SOIL EFFECTS DUE TO SEWAGE-SLUDGE APPLICATION IN AGRICULTURE

Field trials show that increased nutrients (mainly P) and heavy metals in soil due to agricultural use of sewage sludge have mainly to be ex pected when sludge is applied in too high amounts. The biological effe cts of heavy metals on plants are determined by their solubility which is increased by pH decrease and degradation of organic matter. Normal ly, it should be expected that the microbial activity in soil would in crease with the application of organic matter in sewage sludge. But, a lso negative effects on the microflora by simultaneous enrichment of o rganic matter or inorganic and organic pollutants were found. The most important deleterious effects on soil microorganisms are the reductio n in size of the total biomass, a reduced nitrogen fixing activity and changes in the composition of microbial populations of the soil. At w hat stage heavy metal toxicity to soil microorganisms or to microbial processes in soil is likely to become evident is unfortunately still u ncertain. Again, the soluble fraction of heavy metals (also of organic pollutants) is a determing factor. To minimize the shown environmenta l risks an adequate sludge management is proposed.