Contrasting effects of municipal compost on alfalfa growth in clay and in sandy soils: N, P, K, content and heavy metal toxicity.

The limits for loading soils with Tunisian urban compost for cultivating Medicago sativa were determined in a 6-month experiment in a greenhouse. Mature municipal solid waste compost (MSWC) from Tunis city was applied to clay and sandy soils from cultivated fields at rates equivalent to 40, 80, 120tha(-1). In the absence of MSWC, the shoot biomass (dry weight) cumulated over four cuts was 2-2.5 lower in sandy soil than in clay soil. It was 20-25% augmented upon MSWC addition in clay soil, independently of MSWC dose. The opposite trend was observed in sandy soil, the shoot yields being diminished by MSWC in a dose dependent manner. In MSWC-amended clay soil but not in sandy soil, Cd, Cu, Zn, and Pb concentrations in shoots remained below or close to the tolerated values according to EEC norms. The MSWC might be used as conditioner for clay soil, but not for sandy soil.