colocation of the waste with a major energy conversion facility is less likely.
Most of the wastes from the existing U.S. agricultural system occur not in central facilities of the type just described but in widely dispersed locations. Their disposal is not seen as a costly process. In fact, the wastes produced at dispersed sites often are used to provide nutrients and other factors important for soil quality (e.g., crop residues are plowed under, and the nutrients are returned from manure on sparsely grazed ranges). While it is possible (though costly) to collect wastes from such dispersed locations and convert them to fuel, it is not clear that there will always be a net energy gain from doing so. If these wastes are not used to maintain soil quality, they must be replaced by other inputs, such as fertilizers. The result may be a net energy loss.
It is interesting, though perhaps not in itself important, that the energy estimated to be available from all agricultural wastes (about 2.2 quads) is about equal to the present energy inputs for agriculture. This fact does gain some importance when it is considered in light of the trade-offs possible in agriculture between energy recovery from wastes and reuse of wastes at the site of production to reduce energy inputs. The degree to which the use of wastes in the field can replace the use of fertilizers or other farm chemicals is not really known, even though it is widely debated.37
It is possible that in future years, through some combination of reuse of wastes and conversion of wastes to fuels, the agricultural sector will be made essentially self-sufficient in energy. However this balance works out in detail, the agricultural sector will not be a major producer of fuels for other U.S. energy needs. The resource is simply too small to have significant impact on a national scale. Conversion of waste materials, however, may reduce or eliminate agricultural energy demands and thus eventually make a net contribution of a few quads to the U.S. energy system.
Existing farming methods could be used in “energy farms” to produce significant amounts of biomass. Since in an energy farm the goal is to produce the maximum amount of biomass, crops such as eucalyptus trees, rubber plants, or sunflowers might be used because of their rapid growth and high energy content. The energy efficiency of land agriculture is quite low; typically less than 0.5 percent of the solar energy is stored as biomass. However, the photosynthetic process is capable of higher efficiencies, up to 12 percent being noted in some experiments. The efficiency of energy farms could be greatly improved if crops can be developed to perform