Determining the amount of sludge needed to supply a given amount of nitrogen is complicated because liquid and cake sludges contain differing amounts. Because some nitrogen remains in soils after sludge applications, soil tests should be performed before each growing season. By matching sludge applications to soil and crop needs, nitrate contamination of groundwaters can be avoided. Heavy metal concentrations will determine when sludges can no longer be safely applied. Sites can generally not be used for additional applications when cumulative loadings reach EPA limits.
Heavy metals are relatively immobile and remain in soils long after sludge applications have ceased.