Phosphorus availability to rice (Oriza sativa L.)-wheat (Triticum estivum L.) in a Vertisol after eight years of inorganic and organic fertilizer additions.

Integrated use of inorganic fertilizer N and well decomposed cattle manure (CM) or 30-35 days old Parthenium (Parthenium hysterophorus L.), a weed grown off site as green manure (GM) under repeated applications of fertilizer P and urea N for eight years in a rice (Oriza sativa L.)-wheat (Triticum estivum L.) sequence was studied on transformation of fertilizer P applied to soil at the National Research Center for Weed Science, Jabalpur, India. Based on the results, it appeared that, repeated applications of 52 kg super-phosphate P resulted in a marked increase in Olsen P linearly with time. Conjunctive use of urea fertilizer N with organic manure resulted in a larger increase in Olsen P in the Vertisol. Studies further revealed that the greater accumulation of fertilizer P applied in excess to crop removal occurred in inorganic P in the plots receiving only fertilizer N. However, plots receiving fertilizer N along with organic manures led to P accumulation predominantly in organic forms. The study suggests that these two pools of P acted as a sink when fertilizer P was applied in excess to crop removal and are bio-chemically active. The Olsen P status after 8 cycles of rice-wheat crops revealed that the average amount of fertilizer P required after adjusting for crop uptake to increase Olsen P by 1 mg kg(-1) soil was 7.2 kg Pha(-1) in the plots receiving only fertilizer N. Whereas, application of 5t FYM or 6t GM reduced it to 4.6 kg Pha(-1). The plots receiving manure always maintained a greater concentration of Olsen P. The application of CM or GM with fertilizer N enriched short-term inorganic P as well as long-term organic P fertility. After eight years, larger concentrations of organic P in the subsurface layer (16-30 cm), compared to initial values, indicates downward movement of P in organic forms.