Organic residues affect phosphorus availability and maize yields in a Nitisol of western Kenya

The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into t he soil at 5 Mg ha-l for six consecutive seasons in 3 years and responses c ompared with those following the application of 120 kg N ha(-1), 0 kg P ha( -1) (0P); 120 kg N ha(-1), 10 kg P ha(-1) and 120 kg N ha(-1) 25 kg P ha(-1 ) as urea and triple superphosphate (TSP); K was supplied in all treatments ; Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Call iandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tit honia compared to OF, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton tre atment. P recovered in the aboveground biomass and resin P, immediately aft er the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35-77%) treatments than in the inorganic fer tilizer treatments (21-27%). The P content of organic residues, and the sol uble C:total P ratio, were the main residue parameters predicting soil P av ailability and maize yield. All organic residues used in this study can rep lace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of C roton, Lantana and Tithonia.