PHYSICAL AND CHEMICAL CHARACTERIZATION OF 4 COMPOSTED URBAN REFUSES

The physical and chemical characteristics of four composted urban refu ses (one of them amended with CaCO3 during the composting process) fro m Spanish industrial composting plants were studied from the point of view of their use as organic fertilizers. The four composts were very fine in texture with low bulk density and high salinity. The pH was cl ose to neutrality; the organic matter content ranged from 42 to 60% an d the C/N ratio from 16 to 22 (7 for the amended urban refuse). Most o f the total N was in organic forms; NH4+-N largely predominated over N O3--N in the composts, except for the amended composted refuse where t he predominant inorganic form was NO3--N. Calcium was the most abundan t nutrient followed by K, Na, Mg and P. Most of the Ca and Na were in available forms; available K and Mg were lower and available P very sm all. Total Al and Fe were extremely abundant followed by Zn, Mn, Pb, C u, Cr, Ni and Cd. The percentage of available Mn was very high, follow ed by available Cu and Pb in two of the composts, and available Zn and Cd. Available Al, Fe, Ni and Cr were very low or negligible. Most of the total Zn, important percentages of total Pb, Mn, Al and Fe, but ve ry low proportions of total Cr and Ni and only traces of Cd, were comp lexed with organic matter; these compounds seemed to be soluble organo -metallic complexes, except part of those formed by Al and Cd that cou ld be stable complexes. Although the four composted refuses were unbal anced with regard to the main nutrients they all had potential agronom ic value. Total C contents and C/N ratios in the three non-amended com posts were in the range for stabilized composts; however, the NH4+-N c ontent seemed to point to the presence of non-stabilized substances.