Food crops are an important source of minerals that are important to the health of human beings. The main source of these minerals is the soils that they are grown in. The quality of these soils is ameliorated using additives such as fertilizers and manure. Sewage sludge contains important nutrients such as nitrogen, phosphorus and potassium in enough quantities to warrant it being used as manure. Some scale farmers who are close to major urban centers use it as a form of manure. However, sludge may also contain harmful heavy metals which when assimilated by the plants will enter the food chain.