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Soil can take hundreds or thousands of years to mature. Therefore, once fertile topsoil is lost, it is not easily replaced. Soil degradation refers to deterioration in the quality of the soil and the concomitant reduction in its capacity to produce. Soils are degraded primarily by:
• organic matter loss,
• nutrient loss and
Such processes often arise from poor soil management during agricultural activities. In extreme cases, soil degradation can lead to desertification (conversion of land to desert-like conditions) of croplands and rangelands in semi-arid regions.
Erosion is the biggest cause of soil degradation. Soil productivity is reduced as a result of losses of nutrients, water storage capacity and organic matter. This leads to soil compaction and poor soil tilth.
Human activities such as construction, logging, and off-road vehicle use promote erosion by removing the natural vegetation cover protecting the soil.
Agricultural practices such as overgrazing and leaving plowed fields bare for extended periods contribute to farmland erosion. Each year, an estimated two billion metric tons of soil are eroded from farmlands in the United States alone.
The soil transported by the erosion processes can also create problems elsewhere (e.g. by clogging waterways and filling ditches and low-lying land areas).
The two main agents of erosion are wind and water, which act to remove the finer particles from the soil.
Wind erosion occurs mostly in flat, dry areas and moist, sandy areas along bodies of water. Wind not only removes soil, but also dries and degrades the soil structure.
During the 1930s, poor cultivation and grazing practices - coupled with severe drought conditions - led to severe wind erosion of soil in a region of the Great Plains that became known as the "Dust Bowl". Wind stripped large areas of farmlands of topsoil, and formed clouds of dust that traveled as far as the eastern United States.
Water erosion is the most prevalent type of erosion. It occurs in several forms:
• rain splash erosion,
• sheet erosion,
• rill erosion and
• gully erosion.
Rain splash erosion occurs when the force of individual raindrops hitting uncovered ground splashes soil particles into the air. These detached particles are more easily transported and can be further splashed down slope, causing deterioration of the soil structure.
Sheet erosion occurs when water moves down slope as a thin film and removes a uniform layer of soil.
Rill erosion is the most common form of water erosion and often develops from sheet erosion. Soil is removed as water flows through little streamlets across the land.
Gully erosion occurs when rills enlarge and flow together, forming a deep gully.
When considerable quantities of salt accumulate in the soil in a process known as salinization, many plants are unable to grow properly or even survive.
This is especially a problem in irrigated farmland. Groundwater used for irrigation contains small amounts of dissolved salts. Irrigation water that is not absorbed into the soil evaporates, leaving the salts behind. This process repeats itself and eventually severe salinization of the soil occurs.
A related problem is water logging of the soil. When cropland is irrigated with excessive amounts of water in order to leach salts that have accumulated in the soil, the excess water is sometimes unable to drain away properly. In this case it accumulates underground and causes a rise in the sub-surface water table. If the saline water rises to the level of the plant roots, plant growth is inhibited.
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