Economic activity producing primary goods and services often generates secondary “products” or residuals. For those residuals that are highly toxic, the only alternatives are complete destruction or long-term storage in some safe and efficient manner. But for certain residuals that are biodegradable, such as animal wastes and sewage sludges (relatively free of heavy metals), recycling is possible, particularly through soil incorporation and eventual plant uptake. These residuals, on a dry weight basis, will normally contain some nitrogen, phosphorus and potash (less than 10 percent total), and organic matter of at least 50 percent.