The relative contribution of plant residues and fertilizer to the phosphorus nutrition of wheat in a pasture cereal system

Wheat plants (Triticum aestivum cv. Warigal) here grown in a solonised brown soil (Calcixerollic xerochrept) which had been previously cropped to medic (Medicago trunculata cv. Paraggio). The 33P-labelled medic residues and 32P-labelled monocalcium phosphate were added to the soil in factorial combination. Amounts of 31P, 32P and 33P in the wheat plants and in the soil microbial biomass were determined. Addition of residues depressed wheat dry weight, 31P and 32P uptake, while simultaneously increasing amounts of 31P and 32P incorporated into the microbial biomass. Addition of fertiliser had no effect on the proportion of plant P taken up from the residues, but significantly increased the proportion of microbial P derived from this source. The 31P held in the microbial biomass was significantly increased by both residue and fertiliser P addition, with the former having the larger effect. Of the total P applied to the soil, medic residues contributed approximately one-quarter of that supplied by the fertiliser. Of the total P in the wheat plant, medic residues supplied approximately one-fifth of that supplied by the fertiliser.