Crops

As considered here, agricultural crops are managed relatively intensively for the sustained productivity of food and materials useful to humans. In this sense, agricultural systems can involve the cultivation of plants and livestock on farms, as well as the cultivation of fish and invertebrates in aquaculture and the growing of trees in agroforestry plantations.

Agricultural systems can vary tremendously in the intensity of their management practices. For example, species of terrestrial crop plants may be grown in mixed populations, a system known as polyculture. These systems are often not weeded or fertilized very intensively. Mixed-cropping systems are common in nonindustrial agriculture, for example, in subsistence agriculture in many tropical countries.

In contrast, some monocultural systems in agriculture attempt to grow crops in single-species populations. Such intensively managed systems usually rely heavily on the use of fertilizers, irrigation, and pesticides such as insecticides, herbicides, and fungicides. Of course, heavy, sophisticated, energy-requiring machinery is also required in intensive agricultural systems to plow the land, apply agrochemicals, and harvest the crops. The intensive-management techniques are used in order to substantially increase the productivity of the species of crop plants. However, these gains are expensive in of both terms money and environmental damage. The U.S. Environmental Protection Agency estimates that pollution from agricultural runoff—sediment, animal wastes, pesticides, salts, and nutrients—account for 70% of the river miles in the United States that are impaired by pollution, 49% of the impaired fresh-water lake acreage, and 27% of the impaired square miles of marine estuary (inlets where rivers mingle with the ocean). According to the U.S. Department of Agriculture, an average of 11,600 lbs (5270 kg) of soil is being eroded from every acre of cultivated U.S. cropland; even this is a no-table improvement over soil-loss rates of 20 years ago. Soil-loss rates are even higher in poorer parts of the world.