ABSTRACT
The recycling of wastes has become a major concern during the last decades of this century. It is well established that some wastes, especially those rich in organic compounds, have beneficial effects on several soil characteristics, and they often contain nutrients in plant-available forms which give them a definite agronomic value. These circumstances have induced farmers to use such wastes as soil amendments, either in their original form or after some composting process. Since the plant nutrient content is often low, large amounts of these wastes or composts are frequently added to soils, which may cause a buildup of any undesirable component present if high doses are used. A well-known example of this is the case of sewage sludge, which frequently contains significant quantities of heavy metals, so that some restrictions to its use in soils exist in most countries. Such restrictions refer to maximum permissible concentrations of potentially toxic elements in soil after application of sewage sludge (which depend on the soil pH ), maximum annual rates of addition of metals, and maximum concentrations in the sludges themselves.
