Fig. 7. Radiant energy from the sun is harnessed to produce plants of many kinds.
These primary producers, in turn, provide energy and structural materials for the
animals of lower trophic levels. Such a system must receive subsidies from outside
the farm, such as fertilizer, to replace the materials that are lost when such a
product as poultry, beef, or hay is sold and moved to another location. These
subsidies can be reduced, however, if efficient use is made of farm wastes by
recycling them to produce fuel and soil amendments.