Response of Sorghum (Sorghum bicolor (L.) Munch) and Chemical Characteristics of Soil to Organic and Inorganic Fertilizers on Kenyan Lower Midlands Acid Soil

Abstract

Soil acidity, low level of nitrogen (N), phosphorus (P) and soil organic matter are major constraints to sorghum production in Kenya. We investigated the effect of farmyard manure (FYM) and combined application of inorganic nitrogen and phosphate fertilizers applied as calcium ammonium nitrate and triple superphosphate respectively, on soil pH, available P, total N, organic carbon and sorghum grain yield, on a smallholder farm in Siaya County, Kenya. The experiment was a randomized complete block design consisting of four treatments i.e., FYM applied at 0 and 4 t ha-1 in a factorial arrangement with inorganic fertilizers applied to provide 0 and 26 kg P ha-1 + 75 kg N ha-1, replicated three times. The study was conducted during the long rains of March to June and short rains September to December, 2017. Farmyard manure was applied only during the long rains season of 2017, while P and N fertilizers were applied in both cropping seasons. Soil sampling was done before treatment applications and at harvest time. Soil analysis revealed that the soil was acidic (pH = 4.9) with low available P (5.3 mg kg-1 and N (0.08%) but moderate amounts of C (2.0%) and Ca (3.4 cmol kg-1). The effect of sole application of FYM on soil P, N and grain yield were lower than those of the inorganic fertilizers. Farmyard manure increased soil pH because of its alkalinity (pH = 7.1) while triple superphosphate reduced it due to release of phosphoric acid into the soil. The mean annual (average of two seasons) grain yield increments, above the control with no nutrient inputs, due to 4.0 t FYM, 26 kg P ha-1 + 75 kg N ha-1 and 4.0 FYM + 26 kg P ha-1 + 75 kg N ha-1 were 64, 191 and 259%, respectively. Therefore combined applications of FYM and N and P fertilizers have the potential to increase sorghum grain yield on Kenyan acid soils.