Physical and chemical characterization of four composted urban refuses

The physical and chemical characteristics of four composted urban refuses (one of them amended with CaCO3 during the composting process) from 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 close to neutrality; the organic matter content ranged from 42 to 60% and the C/N ratio from 16 to 22 (7 for the amended urban refuse). Most of the total N was in organic forms; NH4+-N largely predominatedNO3−-N in the composts, except for the amended composted refuse where the predominant inorganic form was NO3−-N. Calcium was the most abundantnutrient 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 small. Total Al and Fe were extremely abundant followed by Zn, Mn, Pb, Cu, Cr, Ni and Cd. The percentage of available Mn was very high, followed 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 very low proportions of total Cr and Ni and only traces of Cd, were complexed with organic matter; these compounds seemed to be soluble organo-metallic complexes, except part of those formed by Al and Cd that could be stable complexes. Although the four composted refuses were unbalanced with regard to the main nutrients they all had potential agronomic value. Total C contents and C/N ratios in the three non-amended composts were in the range for stabilized composts; however, the NH4+-N content seemed to point to the presence of non-stabilized substances.This research was supported by the COTOP (Xunta de Galicia, Spain).Peer reviewe