Partially acidulated reactive phosphate rock (PAPR) fertilizer and its reactions in soil

Abstract

The fertilizer reaction products formed during the dissolution of PAPR fertilizer applied to two soils of contrasting P retention were studied using electron microscopy and energy dispersive x-ray spectrum (EDS) analysis combined with common mineralogical analysis procedures. Monocalcium phosphate (MCP) and phosphate rock (PR) residue were the main components of the PAPR fertilizer at the time of application. Dissolution of MCP in PAPR led to the formation of dicalcium phosphate dihydrate (DCPD) which was morphologically distinct from the soil matrix and easily analyzed by the microprobe. The amount of DCPD at the granule site was greater in the soil of low P retention than in the soil of high P retention, consistent with a greater transfer of fertilizer-P to the soil in latter case. The mean crystallite size of carbonate apatite in the fertilizer residue suggested a preferential dissolution of very fine crystallites at acidulations less than 50% during phosphoric acid acidulation. Electron microscopy showed dissolution of primary mineral grains (e.g. clinopyroxene) in soil adjacent to the fertilizer pellet during the dissolution of PAPR fertilizer. The precipitated products in the soil near the PAPR fertilizer pellet were complex compounds containing P, Ca, Al, Fe and Si. There was no evidence of pure phases such as variscite and strengite. These changes were analogous to those occurring in and around fully acidulated P fertilizers (TSP) in soil; however, in the case of PAPR, the reactive phosphate rock residue and DCPD at the site of application may provide a continuous source of P for plants over a long period.