Chemical Properties of Agricultural Soils after Applications of Municipal Sewage Sludge

An alternative technique for disposing and managing municipal sewage sludge is its application to soils associated with crop production. The aim of the present study was to evaluate the suitability of the existing sludge application routes in terms of the fertility of the soil and the accumulation of heavy metals. A chemical analysis was carried out on four agricultural soils located in the north-central region of Algeria, which had been subjected to an irregular application of sewage sludge for more than ten years. Results showed high levels of organic matter, nitrogen, phosphorus and potassium in the sludge samples. The amended soils are characterised by an increased content of organic matter and an improved availability of nutrients, including nitrogen, potassium and phosphorus. In addition, the pH of the soil solution is close to neutral, there is an increase in the cation exchange capacity and there is no change in the electrical conductivity. The total concentrations of cadmium, copper, nickel and zinc were found to be consistently low and below the acceptable limit values for soil. Copper accumulation in P3 soil was slightly above standard (122.4 mg.kg-1). However, amended soils showed remarkable Cu (+167%) and Ni (+84%) accumulation. Whereas the total concentration of Cd is slightly lower than in the control soil, its speciation indicates a very high bioavailability. The results show that sewage sludge application has a positive effect on the availability of essential elements and the improvement of soil properties. However, inappropriate use and/or inaccurate application rates can lead to soil contamination with potentially toxic heavy metals.