NUTRIENTS LIMITING SOYBEAN (Glycine max L) PRODUCTION IN ACRISOLS AND FERRALSOLS OF KAKAMEGA AND BUSIA COUNTIES

Abstract

Soybean is a good source of protein and calories and therefore important for food security. Soybean yields in western Kenya stand at 0.6 t ha-1 season-1against a potential of 3 to 3.6 t ha-1 season-1. This has been attributed to low soil fertility and a lot of work has been carried out on nitrogen (N) and phosphorus (P) nutrition but yields are still low leading to suspicion that other nutrients could also be limiting. To investigate this, two trials were set up. The first was a nutrient omission trial set up in the greenhouse to diagnose nutrients limiting soybean production in five distinct locations; Kakamega (Shikhulu and Khwisero sub –locations), Masaba central, Butere and Butula sub counties. Each of the soils collected from the different sites were subjected to ten nutrient solution treatments; 6 macronutrients (N, P, K, Ca, Mg and S) tested separately, micro-nutrients (B, Mo, Mn, Cu, Zn) in one treatment, a complete (macro and micro-nutrients), a control (distilled water) and a lime treatment. The experiment was laid out in Completely Randomized Design. Omission of nutrients from the nutrient solutions resulted in their low concentrations in plant tissues from all the soils. Potassium for instance had, 0.97, 0.78, 1.24, 1.14 and 1.78% in Masaba central, Khwisero, Shikhulu, Butere and Butula respectively. These concentrations were lower than the sufficient ranges in all the soils except in Butula. Significantly (P ≤ 0.05) lower shoot dry weights (SDWs) were obtained from omission of K, Mg, P and micro-nutrients in soils from Masaba central and Butula, Mg and P in soils from Khwisero and K, Mg and P in soils from Shikhulu and Butere. Lime application significantly (P ≤ 0.05) improved soil pH and SDWs in soils from Masaba central and Butere. In conclusion, K, Mg, P and micro-nutrients are limiting in most of the soils. In the second trial, field experiments were carried out in Masaba central and Butula to assess the effect of manure and Sympal (0:23:15 plus 10% CaO, 4% S, 1% and MgO) application as sources of macro-nutrients and micro-nutrients on soybean performance and to assess their effect and lime application on selected soil chemical properties and soybean yields. The treatments included; control, Sympal, manure, Sympal plus manure, Sympal plus micro-nutrients (B and Mo), manure plus micro-nutrients, Sympal plus lime, manure plus lime, Sympal plus micro-nutrients plus lime. The experiment was laid out in completely randomized block design. Sympal and farmyard manure were applied at 30 kg P/ha, lime at 2 t/ha, B at 0.44 kg/ha and Mo at 0.57 kg/ha. Manure significantly improved soil pH (8%) and total N (12.5%) in Masaba central and available P by 28% in Masaba central and 31% in Butula. Sympal, significantly improved soil pH by 8.1% in Butula and available P by 51% in Masaba central and 58.3% in Butula. Liming significantly (P ≤ 0.05) improved soil pH by 11.7% in Masaba central and 16.4% in Butula and total N in combination with Sympal by 12.5% in Masaba central. Number of active nodules was significantly improved by micro-nutrients in Masaba central and Sympal plus manure in both sites. Manure significantly increased nodule dry weights, 2.0 and 4.03 g in Masaba central and Butula respectively. Micro v nutrients and lime in combination with Sympal and manure significantly improved soybean grain yields in both sites. From the results, fertilizers that supply macro (P, Ca, Mg, and S) and micro (B and Mo) nutrients and lime may be adopted to improve soil pH, available P, total soil N and soybean grain yields.