Soil provides the principal natural source of nutrients, and fertilisers supply the additional input for increased crop yields (Olsen, 1987; Jenkinson 1982). However, agricultural exploitation of soils with marginal 'available' essential metal pools has led to the development of deficiency problems in certain types of crops which have either a high demand for particular elements, or which possess relatively inefficient uptake mechanisms. Such problems become particularly pronounced in perennial crops (such as fruit or timber trees) and require routine correction by supplementing natural metal sources. Artificial fertilisers, especially those containing N, P, K and Ca, are extensively used (Sutcliffe and Baker, 1981; Bockman et al., 1990) and have been shown to stimulate growth (Trueby and Raba, 1991). Plants take up the macronutrients through the roots, however, deficiencies of micronutrients are usually corrected by spraying on the leaves, as some trace elements can be absorbed in this way.

Phosphate fertilisers also contain a number of other elements found in the parent phosphate rock (Bowen, 1979; Bockman et al., 1990; Jackson and Alloway, 1992). Cadmium originating from sedimentary rock is particularly undesirable, and processes for the removal of Cd from such fertilisers are being developed (Bockman et al., 1990). Fertilised soils have shown increases in Cd content after a number of years, but there appears to be little evidence for long-term Cd-increase in crop plants, except possibly for wheat (Jones and Johnston, 1989). Mortvedt (1984) determined the uptake of Cd and Zn by several vegetable crops heavily fertilised with triple superphosphate over a ten year period. Cd levels were found to be similar in fertilised and unfertilised snap bean seed, beet blades and roots, and in sweet corn leaves and grain. However, Zn concentrations were found to decrease with P application in all tissues except cabbage heads and cores. Claims that fertilisers promote the uptake of Al by plants have been refuted (Akerstrand et al., 1988).

A number of waste materials have been considered for use as fertilisers, for example municipal sewage sludge can be a valuable source of N and P and some trace elements such as Cu, Fe, Mn and Zn. However, some sludges contain high metal concentrations that can be toxic to plants, cause long-term soil contamination and introduce toxic metals into the food chain (Jackson and Alloway, 1992). The long-term application of sewage sludge and consequent metal uptake by plants has been reviewed by Juste and Mench (1992). These authors concluded that sludge-borne metals do not cause toxicity to the majority of crops and that Zn is the most available of the sludge-borne metals.

Recently Chaney and Ryan (1992) and Henry and Harrison (1992) have reviewed the effects of heavy metals in sewage sludges and MSW-composts (municipal solid waste) used as soil amendments. Chaney and Ryan found that in contrast to sewage sludge, MSW-composts contain phytotoxic levels of boron. Application can also raise the pH of the soil-compost mixture, which can result in compost-induced Mn-defi-ciency. Chaney and Ryan conclude that uncontaminated sludges and MSW-composts comprise no risks in relation to Cd uptake by crops and vegetables, thus there is no food chain Cd risk to humans consuming western diets. This largely comes about because Zn is a natural limiting factor in the following way: either soil pH is kept at a reasonable level for crop production limiting Cd uptake; or if the soil pH drops enough to allow Cd uptake, then Zn-phytotoxicity reduces the yield and the Cd risk.

The potential of ammonia-based flue gas desulphurisation waste solution as a nitrogen fertiliser has been assessed by Gissel-Neilson and Bertelsen (1989) by field trials using barley and rye grass. The solution had the same fertiliser value as calcium-ammonium-nitrate. The toxic effects of sulphite were reduced by avoiding direct contact of the solution with the plants.

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