Abstract
Studies on sewage sludge (SS) have confirmed the possibilities of using this waste as fertilizer and/or soil conditioner in crop production areas. Despite restrictions with regard to the levels of potentially toxic elements (PTE) and pathogens, it is believed that properly treated SS with low PTE levels, applied to soil at adequate rates, may improve the soil chemical and microbiological properties. This study consisted of a long-term field experiment conducted on a Typic Haplorthox (eutroferric Red Latosol) treated with SS for seven successive years for maize production, to evaluate changes in the soil chemical and microbiological properties. The treatments consisted of two SS rates (single and double dose of the crop N requirement) and a mineral fertilizer treatment. Soil was sampled in the 0-0.20 m layer and analyzed for chemical properties (organic C, pH, P, K, Ca, Mg, CEC, B, Cu, Fe, Mn, Zn, Cd, Ni, and Pb) and microbiological properties (basal respiration, microbial biomass activity, microbial biomass C, metabolic quotient, microbial quotient, and protease and dehydrogenase enzyme activities). Successive SS applications to soil increased the macro- and micronutrient availability, but the highest SS dose reduced the soil pH significantly, indicating a need for periodic corrections. The SS treatments also affected soil microbial activity and biomass negatively. There were no significant differences among treatments for maize grain yield. After seven annual applications of the recommended sludge rate, the heavy metal levels in the soil had not reached toxic levels.
Highlights
The fast development of metropolitan regions generates ever- increasing amounts of all sorts of residues
Samples of the sewage sludge (SS) used during the long-term experiment were analyzed and the data used to estimate the total amounts of chemical elements applied to the soil, considering both SS rates 10 and 20 t ha-1 (Table 2)
The P/K ratios (5:1) of sewage sludge were well above the recommended ratio (1.5:1) for maize in Oxisols, according to Raij et al (1996). These high P/ K ratios evidenced the need for K complementation in fields fertilized with SS, to supply enough K to maize and N:P:K in adequate proportions, as proposed by Pratt et al (1977) and confirmed by Simonete et al (2003) under greenhouse conditions
Summary
The fast development of metropolitan regions generates ever- increasing amounts of all sorts of residues. The proposal of using SS in crop fields as a sustainable way of SS disposal is based on the high organic C and nutrient levels. Sewage sludge has been used as organic fertilizer at rates equivalent to its NPK contents, such as the commercial fertilizer formula 4: 2.5: 1 (N: P: K). This organic residue has been recommended as soil conditioner, since it increases soil water retention capacity, porosity and aggregate stability, due to its high organic matter content (Jorge et al, 1991)
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