Soil, a mixture of mineral, plant, and animal materials, is essential for most plant growth and is the basic resource for agricultural production. Soil-forming processes may take thousands of years, and are slowed by natural erosion forces such as wind and rain. Humans have accelerated these erosion processes by developing the land and clearing away the vegetation that holds water and soil in place. The rapid deforestation taking place in the tropics is especially damaging because the thin layer of soil that remains is fragile and quickly washes away when exposed to the heavy tropical rains (see Decertification). Globally, agriculture accounts for 28 percent of the nearly 2 billion hectares (5 billion acres) of soil that have been degraded by human activities; overgrazing is responsible for 34 percent; and deforestation is responsible for 29 percent.
In addition to reducing deforestation and overgrazing, soil conservation involves reforming agricultural soil management methods. Some of the most effective methods include strip-cropping, alternating strips of crop and uncultivated land to minimizer erosion and water runoff; contour farming, planting crops along the contours of sloping lands to minimizer erosion and runoff; terracing, which also reduces erosion and runoff on slopes; growing legumes, such as clover or soybeans, to restore essential nitrogen in the soil (see Nitrogen Fixation); and minimising tillage, or lowing, to reduce erosion.
