EVALUATION OF COMPOST BY SELECTED CHEMICAL AND BIOLOGICAL METHODS

Compost prepared on the basis of sewage sludge may be abundant both in nutrients essential for plants and in heavy metals. Additions of composts to soil increase mobility of metals and their potential transfer into plants. That is why, apart from the determination of the total metal content in compost, their bioavailability for plants should also be assessed. One-step extractions are very popular chemical methods estimating bioavailability. Bioavailability of metals may be connected with a potential phytotoxic effect. Biological methods (germination test) are usually applied in the evaluation of compost phytotoxicity. The aim of the present study was to determine the impact of various composts on metal bioavailability and germination index. The research objectives were achieved using 3 mature composts. The composts were prepared using sewage sludge, wheat straw, sawdust, waste of hemps, pine bark and wood cuttings. The amounts of bioavailable metals (Ni, Cu, Mn, Zn, Fe) were determined using one step extractions. They were conducted using-the following extractants: H2O, CaCl2, CH3COOH, HCl, DTPA and EDTA. The germination index (GI) was determined using seeds of cress (Lepidium sativumm L.). It was found that the lowest amounts of metals were recorded using H2O and CaCl2 extraction, but the highest when applying EDTA solution and HCl. The highest number of significant correlations between GI values and amounts of metals extracted With CaCl2 solution may indicate an important role in the estimation of bioavailable contents of metals in composts. The composition of investigated composts statistically influenced GI values, which ranged from 43.5% to 105.2%. Composted sewage sludge with sawdust and wheat straw had the strongest inhibitory effect on cress germination. The obtained data suggest that the phytotoxic effect of composts depends not only on sewage sludge, but also on other composted wastes.