Soil degradation
Soil degradation refers to the decline in soil quality and productivity, primarily caused by human activities such as agriculture, overgrazing, and deforestation, as well as natural processes. A significant global concern, it has led to the loss of approximately 1.2 billion hectares of land since World War II, with millions of hectares rendered unsuitable for crop production. The degradation is most severe in Asia and Africa, where water and wind erosion are prevalent, but it also affects regions like Central America and parts of Europe due to industrial waste and agricultural practices.
This phenomenon can be categorized into three main types: physical, chemical, and biological degradation. Physical degradation involves the compaction and erosion of soil, while chemical degradation is characterized by nutrient depletion, often exacerbated by practices like monocropping and poor irrigation methods. Biological degradation results from the loss of organic matter essential for soil health, which can be accelerated by factors such as overgrazing and land clearing. Understanding soil degradation is critical for developing sustainable land management practices that can help preserve soil health for future agricultural use.
Soil degradation
Soil degradation is a decline in soil quality, productivity, and usefulness because of natural causes, human activities, or both. Degradation may be caused by unfavorable alterations in one or all of a soil’s physical, chemical, and biological attributes.
Background
In 1992, for the first global study of soil degradation, the World Resources Institute in Washington, DC, reported that 1.2 billion hectares of land worldwide had been seriously degraded from World War II to the present. It also stated that 9 million hectares of once usable land could no longer support crops.
Natural Processes and Human Activities
Of the total lands lost to soil degradation, almost two-thirds is in Asia and Africa; most of the loss is attributable to water and wind erosion resulting from agricultural activities, overgrazing, deforestation, and firewood collection. There are also seriously degraded soils in Central America, where degradation is caused primarily by and overgrazing. In Europe, industrial and urban wastes, pesticides, and other substances have poisoned soils in much of Poland, Germany, Hungary, and southern Sweden. In the United States, the US Department of Agriculture estimates that a quarter of the nation’s croplands have been depleted through deep plowing, removal of crop residue, conversion to permanent pasture, and other conventional agricultural practices. Although unwise management practices contribute significantly to soil degradation, soil degradation also involves three natural soil processes: physical, chemical, and biological degradation.
Physical Soil Degradation
Physical soil degradation involves deterioration in soil structure, leading to compaction, crusting, accelerated erosion, reduced water-holding capacity, and decreased aeration. Soil compaction is the compression of soil particles into a smaller volume. Excessively compacted soil suffers from poor aeration and reduced gas exchange, which can restrict the depth of root penetration. Soil compaction also causes accelerated runoff and of soils.
Crusting is the formation of a hard layer a few millimeters or a few tens of millimeters thick at the soil surface. Crusts affect drainage, leading to waterlogging at the soil surface and to salinity or alkalinity problems. Once crusts called “duricrusts” form, soil moisture recharge declines, and vegetation cannot root. Sheet and gully erosion increases as the land fails to absorb precipitation. Hard layers can also form below the cultivation depth and are called hardpans (other names are plow soles, traffic pans, and plow pans). These compacted layers can restrict root growth, making crops and trees vulnerable to drought and lodging (falling over).
Chemical Degradation
Chemical degradation comprises changes in soil’s chemical properties that regulate nutrient availability. Nutrient depletion is the major factor in chemical soil degradation. Soil nutrient depletion may be caused or exacerbated by many factors, including monocropping, of nutrients, and salt buildup.
A historic example of nutrient depletion is the depletion of soils in the southeastern United States by the growing of cotton. As late as 1950, “King Cotton” was the most valuable farm commodity produced in Alabama, Arkansas, Georgia, Louisiana, Mississippi, South Carolina, Tennessee, and Texas. In the eighteenth and nineteenth centuries, the growing of cotton ruined soil fertility as it spread westward from the Atlantic to the Texas panhandle. Cotton growth without regard to topography in hilly regions contributed to soil erosion. was eventually removed from many fields, which further depleted nutrients. One reason that peanuts became a major crop in the South is that they were nitrogen-fixing plants that could grow in soils depleted of nitrogen by cotton.
Nutrient leaching is another problem. Continuous irrigation can leach nutrients and cause salt buildup in soils where drainage is poor. Leaching can move essential but soluble nutrients past the root zone deeper into the soil and into groundwater. In addition, the water used to irrigate soil often contains salts that can accumulate to toxic levels and inhibit plant growth where evaporation occurs readily. Thick crusts of salt on farmland in Pakistan, Australia, Ethiopia, Sudan, and Egypt have made soil unfit for crops.
Laterization refers to the product and process of wetting and drying that leads to the irreversible consolidation and hardening of aluminum- and iron-rich clays (plinthitic materials) into hardpans, sometimes of great thickness. In Greek plinthos means “brick.” Laterization is particularly common in the humid and subhumid tropics.
Biological Degradation
The loss of organic matter and soil nutrients needed by plants can occur in any environment, but it is most dramatic in hot, dry regions. Organic matter is important in maintaining soil structure, supporting microorganisms, and retaining plant nutrients. Because organic matter is near the soil surface, it is generally the first soil component to be lost. Organic matter may be lost through brush fires, stubble-burning, overgrazing, or the removal of crops, fodder, wood, and dung. Loss of organic matter can be accelerated when soil moisture is reduced, when soil aeration is increased, or both. For example, peat soils that are drained decompose rapidly and subside. In drier climates, the loss of organic matter reduces the soil’s moisture-holding capacity and lowers soil fertility, which leads to lower crop yields and thus to less organic matter being returned to the soil.
Tropical forests such as those of the Amazon basin in South America seem lush, so people widely assume tropical soils to be fertile and high in organic matter. Although tropical forests do produce considerable organic matter, the amount that stays in the soil is surprisingly small, and the soils actually have low nutrient levels. Soil microorganisms in the rainforest break down the organic matter and release nutrients that are absorbed by growing plants. However, warm temperatures and high rainfall cause accelerated nutrient loss if plants are absent. Nutrients that would buffer the pH of the soil are lost. Consequently, the clearing of rainforests exposes the soil to erosion, leaching, acidification, and rapid nutrient depletion.
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