It is timely to examine energy resources as related to agriculture. Agriculture is the only large industry that converts solar energy into materials useful to man; photosynthesis is the direct and indirect source of most of man's sustenance. In primitive agricultural systems, with low inputs of outside energy, the efficiency of solar energy conversion was high. However, crop yields under primitive agricultural systems were low. As populations of the world increased and most readily available agricultural lands were utilized, man was unable to feed himself without higher crop yields. Fertilizer nutrients then became the major factor limiting agricultural production. In the industrialized countries where labor resources were scarce, agriculture was mechanized which resulted in additional inputs of energy. With these high energy inputs modern mechanized agriculture is much less efficient in the conversion of solar energy to agricultural products.
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