Effect of different sowing times and potash levels on insect pest populations, nutrient availability, uptake, yield and economics of soybean (Glycine max)

A field experiment was conducted during rainy (kharif) seasons of 2012–15 at Kasabe Digraj, Sangli, Maharashtra, to study the effect of different sowing times and potash levels on insect-pest populations, nutrient availability, uptake, yield and economics of soybean [Glycine max (L.) Merr.]. The treatments consisted of 3 sowing times, viz first, second fortnight of June and first fortnight of July as a main treatments; and 0, 15, 30 and 45 K2O kg/ha as subtreatments. The stem length tunnelling percentage, caused by stem fly and Spodoptera signifi-cantly increased with the late sowing and ranged between 12.84–14.12 among the sowing times. The reduction in population of aphids, Spodoptera and stem tunneling was recorded in 45 K₂O kg/ha applied to soybean as compared to the control. The effect of different sowing times was found non-significant on soil properties except electrical conductivity (EC). Different levels of potash given to soybean exerted significant influence on EC of soil, available nitrogen (N), phosphorus (P) and also potassium (K) in soil after crop harvest. The highest EC of soil and available K were recorded in treatment where potash was applied @ 45 kg/ha as compared to rest of the levels of this nutrient tried and at par with application of potash @ 30 kg/ha in respect of EC, available N, P and K content in soil. Significantly highest total uptake of N, P and K of soybean was observed in the first fortnight of June as compared to the first fortnight of July. The highest total uptake of N, P and K by soybean was recorded in treatment with potash applied @ 45 kg/ha as compared to the remaining levels of potash. Significantly highest grain yield (1.99 t/ha) of soybean was registered in plot sown during the first fortnight of June over all those sown thereafter. Similarly, it also remained highest in treatment receiving potash @ 45 kg/ha. The sowing of the soybean crop during the first fortnight of June resulted in significantly higher net monetary returns of 15.49 × 10³/ha with significantly highest benefit: cost (B: C) ratio of 1.58. The benefit: cost ratio of potash applied @ 30 and 45 kg/ha remained at par. Application of 5 tonnes FYM/ha with 50, 75 and 45 kg N, P₂O₅ and K₂O kg/ha to soybean sown in first fortnight of June is recommended as a revised fertilizer nutrient dose for medium deep black soils for minimizing insect-pest populations, getting higher yield, monetary returns and maintaining soil fertility.