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

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 (Pext) and maize (Zea mays L.) yield (MY). 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 Pext was explained by the P added (Padd) from the amendments. Response of MY to Padd was best described by logarithmic function as: MY=0.78 ln(Padd)+0.04 (R^2=0.91***) for crop 1, and MY=0.93 ln(Padd)-0.5 (R^2=0.90***) for crop 2. Response of MY to Pext 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 MY 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 MY were isolated, it appears that reasonable MY 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: (Pext treatments-Pext 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 Pext can come from either P released from organic inputs or increased availability of native soil P following addition of organic and inorganic fertilizers.