Summary and Conclusions

Soybean occupies a very significant place in global oilseed production as well as being the predominant source of protein in the vegetarian diet of people. The productivity of soybean in many countries is around 2 t ha-1, with a gap of 1 t ha-1 between potential and observed yield. Emerging mul-tinutrient deficiencies (e.g. of nitrogen, phosphorus, sulphur, zinc, iron, boron) in soils, coupled with the application of only nitrogen and phosphorus to major crops by farmers (and that at lower rates than recommended), is one of the major reasons for lower productivity. Sulphur deficiency is also widespread in soybean-growing areas because of the farmers' preference for DAP as a source of phosphorus rather than sulphur-containing SSP.

Research in the recent past has conclusively shown that soybean productivity can be increased and sustained through optimum nutrient management. The application of nitrogen at 35 kg ha-1 as a starter is recommended on nitrogen-deficient soils. Optimum rates of application of phosphorus and potassium depend upon their availability in the soil. If soils contain higher available phosphorus, maintenance fertilization is recommended. Repeat applications of phosphorus and sulphur lead to their accumulation in the soil, which may reduce the rates of application in subsequent crops. Therefore, these fertilizers should be applied on the basis of soybean-based cropping systems as a unit, rather than as a single crop.

Among the micronutrients, zinc deficiency is most widespread. An application of 5 kg Zn ha-1 as zinc sulphate to soybean has been found to adequately meet the zinc requirements of both soybean and wheat crops in soybean-wheat rotation on black soils of central India. The application of 4 kg borax ha-1 has been found to be effective in correcting boron deficiency on black soils of India. Keeping in mind multinutrient deficiencies, the balanced application of all deficient nutrients at the recommended rates may be essential for sustaining higher productivity. Among all of the nutrient management options, the conjoint use of organic manures, biofertilizers and fertilizers in INM has proved efficient and economical in achieving high yields of soybean. The application of fertilizer nutrients based on target yield and soil test value could be helpful in saving on costly fertilizers. Many small and marginal farmers in developing countries feel that the application of fertilizers is risky as there are no guarantees that they will be able to harvest a good soybean crop due to other associated problems (e.g. pests, weeds and waterlogging) in soybean production. Therefore, improved nutrient management technologies should be recommended and popularized among farmers along with other pest, weed and water management options as a package of practices.

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