11Introduction
Traditionally, manures and slurries have been recycled to soil (Chambers etal.,
2001). More recently, sewage sludge, the waste product of sewage treatment plants,
has also been used, although there are some concerns regarding the content of heavy
metals or other toxins in some sludges (Smith, 1996). The main aim of adding such
wastes to soil is to utilize their nutrient content, especially nitrogen, phosphorus, and
potassium. The amount of nutrient varies depending on the source and type of waste,
but typical contents are 1 to 2% N, 0.5 to 1% P, and 0.5 to 1.0% K. The potassium
tends to be immediately available for plant use, but the nitrogen and phosphorus
are released more slowly by microbial action. The organic nature of ...