Laboratory studies on the characterization of soil-fertilizer P reaction products were carried out by reacting three - soils occurring in a toposequence in the plateau region of Bihar (India) with saturated solutions of diammonium orthophosphate (DAP), triple superphosphate (TSP) and ammonium polyphosphate:(APP) for 1 hour and 24 hours. The reaction products (precipitates) formed in the solutions after 120 days of incubation were isolated and identified through X-ray diffraction technique. Results indicate the formation of Brushite [CaHPO4 . 2H(2)O), Strengite (FePO4 . 2H(2)O), Variscite (AIPO(4) . 2H(2)O) and Fe-4 (P2O7)(3) as major soil-fertilizer P reaction products in these soils with ortho-and polyphosphates as source of phosphorus. Pot cultures were used to evaluate the relative efficiency of reaction products (Struvite, Brushite, Variscite and Strengite), orthophosphates (DAP and SSP) and polyphosphate (APP) as sources of P for gram (Cicer arietinum L.) in a typical acid soil. Results indicate significant response of gram to different sources and level of added P The dry weight and P uptake at 0, 6 and 12 mg P kg(-1) soil were 0.406, 0.519 and 0.609 (g pot(-1)); and 0.289, 0.428 and 0.575 (mg P pot(-1)), respectively. Among the sources, struvite proved to be superior or equally effective as APP, DAP or SSP as sources of P for gram. Uptake of P also varied significantly with different P sources and levels of P application. Strengite was least effective in enhancing yield and P uptake by the crop.
