Hard Minjingu Phosphate Rock: an Alternative P Source for Maize Production on Acid Soils in Tanzania

The phosphate rock (PR) deposit at Minjingu in northern Tanzania consists of two forms, locally called hard Minjingu phosphate rock, MPR (4.8 × 106 Mg with 10.6% P) and soft MPR (3.3 × 106 Mg with 13.3% P). Extensive chemical and instrumental analyses have shown that the two MPRs differ mainly in consistency, while their reactivities are comparable. Soft MPR in direct application has been extensively evaluated with good results under greenhouse and field conditions whereas hard MPR has only been tested in a greenhouse experiment. The agronomic value of directly applied hard MPR on maize growth was therefore tested under field conditions on four acidic soils low in available Ca and P at Magadu (Ultisol), Mlingano (Oxisol), Nkundi (Ultisol) and Sasanda (Andisol). The treatments tested were hard MPR, soft MPR, triple superphosphate (TSP) and a control. Each P source was applied at a rate of 80 kg ha−1 P at Magadu, Mlingano and Nkundi but 160 kg ha−1 P at Sasanda. Other nutrient deficiencies were corrected in each soil with appropriate fertilizers that were applied in all three years of experimentation. The soils gave significant positive responses to application of the three P sources but TSP application resulted in significantly higher P concentrations in leaves and grain yields than MPR addition in the first year. However, in the second and third years the performance of MPRs approached that of TSP and the relative agronomic effectiveness (RAE) of MPRs increased from 50 – 70% in the first year to 80 – 95% in year three. Moreover, crop performances on hard MPR-treated plots and soft MPR-treated plots were not significantly different. This important result suggests that hard MPR can replace soft MPR and even TSP on acidic soils low in available Ca and P. However, more testing is needed to ensure confident delineation of soils that respond to direct application of hard MPR from non-responding soils. The rather poor performance of MPR for the first year must also be improved.