THE EFFECT OF C-P RATIO OF PLANT RESIDUES ADDED TO SOILS OF CONTRASTING PHOSPHATE SORPTION CAPACITIES ON P UPTAKE BY PANICUM-MAXIMUM (JACQ)

Reutilization of P from Setaria sphacelata residues having C:P ratios of 704:1, 227:1, and 77:1 was studied using a reverse P-32-isotopic di lution technique in the greenhouse. Residues were incorporated into tw o highly weathered Malaysian soils (Ultisols and Oxisols) treated with five levels of inorganic P labeled with P-32 and cropped with Panicum maximum (Jacq.). Yield of dry matter, total P, and P-32 activity of P anicum was monitored through four cuttings spanning a period of approx imately 13 weeks. Panicum yields and residue-P uptake were depressed b y high and medium C:P ratio residues at the first cutting but recovere d in subsequent cuttings. General adequacy of P concentrations in the affected tissue and its failure to respond to applied inorganic P amon g other factors suggest that the initial yield depression was not indu ced by P immobilization. The fractional percentage of P derived from r esidues increased with increasing P content of residues at all cutting s. Similarly, L-values were consistently greater on residue-treated so ils. Residue P had greater effects on the soil with lower than on that with higher P-sorbing capacity. Changes in residue P uptake, L-values , and extractable P in soil between successive cuttings suggest a rapi d release of a large amount of easily mineralizable residue P within t he first 4 weeks, with the subsequent P release being much slower by c omparison.