Long-term effects of inorganic fertilizers and manure on phosphorus reaction products in a Typic Ustochrept

Phosphorus availability from residual P sources in soil is virtually the chemistry of metastable P compounds, i.e., their solubility and interaction with other soil and plant components. Identification of these compounds in soils under long-term application of fertilizers and farmyard manure is unstated and is prerequisite to comprehend the processes of P transformations and cycling in soil-plant system. X-ray diffraction analysis of surface and subsurface soils fertilized for 32 years under a maize–wheat–cowpea (fodder) cropping sequence revealed that P reaction products in soil depends upon the kind of fertilizer added to the soil as well as on management practices. Precipitation of applied P occurred mainly as octa-calcium phosphate, hydroxy apatite and variscite. In addition to these compounds, precipitation as \(\hbox{CaNH}_{4}\hbox{PO}_{4}\cdot\hbox{H}_{2}\hbox{O}\) was also observed in soil receiving diammonium phosphate rather than super phosphate as a fertilizer P source. Continuous application of farmyard manure (FYM) for three decades inhibited the conversion of applied P to more stable forms such as variscite and hydroxy apatite. The different P reaction products govern the availability of residual soil-P to growing plants. The reaction products formed were associated only with the sand and silt fraction of soil. Phosphorus in the clay fraction is mainly P adsorbed on exchange complexes or entrapped in lattice of silicate minerals.