THE FATE OF SLUDGE PHOSPHORUS IN SOIL-PLANT SYSTEMS

Sewage sludges have been used for many years as sources of P for agric ultural crops, but there is a lack of information regarding the propor tion of sludge P that can be used by crops. The aim of this work was t o assess the importance of soil available P and sludge origin on the u tilization of sludge P by plants. First, the changes in soil P isotopi cally exchangeable within 1 min (E1min) were measured in incubated soi l-sludge mixtures using two soils and four sludges. Then, the uptake o f sludge P by ryegrass (Lolium perenne L.) was measured on the same sl udge-amended soils. The application of sludges increased E1min to valu es lower or equal to those obtained following the application monocalc ium phosphate. Similarly the utilization of sludge P by ryegrass was s ystematically lower than the utilization of P derived from a water-sol uble fertilizer. In both soils, the lowest utilization of sludge P was observed for the two FeSO4 flocculated and anaerobically digested slu dges, while the primary sludge and the aerobically digested sludge rel eased somewhat higher quantities of P to ryegrass. In the clayey soil, the amount of sludge P taken up by the crop was significantly related both to the sludge and soil available P content, whereas no such rela tion was observed in the loamy soil because of its high available P co ntent. The origin of the sludge and the soil available P content must therefore be taken into account when advising sludge application to cr ops to adjust P inputs to plant needs.