In 1992, for the first global study of soil degradation, the World Resources Institute in Washington, D.C., reported that 3 billion acres of land worldwide had been seriously degraded since World War II. They also stated that 22 million acres of once usable land could no longer support crops.
Natural Processes and Human Activities
Of the total acreage 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 deforestation 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 U.S. 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 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 erosion 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 water logging 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 hard pans (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).
|Soil limits to agriculture|
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.
Topsoil was eventually removed from many fields, which further depleted nutrients.One reason that peanuts became a major crop in the South is that they are nitrogen-fixing plants that can 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 into hard pans, sometimes of great thickness, called plinthitic materials (Greek plinthos means “brick”). Laterization is particularly common in the humid and subhumid tropics.
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 rain 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 rain forest 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 rain forests exposes the soil to erosion, leaching, acidification, and rapid nutrient depletion.