Changes in soil phosphorus availability with poultry compost age

Composting reduces the available nitrogen (N) content of organic materials by immobilizing it and converting it to a slow release form. The effect of composting on phosphorus (P) is less clear. Adding compost to soil can increase water extractable soil P by direct addition, dissolution, displacing sorbed, or reducing sorption capacity for P. Organic acids can affect soil P and vary with compost maturity. The objective of this study was to examine the effect of poultry manure compost on available soil P levels. Compost of different maturities was studied to evaluate the effect of biological activity on extractable P levels in two contrasting soils, a loam and clay. Turkey litter was mixed with orchard grass hay at a 3:1 volume ratio and turned frequently, and temperature, carbon dioxide, and oxygen were measured regularly. Compost samples were taken at day 0 when the compost was made, and at weeks 2, 4, and 8. Compost maturitydetermined by the Dewar self-heating and oxygen uptake tests indicated that two-week old compost had greater biological activity than compost at 4 or 8 weeks. Compost samples were added to a loam or clay soil at 0.15 and 0.30-g total P kg soil(-1) and then incubated for 8 weeks. At I d and at 2, 4, and 8 weeks, water-extractable and Mehlich-1 extractable P were determined. The effect of compost age was most pronounced in the loam on day 1 with water-extractable P compared to the Mehlich-1 extractable P fraction. These data suggest that water-extractable P may increase when loam soils are amended with biologically active, immature compost or when the sorption capacity of the soil is not sufficient to offset the effects of the compost addition. Because water-extractable P is implicated in runoff events, caution should be exercised in applying immature composts to critical source areas within the watershed, which are most vulnerable to P loss in surface runoff and erosion.