Value of a heat-treated sludge in the phosphorus fertilization

Abstract

Sludges produced by a thermal conditioning treatment were studied to evaluate the bioavailability of the phosphorus they contained. First, the direct availability of P was estimated by means of a rye-grass test in a sand medium. Second, the flux of P from the sludge to the plant when the soil fixing capacity competed with the plant for a part of the phosphorus supplied was determined using a P-depleted soil. The heat-treated sludge had half the P-availability of most of digested sludges, i. e. 11 per cent of the efficiency of monocalcium phosphate after 30 days (20 °C) of root-soil contact and 36 per cent after 155 days (20 °C). In a loamy soil, the bioavailability of sludge P was 30 per cent compared with monocalcium phosphate after 50 days (20 °C), and increased to 37 per cent when wheat straw was supplied. It was more than 40 per cent after 150 days. It is suggested that heattreated sludges could be used for maintenance fertilization on soils of good P status, especially those with high P retention capacity.