FINGER MILLET (Eleusine coracana) AND SOYBEAN (Glycine max) RESPONSES TO NPK FERTILIZER IN ZINC-AMENDED SOILS OF WESTERN KENYA

Abstract

Low agricultural productivity in SSA is attributed primarily to declining soil fertility. About 65% of the agricultural soils in SSA are degraded as a result of nutrient depletion without adequate replenishment. Under normal circumstances within the region, Nitrogen (N), phosphorus (P) and potassium (K) are the commonly addressed deficiencies through fertilizer application. However; crops are becoming less responsive to that strategy. Zn deficiency has been identified to contribute significantly to that low response of crops to NPK fertilizer leading to low economic returns on fertilizer use. Despite being droughttolerant crops, finger millet (Eleusine coracana) and soybean (Glycine max) yields are consistently lower than 1000 kg ha-1 in western Kenya with or without NPK fertilizer application. Therefore, on-farm trials were conducted in Bungoma and Siaya Counties of western Kenya during two subsequent cropping seasons (long & short rains of 2019). The study evaluated responses of two varieties of finger millet (U15 and SEC915) and soybean (SB19 and SB134) to NPK after addition of Zn. Zn was applied at 0, 1.5, and 3 kg ha-1 and NPK at blanket rates of 100 kg ha-1 N, 30 kg ha-1 P, 50 kg ha-1 K. Soil parameter that include soil nutrient status at 0, 90 and 134 days and plant tissue parameters including leaves and grain nutrients contents were measured at different stages of soybean and millet growth and development. Results indicate that addition of Zn to NPK fertilizer application is a valuable strategy to improve N bioavailability, mobilize P and exchangeable K in soils. However, no significant difference (p > 0.05) was observed between the treatments. Further, the strategy significantly (P < 0.05) increases P and Zn leaf content in finger millet while in soybean significant (P < 0.05) increases in P, K and Zn leaf content were observed. Results also showed that application of Zn alongside NPK fertilizer significantly (p < 0.05) increased the grain Zn content and uptake which translated to larger grain yields. Further, the largest agronomic Zn fertilizer efficiency (AEZn) was realized when Zn was applied at 1.5 kg ha-1 for both crops. For all Zn fertilizer treatments, there were strong and positive relationships between grain yields and Zn uptake. Apparent Zn recovery was low for both crops in the first season which led to an accumulation of this micronutrient in the soil. This later increased the root-shoot translocation of Zn into finger millet and soybean leaves. Generally, the application of Zn alongside NPK fertilizer was profitable for both crops at all sites. The study recommends applying Zn along with NPK fertilizer in Zn-depleted soils to increase finger millet and soybean yields. Further research should be carried out to establish these other limitations that hinder both crops from reaching their potential yield.