Agricultural utilization of sewage sludge: effect on the chemical and physical properties of soils and on the productivity and recovery of degraded areas

Abstract

This study is an overview of the agricultural recycling of sewage sludge and its impact on the chemical and physical properties of soils and on the productivity and recovery of degraded areas. Sewage sludge contains some of the essential plant nutrients (nitrogen, phosphorous, and micronutrients); it also has variable humidity content and is rich in organic matter. Sewage sludge also acts as a soil conditioner, improving soil structure and aggregation, thus decreasing density and increasing aeration of soils.Sewage sludge can complement other crop fertilization techniques by reducing the use of chemical fertilizers and fertilization costs as it increases nutrient availability in soils. As a result, the sludge can enhance plant development and productivity. Sludge application in degraded areas leads to a rapid growth of gramineous and leguminous plants. Plants growing in sludge-applied areas tend to be more vigorous and to cover larger areas (percentage); they also tend to have higher productivity and a better development of the root system. Soil recovery by liming and mineral fertilization can also lead to vegetation regrowth; however, the poor physical and biological soil conditions can deteriorate the cover plants before the soil is actually recovered. Sewage sludge must be processed before being used and cannot be applied directly to agricultural or forested land until biological treatments reduce the sludge organic content and promote organic matter stabilization. In the State of Paraná, Brazil, the direct application of sewage sludge on horticultural and other products that are ingested raw, is not recommended. Sludge fertilization is recommended for corn, wheat, sugarcane, sorghum, fruitiferous plants, and for forest plant species used to recover degraded areas. State regulations determine the levels of heavy metals allowed in the sludge and the maximum dosage of 50 ton biosolid matter/ha, for a 10-year period.