Composition of a Spanish sewage sludge and effects on treated soil and olive trees

The effects of sewage sludge (SL) application on the soil and olive trees (Olea europaea L., cultivar: cornicabra) were studied. The plants were grown in 8.5 L pots and subjected to the following treatments: 0, 3.66, 7.32, 14.65, 29.3, 58.6, and 117.2 g SL kg−1 soil that corresponded, respectively, to 0, 4, 8, 16, 32, 64 and 128 Mg ha−1 dry weight of sewage sludge. The application of SL at the rates 64 and 128 Mg ha−1 produced leaf tip burning and leaf drop after 120 days, although cumulative metal pollutant loading rates was below USEPA and European regulations. This toxicity symptom could be caused by the high sodium levels in the leaves (over 0.19%), which can damage olive tree development. The Na contents of leaves were well correlated with soil Na content (r2: 0.91). In general, SL rates significantly increased the level of Cr, Ni, Cu, Zn, Cd and Pb in soil and plants, but these concentrations were in the normal ranges, except for the Zn concentration, which was over the critical soil content for the rates of 32, 64, 128 Mg ha−1 but not in the leaves. Results suggested that regulations about the utilization of sewage sludge on agricultural land should consider the limit values for salt, and not only metals, that may be added to soil, in order to minimize the risk of negative effects to plant health.