Abstract
Phosphorus sorption characteristics of an alkaline sandy loam (Udic Ustochrept) amended with cowpea (Vigna unguiculata), dhaincha (Sesbania aculeata) and sunhemp (Crotolaria juncea) green manures were determined after 20 and 40 days of incubation. The amount of P sorbed required to establish 0.2 µ.g P ml-1 as the critical value of P in solution of anoxic soil decreased with addition of green manure (cowpea > dhaincha > sunhemp > control) and with increasing period of flooding. Anaerobic decomposition of green manure reduced the bonding energy and P adsorption maxima, possibly due to release of P during mineralization of plant residues. In addition, accumulation of intermediate organic acids complexed metal cations, thereby inducing solubilization of soil native P or reduced fixation of added inorganic P form. Thus green manure incorporation significantly reduced P sorption capacity of anoxic soil, and this factor should be considered in soil P management practices. P sorption isotherms could be used to predict P levels for optimum yield, and for measuring amounts of inorganic P fertilizer substituted by green manure.
