A greenhouse experiment was carried out to evaluate the effect of urban waste compost on soil fertility and on total and extracted contents (DTPA 0.005 mol L-1 pH 7.3) of the elements Fe, Mn, Zn and Ni in a sandy Red-Yellow Podzolic soil (Ultisol). The yield of dry matter of Sorghum bicolor plants and the content of P, K, Ca, Mg, Fe, Mn, Zn and Ni were evaluated in plant tops as a function of compost rates of 0, 12.5, 25, 50 and 100 t ha-1, in the absence and presence of mineral fertilizers. Dry matter yield increased with the compost rates only in the absence of mineral fertilization. Ni was not detected in the plant tops of sorghum, whereas Fe, Mn and Zn contents were related with the DTPA-extracted contents of these elements in the soil. The application of urban waste compost increased pH, organic matter, exchangeable bases, P-resin and CEC values of the soil, reducing the potential acidity (H + Al). The total contents of Fe, Mn and Zn in the soil increased with the compost rates, whereas the total Ni content increased only in the absence of mineral fertilization. The contents of Zn and Ni in the soil, extracted with DTPA, increased with the compost rates both in the absence and in the presence of mineral fertilization; Mn and Fe contents, however, decreased in the presence of mineral fertilization and were not affected in its absence. Although accumulation of the heavy metals in the plants was not detected, the content of some elements in the soil increased; therefore, attention should be given to the possibility of soil contamination when conducting successive applications of urban waste compost.
organic fertilization; DTPA; heavy metals; MSW; soil acidity