Phosphorus availability and maize response to organic and inorganic fertilizer inputs in a short term study in western Kenya

Abstract

The use of organic materials as P sources is of considerable interest in smallholder farming systems in tropical Africa, mainly because of their potential as alternatives to inorganic P fertilizers. Field studies conducted in a Nitisol of western Kenya in 1995 (crop 1) and 1996 (crop 2) compared effects of organic and inorganic fertilizers on resin extractable P availability (P ext) and maize ( Zea mays L.) yield (M Y). Leaf biomass and small twigs of Tithonia diversifolia, Croton megalocarpus, Lantana camara L., Senna spectabilis, Calliandra calothyrsus, and Sesbania sesban, were applied at 5 Mg ha −1 (DW), supplying an estimated 9–15 kg P ha −1 and 30–212 k N ha −1. The inorganic fertilizer was triple superphosphate (TSP), applied at 0 (control), 10, 25, 50, and 150 kg P ha −1 with each plot receiving 120 kg N ha −1 as urea. All plots received a blank application of 100 kg K ha −1 as potassium chloride. Between 92 and 98% ( P<0.001) of the variation in P ext was explained by the P added (P add) from the amendments. Response of M Y to P add was best described by logarithmic function as: M Y =0.78 ln( P add )+0.04 ( R 2=0.91 ∗∗∗ ) for crop 1, and M Y =0.93 ln( P add )−0.5 ( R 2=0.90 ∗∗∗ ) for crop 2. Response of M Y to P ext was best described by a linear function with R 2 ranging from 0.84 to 0.89 for crop 1, and 0.76 to 0.81 for crop 2. Effects of Tithonia and Croton on M Y were similar to effects of 50 kg P ha −1+120 kg N ha −1 as inorganic fertilizer. Although the confounding effects of nutrients other than P in the organic materials on M Y were isolated, it appears that reasonable M Y can be achieved if adequate amounts of high quality organic materials such as Tithonia and Croton are used as P sources. A term, ‘phosphorus availability index’ (PAI), calculated as: (P ext treatments−P ext control)/P added, was introduced to describe the P availability capacity of the amendments. The PAI values suggested a greater propensity for net P mineralization following addition of all organic materials (except S. spectabilis) than for the inorganic amendments. The PAI results indicate that improvements of soil P ext can come from either P released from organic inputs or increased availability of native soil P following addition of organic and inorganic fertilizers.