Over the 10,000 years since agriculture began to be developed, peoples everywhere have discovered the food value of wild plants and animals, and domesticated and bred them. The most important crops are cereals such as wheat, rice, barley, corn, and rye; sugarcane and sugar beets; meat animals such as sheep, cattle, goats, and pigs or swine; poultry such as chickens, ducks, and turkeys; animal products such as milk, cheese, and eggs; and nuts and oils. Fruits, vegetables, and olives are also major foods for people. Feed grains for animals include soybeans, field corn, and sorghum. Separate articles on individual plants and animals contain further information.
Tillage operations that prepare the soil for planting and control weeds also expose bare soil to possible erosion by wind and water. Erosion removes fertile soil and contributes to problems of air and water pollution. Several techniques are used to combat erosion. Crop farmers are shifting toward reduced-tillage techniques to prepare their seedbeds. Many keep a mulch cover of residues to shield their soil from wind and water erosion. Crop rotation and cover crops are also being increasingly used to hold soil in place between plantings. Still, many small-seeded crops require a finely worked, pulverized seedbed, and soil erosion cannot be completely eliminated.
Fertilizers from both natural and manufactured sources can also contribute to water pollution. Soluble-nitrogen forms can leach into groundwater, and all fertilizers, including animal manures, can be carried along in surface runoff. Crop farmers are paying more attention to reducing fertilizer loss, partly for environmental reasons but also because of fertilizer prices. The amount of water pollution contributed by fertilizer is generally low compared to other sources, and it can be reduced by avoiding excessive fertilization and by reducing soil runoff. Despite some pollution risks, the use of fertilizers, both organic and manufactured, is essential to enrich heavily used croplands.
Crop farmers must continually battle the insects and diseases that attack their plants. When used correctly and with care, chemical insecticides and fungicides are helpful in improving the quality and dependability of food supplies, but some have had detrimental effects on the...