Establishment of an agronomic database for Minjingu phosphate rock and examples of its potential use

A compilation of all presently available agronomic data on the Tanzanian Minjingu phosphate rock (MPR) has shown to be a valuable tool for evaluating the interactions between tropical soils and MPR or water-soluble P sources (WSP). The effect of a delayed dissolution of MPR products compared to WSP was clearly demonstrated by the slope of the regression curve relating the yield response to MPR with the yield response to WSP. In the first 2 years, the average effectiveness of MPR is 74 and 94% of WSP respectively, whereas from the third year onwards the relative effectiveness is around 104%. From the analysis of the effect of soil properties on the yield response, it was found that low soil pH and low amounts of available P are the most favourable conditions for the effective use of MPR. However, beneficial effects were also indicated on less acidic soils if they are strongly P deficient. MPR application seems to have superior effect on soil properties such as pH, exchangeable Ca and Al compared with WSP. The effect of MPR application on available P and NaOH extractable P is significantly lower compared with WSP but the difference seems relatively small. An estimation of the distribution of the cultivated area of Tanzania into four suitability classes for direct application of MPR seems to be a useful tool in the process of selecting target areas where direct MPR application should be promoted.