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

摘要

The use of organic materials as P sources is of considerable interest in smallholder fanning 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 My to Padd was best described by logarithmic function as: M-Y = 0.78 In(P-add) + 0.04 (R-2 = 0.91***) for crop 1, and M-Y = 0.93 In(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 My 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: (Peat 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.