Agronomic effectiveness of unacidulated and partially acidulated Minjingu rockphosphates on Stylosanthes guianensis

The majority of soils in Africa are P-deficient, bur the high cost of conve ntional, water-soluble P fertilisers limits their use by resource-poor farm ers. Rockphosphates are a low-cost alternative. The relative agronomic effe ctiveness of unacidulated (RP), 25% partially acidulated (PARP25), 50% part ially acidulated (PARP50) Minjingu rockphosphate and triple superphosphate (TSP) was evaluated at rates of 0-80 kg/ha P on style (Stylosanthes guianen sis) on an Ultisol in the Ethiopian highlands. The fertilisers were applied once and their effects were followed for 4 consecutive harvests. Style dry matter (DM) yields were below 3 t/ha at each cut without applied P and rea ched 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 style 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 whi le those for P uptake were 106% for RP, 62% for PARP25 and 85% for PARP50. Significant (P < 0.05) effects of P on style DM and P uptake were observed at all harvests. It is concluded that raw Minjingu rockphosphate is highly effective on style in these soils. This rockphosphate could be used to incr ease forage production for increased and sustainable crop-livestock product ivity on the P-deficient Ultisols.