Abstract:

The majority of soils in Africa are P-deficient, but the high cost of conventional, water-soluble P fertilisers limits their use by resource-poor farmers. Rock phosphates are a low-cost alternative. The relative agronomic effectiveness of unacidulated (RP), 25% partially acidulated (PARP25), 50% partially acidulated (PARP50) Minjingu rock phosphate and triple superphosphate (TSP) was evaluated at rates of 0–80 kg/ha P on stylo (Stylosanthes guianensis) on an Ultisol in the Ethiopian highlands. The fertilisers were applied once and their effects were followed for 4 consecutive harvests. Stylo dry matter (DM) yields were below 3 t/ha at each cut without applied P and reached as high as 4.8 t/ha when P was applied. Over all 4 cuts, RP was 100%, PARP25 was 89% and PARP50 was 103% as effective as TSP in increasing stylo herbage yields. The corresponding relative responses in P uptake were 103, 79 and 92% for RP, PARP25 and PARP50, respectively. The substitution rates for herbage yields were 100% for RP, 79% for PARP25 and 106% for PARP50 while those for P uptake were 106% for RP, 62% for PARP25 and 85% for PARP50. Significant (P < 0.05) effects of P on stylo DM and P uptake were observed at all harvests.