is risky because the seed can be smothered or the surface can crust. Broadcast the treated seed uniformly, then run the rotavator at shallow depth, about 5–7 cm, to cover it. This uses the single pass for both seed covering and soil loosening. Open shallow drainage furrows at 3–4 m intervals so that unexpected rain does not waterlog the crop. Do not sow into standing water or into a fully puddled surface. A light compaction after covering helps seed-soil contact. Because labour is scarce, avoid line sowing that needs extra markers and gap filling; uniform broadcast at the correct seed rate is the better trade-off here.
3. When should the single irrigation be applied, and why not earlier?
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Save the irrigation for the flowering to early pod-fill stage, roughly 30–35 days after sowing. That is the most moisture-sensitive stage; water stress then causes flower drop and poor pod set. Irrigating at 15–20 days would encourage vegetative growth, keep the clay soil saturated, and increase the risk of root rot and waterlogging, while wasting the only available irrigation. Check the field before deciding: if the soil at 10 cm depth is still moist and plants are not wilting by late morning, wait. If leaves wilt before noon and the soil cracks, advance the irrigation slightly to about 25 days. After irrigation, drain excess water rather than leaving it ponded, because blackgram is sensitive to waterlogging.