The effects of combination of minjingu phosphate rock and single superphosphate fertiliser on dissolution of minjingu phosphate rock in soils

Abstract

The effect of combinations of superphosphate and Minjingu phosphate rock (a sparingly water-soluble fertiliser) in the percentage ratios of superphosphate to Minjingu phosphate rock of 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20 and 90:10 was studied in a laboratory incubation experiment. Three acid soils from Rift Valley Province having contrasting chemical properties were used. The treatments consisted of three P sources (superphosphate, Minjingu phosphate rock and mixtures of superphosphate and Minjingu phosphate rock), and three rates of applications (0, 200 and 800 mg P kg –1 dry soil). The fertilisers were separately mixed with each soil at the three rates of application and were brought to field moisture capacity with distilled water. The samples were incubated for 30, 60 and 90 days under laboratory conditions. Soil pH and extractable P were measured for all the periods of reaction, while extractable Ca, Fe and Mn were measured only at the 90 days of incubation period. Results showed that the solubility of Minjingu phosphate rock in the mixtures was enhanced in all the soils. The solubility increased with increasing proportion of superphosphate component in the mixtures. This was attributed to phosphoric acid produced from the hydrolysis reaction in soil of monocalcium phosphate in superphosphate. The results also indicated that the phosphoric acid not only dissolved the phosphate rock in the mixtures but also the Fe and Mn minerals in the soils. Results further showed that application of the fertilisers decreased the extractable Fe and Mn in all the soils. Negative correlation coefficients though only significant for one soil, were obtained between extractable P and Fe, and P and Mn concentrations and these indicated that the dissolved Fe and Mn minerals may precipitate P in solution and thereby reduce P availability.