To supplement the information available from MSW compost studies, studies of sewage sludge and yard waste compost have also been considered. Since MSW composts are similar to sewage sludge composts in many respects, much of what has been learned about the latter can be extrapolated to MSW composts. New regulations for sewage sludge application to land were recently established by the U.S. Environmental Protection Agency. These trace elements occur naturally as a result of the weathering of rocks. They can be leached into surface water or groundwater, taken up by plants, released as gases into the atmosphere, or bound semi-permanently by soil components such as clay or organic matter.
Metals appear in the municipal solid waste stream from a variety of sources.
Batteries, consumer electronics, ceramics, light bulbs, house dust and paint chips, lead foils such as wine bottle closures, used motor oils, plastics, and some inks and glass can all introduce metal contaminants into the solid waste stream. However, in higher amounts they may decrease plant growth. The impact of metals on plants grown in compost amended soils depends not only on the concentration of metals, but also on soil properties such as pH, organic content and cation exchange capacity. Different types of plants also react very differently to metals which may be present. However, like other trace elements, boron is more likely to be deficient in soils than to cause toxicity.