FRACTIONATION OF RESIDUAL PHOSPHORUS IN A HIGHLY WEATHERED SLUDGE-TREATED SOIL - ORGANIC PHOSPHORUS

Numerous studies have shown that, through transformation processes, or ganic phosphorus (P) plays an important role in P cycling and plant nu trition in temperate and tropical soils. Field and laboratory studies were conducted to evaluate the transformation of residual organic P in the 0-15 cm surface layer of a sewage sludge-amended Decatur silty cl ay loam, six years after the final of five consecutive annual sludge a pplications to field plots of Huntsville and Chicago sludges at 20 Mg/ ha and 10 years after a single application of 100 Mg/ha of the same sl udges. Two indicator crops, corn (Zea mays L.) and sudangrass (Sorghum sudanenses L.), were grown on these plots for one season. Treatment p lots-control, fertilizer, Chicago 20 Mg ha/yr, Huntsville 20 Mg ha/yr, Chicago 100 Mg/ha, (single application), and Huntsville 100 Mg/ha (si ngle application)-were sampled and organic P fractions determined. The P fractions followed the order of: moderately labile P > moderately r esistant P > highly resistant P > labile P, both before planting and a fter harvesting irrespective of the soil treatment. Generally, the Chi cago and Huntsville 20 Mg/ha/yr treatments resulted in higher soil org anic P fractions than the same sludges at the 100 Mg/ha (single applic ation) rate. The Huntsville sludge treatments also resulted in higher P fractions than the Chicago sludge treatments at the corresponding ap plication rate. Percentages of the two major P fractions to the total organic P, calculated both before planting and after harvesting, revea led that by the end of harvesting, the moderately labile P had decreas ed by 5.5% while the moderately resistant P had increased by 4.7%, sug gesting that transformation of the organic P could have occurred durin g cropping.