Use of composted sewage sludge as horticultural growth media: effects on germination and trace element extraction

The increasing demand for growth media for greenhouse horticultural uses, the rising new uses of substrates, and the scarcity and cost of traditional sources, such as sphagnum peat moss in Mediterranean countries, have focused research on new substrate materials. Use of composted sewage sludges as a component of growth media could be a feasible alternative, with the benefit of their high availability and their very low cost. In this study, the effects of composted sewage sludge, blended with sphagnum peat moss in growth media, on germination and trace element extraction of cauliflower (Brassica oleracea var italica Plenc cv. Tenere), broccoli (Brassica oleracea var. Botryti cv. Marathon), and lettuce (Lactuca sativa c.v. Malta) were studied. Four treatments were established, on the basis of the addition of increasing quantities of composted sewage sludge to peat (0, 15, 30, and 50%, v/v). Germination percentage and trace elements (Fe, Cu, Mn, Zn, B, Pb, Ni, Cd, and Cr) extraction and differential root/aerial part distribution were studied. Increasing sewage sludge treatments (especially 30 and 50%) reduced germination of lettuce and broccoli. In cauliflower seedlings, an increment of germination was observed for the 15 and 30% treatments compared with the control. The contents of Fe, Cu, Mn, and Zn in plant tissues increased with the increasing rate of sewage sludge in the media. The highest micronutrient extraction efficiency was observed in broccoli (roots) and lettuce (leaves). Fe and Zn were the micronutrients extracted to a higher degree in all plants. Mn was mainly accumulated in aerial parts. Heavy metal extraction was very low in relation to micronutrients. Cd, Pb, and Cr were mainly accumulated in roots and Ni in the aerial parts.