Phosphate reaction products in major soil series of Haryana

A laboratory experiment was carried out to identify the reaction products of diammonium phosphate (NH4)2HPO4, the major phosphatic fertilizer applied by the farmers in five major soil series of Haryana, India. Saturated solution of diammonium phosphate (DAP) was allowed to react with soil for half an hour (0.5 h) and three days at 25+- 1øC. Subsequently, the suspensions were filtered and incubated for one and three months. The filterates were analysed for P, Ca, Fe and Al immediately after the filteration and after one and three months. The pH of the filterates increased with increase in reaction time as well as incubation period in all the soils. The concentration of P, Ca, Fe and Al of the filterates decreased with the increase in reaction time from 0.5 h to three days in all the soils indicating the probable precipitation in soil itself during the reaction process. The P, Ca, Al and Fe content of the filterates also decreased with the increasing incubation period due to precipitation. No precipitate was formed in solution phase with the longer period (three days) of reaction of DAP with the soils. This indicated that at the time of precipitation, the precipitates formed after the addition of saturated solution of DAP to these soils might have remained alongwith the soil residue. X-ray diffraction analysis of the precipitates showed that struvite I (Mg NH4PO46H2) was the main reaction product in Morni, Elanabad and Kajnu soils. Vashegyite At (Pø43) (OH)3 and newberyite (MgHpø4.3H2O) were the main reaction products in Pataudi and I Faridabad soils after one and three months of aging. The other reaction products were overite (Ca3 Al8 (Pø4)8 (OH)6 15HO), brushite (Ca (H2PO4).H2O), variscite (A1PO4.2H2O), strengite (FePO4.2H2O) and J phosphoferrite Fe (Mn)3 (Pø4)2.3H2O. The DAP also precipitated in all these soils at both the aging periods.