Effect of organic and inorganic phosphorus sources on maize yields in an acid soil in western Kenya

Maize production in western Kenya is commonly limited by P deficiencies and aluminum phytotoxicity. Due to high costs of imported fertilizers and lime, focus is now shifting to solutions that utilize local resources. We tested the effect of three inorganic P sources i.e., triple superphosphate (TSP), Minjingu phosphate rock (MPR) and Busumbu phosphate rock (BPR), each applied in combination with two organic materials (OMs) i.e., farmyard manure (FYM) and Tithonia diversifolia green manure (tithonia), or with urea on soil chemical properties related to soil acidity, P availability and maize yields for three consecutive seasons in western Kenya. The OMs and inorganic P sources were applied to provide 20 and 40 kg P ha−1 respectively in their combination. Where urea was used, the inorganic P sources were applied at 60 kg P ha−1. Maize did not respond to application of TSP, MPR or BPR with urea in the first two seasons. However, after three seasons, maize significantly responded to application of MPR with urea. FYM was more effective than tithonia in increasing the labile inorganic P pools but it gave lower maize yields than tithonia which was more effective in reducing the exchangeable Al. It appears that the ability of an OM to lower the exchangeable Al is more important in increasing maize yields than its ability to increase P availability. The effectiveness of the inorganic P sources in increasing maize yields followed the order of their effectiveness in increasing available P, i.e., TSP > MPR > BPR, once Al phytotoxicity was reduced by application of tithonia but the difference between TSP and MPR was not significant. The extra maize yield obtained by the additional 40 kg P ha−1 from the inorganic P sources was, however, in most cases not substantial enough to justify their use. Economic considerations may therefore favour the use of tithonia or FYM when applied alone at 20 kg P ha−1 than when combined with any of the inorganic P sources used in this study at a total P rate of 60 kg ha−1.