The value of sewage sludge to agriculture and effects of the agricultural use of sludges contaminated with toxic elements: a review.

The effects of organic matter, nitrogen, phosphorus and toxic elements in sewage sludges applied to agricultural land are reviewed. Organic matter may improve the structure and water holding capacity of poor soils and the nitrogen and phosphorus in sludge have fertiliser value. However, and important consideration in applying sludge to land is the limitation of addition of toxic elements and any beneficial effects are secondary to this. Crops can accumulate toxic elements from sludge-amended soils and where heavily contaminated sludges and excessive rates of application are used plants may accumulate concentrations which are phytotoxic. The extent of accumulation varies considerably with plant species and cultivar; cereals and legumes accumulating lower concentrations than leafy plants such as lettuce and spinach. Although guidelines limit the addition of toxic elements in sludges and soils, thus reducing the quantities of these elements accumulated by plants, total concentrations of toxic elements in soil provide no indication of their availability to plants. The procedures applied to the determination of the forms of elements in sludges and soils and attempts to relate forms extractable in a variety of reagents to their availability to plants have been discussed. The factors which influence the forms of nutrient and toxic elements, their long term availability and hence their accumulation by crops are also reviewed.