ABSTRACT
Before the introduction and widespread use of commercial fertilizers in the early twentieth century, application of manures and other by-products to soils was an important way of maintaining soil fertility and productivity. ¤ese applications were made based on the empirical observation that plant growth was stimulated by such applications. ¤e importance of these materials in agricultural production declined during the twentieth century in response to the lower cost of commercial (inorganic) fertilizer per mass of nutrient applied and the concentration of animals in conned animal feeding operations (CAFOs) removed from the farm. ¤e term “waste” was applied to manures and other materials, since these materials were considered to have a negative (i.e., disposal) value in lieu of their use as soil amendments. However, increasing energy costs have recently caused dramatic increases in the cost of commercial fertilizers, especially N fertilizers. In addition, global reserves of phosphate rock are dwindling. At current rates of extraction, known United States reserves will be depleted in 40 years and known global reserves in about 90 years (Vaccari, 2009). ¤is, coupled with current environmental regulations regarding release of nutrients into the environment and the rising costs associated with construction of landlls, has revived interest in land application of waste materials to crop and forest lands as a way of avoiding disposal costs while recycling nutrients to soils. ¤e organic material present in many of these by-products also has the potential to build humus levels in soils and thereby improve long-term productivity.
