This article, from the Encyclopedia of Earth, was written by David Pimental, an entomologist at Cornell. The EoE is a non-technical reference tool with entries about the Earth, environmental science, and the relationship between people and nature. The articles are written and reviewed by experts in those fields.
"Humans began to cultivate food crops about 10,000 years ago. Prior to that time, hunter-gatherers secured their food as they traveled in the nearby environment. When they observed some of the grains left behind at their campsites sprouting and growing to harvest, they began to cultivate these grains. From these humble beginnings agriculture began. Slash and burn, an early type of crop culture, remains today a truly sustainable agriculture, one that is independent of fossil fuel energy. In such a system, about 10 hectares of productive land is held in fallow for each planted hectare. With this rotation system, a hectare is planted once every 20 years, allowing the soil to reaccumulate vital plant nutrients. Although the practice requires large acreages and large labor inputs, the crop yields are adequate. For example, corn with amplerainfall can yield about 2,000 kilograms per hectare (kg/ha).
Over time, human labor in agriculture has decreased, first because of the use of animals and finally with machinery powered by fossil fuels. Currently, plentiful and economical fossil energy supports an era of machinery and agricultural chemicals. About 1,000 liters of oil equivalent are used to produce a hectare of corn with a yield of 9,000 kg/ha. One-third of this energy is used to replace labor, one-third for fertilizers, and one-third for others.
Worldwide, more than 99.7% of human food (calories) comes from the land. Serious environmental impacts, such as soil erosion, water runoff, and pesticide pollution, result from fossil fuel-intensive agriculture. A critical need exists to assess fossil energy limits, the sustainability of agriculture, and the food needs of a rapidly growing world population."
-David Pimental, "Agriculture," in Encyclopedia of Earth