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Heavy metals accumulate in agricultural soils amended with various agricultural and industrial wastes. There is some evidence of metal transport in long-term waste-amended soils, but most data show limited mobility of heavy metals in waste-amended soil profiles. The bioavailability of heavy metals and their mobility in soils are largely determined by their distribution among various solid-phase components. Heavy metals in soils amended with various wastes are redistributed and transferred with time from the labile forms to the more stable forms, and the redistribution processes are dependent upon the source and process of waste, level of waste input, nature of metal, time scale, and soil properties such as pH, Eh, texture, and moisture regime.

Long-term studies on the kinetics of transformation and redistribution of heavy metals in various waste-amended soils, both under laboratory-controlled and field conditions, are needed to determine the dosage limits of various wastes in different soils and under different agricultural practices, and to select crop rotation systems and optimize management protocols to achieve minimum metal availability in soils. This will achieve the lowest toxicity to plants and minimum potential contamination of groundwater from the application of various wastes to soils. This understanding will enable one to assess the merits and disadvantages of irrigation with reclaimed sewage water, the prolonged use of animal waste and sewage sludge, and the use of municipal compost in agricultural land disposal.