The reactivity of Tanzanian Minjingu phosphate rock can be assessed from the chemical and mineralogical composition

Minjingu phosphate rock (MPR) in northern Tanzania with an estimated reserve of 7 million tonnes consists of two types including hard MPR and soft MPR. Both types seem promising for direct application as phosphorus (P) fertilizers but improved knowledge about their reactivity is needed. Since mineral reactivity depends on the chemical and mineralogical composition and crystal size, these properties were investigated on different MPR samples by means of combined chemical, X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM)-microprobe analyses and compared with similar properties of five well-known sedimentary and igneous PR references. As a direct test on reactivity, the solubility of MPR and reference samples in neutral ammonium citrate (NAC) was determined. The chemical composition indicated that the apatites in both MPR types are fluorine deficient francolites. For hard MPR this is in agreement with the mineralogical investigation, while XRD suggests that soft MPR belongs to excess fluorine francolites with low carbonate substitution, which generally have low reactivity. However, the observed very small crystal size together with high substitution rates of carbonate for phosphate and of strontium for calcium can explain the high reactivity of soft MPR. In agreement with the rather high NAC solubility, i.e. 4.2% and 6.2% P2O5 for hard and soft MPR, respectively, these properties place hard MPR and soft MPR among the medium to highly reactive PR, respectively. Consequently, both hard and soft MPR may be considered suitable for direct application as P fertilizers. This is in good accordance with the results of previously performed field and pot trials showing that direct application of hard MPR or soft MPR resulted in maize yields comparable to the yields obtained using water-soluble phosphate (triplesuperphosphate) on acid soils low in available P and Ca in sub-humid to humid Tanzania.