PHYTOTOXICITY DUE TO THE AGRICULTURAL USE OF URBAN WASTES - GERMINATION EXPERIMENTS

The inhibitory effect on germination by the organic fraction of different urban wastes was studied in two experiments: on barley (Hordeum vulgare L cv Reinete) seeds in petri dishes and on ryegrass (Lolium perenne L cv Argo) seeds in containers. The urban wastes were mixed with a calcareous soil. Fresh sewage sludge almost totally inhibited germination of barley seeds when used as substrate; fresh municipal solid waste also inhibited germination, although to a lesser degree. Both inhibitory effects were less pronounced when the products had been composted previously. Similar results were obtained when the experiments were carried out with water extracts although overall inhibition was less. Correlation between germination indices and various parameters of the products used showed that the highest indices were obtained when the organic matter of the samples was the most stable (a smaller proportion of labile carbon, carbon extracted with Na4P2O7, carbon precipitated at pH 2 and water soluble carbon in the materials and extracts). Inhibition was greater when there were greater quantities of NH4+, polyphenols and organic acids of low molecular weight. Yields of ryegrass obtained from high doses of urban wastes (180 t ha-1) mixed with a calcareous soil were less than those of the control at the first harvest, these yields being influenced negatively by the same factors as above; at the second harvest the yields with most of the products used were higher than those of the control, probably as a result of their maturation in the soil. When the water extracts of the products were added to the soil, no significant differences from the control were noted at the first harvest.