SOIL LAYERS AND COMPONENTS Soil is a mixture of inorganic materials (clay, silt, pebbles, and sand), decaying organic matter, water, air, and living organisms. Soil forms when life forms decay, when solid rock weathers and crumbles and when sediments are deposited by erosion. Mature soils are arranged in a series of zones called soil horizons. A cross-sectional view of the horizons is called a soil profile. Some organic litter in the two top layers is broken down into partially decomposed organic material called humus which is an important soil material. It also coats the sand, silt, and clay particles in topsoil and binds them together into clumps. Soil color is important. For example, dark-brown or black topsoil is nitrogen-rich and high in organic matter. Gray, bright yellow, or red topsoil’s are low in organic matter and will need nitrogen fertilizer to support most crops. Rainwater percolates (infiltrates) through the soil layers and occupies many of the pores. As the water seeps down, it dissolves and picks up various soil components in upper layers and carries them to lower layers--a process called leaching. Soils develop and mature slowly. One process is humification--in which organic matter in the upper soil layers becomes humus. In lower layers a soil matures through mineralization, in which decomposers turn organic materials into inorganic ones. SOIL TEXTURE AND POROSITY Soils vary in their content of clay (very fine particles), silt (fine particles), sand (medium-size particles), and gravel (coarse to very coarse particles). The relative amounts of the different sizes and types of mineral particles determine soil texture. A common soil type called loam includes clay, sand, silt, and humus. Soil texture helps determine soil porosity: a measure of the volume of pores or space per volume of soil and the average distance between those spaces. Typically 40-60% of the volume of soil consists of pore spaces that allow air and water to travel thought the soil. A porous soil (with many pores) can hold more water and air than a less porous soil. The average size of spaces or pores in a soil determines soil permeability: the rate at which water and air move from upper to lower soil layers. Loams are the best soil for growing most crops because they hold lots of water but not to tightly for plants roots to absorb. The acidity or basicity of soil determines the types of plants it can support. Acidity and basicity is commonly expressed in terms of pH. When soils are too acidic, the acids can be partially neutralized by an alkaline substance such as lime. In dry regions such as the western and southwestern United States, calcium and other alkaline compounds are not leached away by rain. Soils in such areas may be too alkaline (pH above 7.5) for some crops. If drainage is good, irrigation can leach the alkaline compounds away. Adding sulfur, which concerts to sulfuric acid, reduces soil alkalinity. The burning of fossil fuels, especially coal, releases sulfur dioxide and nitrogen oxides, which form acidic compounds in the atmosphere. These compounds fall back to the earth as acid deposition. SOIL EROSION Soil erosion is the movement of soil components, especially surface-litter and topsoil. The two main movers are flowing water and wind. Some soil erosion occurs naturally. However, farming, logging, building, overgrazing by livestock, four-wheeling off-road, and other human activity that destroy plant cover leave the soil vulnerable to erosion. Losing topsoil makes soil less fertile and less able to hold water. The resulting sediments, the largest source of water pollution, clogs irritation ditches, boat channels, reservoirs, and lakes. Fish die. Water is cloudy and tastes bad. Flood risk increases. Soil, especially topsoil, is classified as a renewable resource because it is continuously regenerated by natural processes. However, in tropical and temperate areas it takes 200-1,000 years for 1 inch of new topsoil to form. THE WORLD SITUATION Today topsoil is eroding faster than it forms on about one-third of the world’s cropland. In Africa soil erosion has increased 20-fold in the last three decades. Central America and the United States have each lost 25% of their productive cropland. Overgrazing (the worst culprit) accounts for 35% of the damage, with the heaviest losses in Africa and Australia. Deforestation causes 30% of Earth’s severely eroded land and is most prevalent in Asia and South America. Unsustainable methods of farming account for 28% of such erosion, with two-thirds of the damage found in North America. The world is losing about 7% of its topsoil from potential cropland each decade. In mountainous areas, such as the Himalayas and the Andes, farmers have traditionally built elaborate systems of terraces. Terracing allowed them to cultivate steeply sloping land that would otherwise quickly lose its topsoil. For thousands of years people in tropical forests have successfully used slash-and-burn cultivation to feed their small populations. In recent decades, however, growing populations, mostly from forest immigrants, and poverty have forced many such farmers to reduce the fallow period of their fields to as little as 2 years, instead of the years needed for the soil to regain its fertility. Once-forested hills in many LDCs, such as Madagascar, have been stripped of trees by poor people for local fuel and timber exports. Because new trees are seldom planted in LDCs, the topsoil quickly washes away. In MDCs many farmers have replaced traditional soil conservation practices with commercial inorganic fertilizers and irrigation water. However, a 10-fold increase in fertilizer use and a tripling of irrigated cropland between 1950 and 1992 have only temporarily masked the effects of erosion and nutrient depletion. SPREADING DESERTIFICATION Desertification is a process whereby the productive potential of arid or semiarid land falls by 10% or more and is caused mostly by human activities. The regions most affected by desertification are all cattle-producing areas and include sub-Saharan Africa, the Middle East, western Asia, parts of Central and South America, the western half of the United States, and Australia. Practices that leave topsoil vulnerable to erosion and drying include overgrazing; deforestation, surface mining, irrigation techniques that lead to increased erosion, salt buildup, farming on land with unsuitable terrain or soils; and soil compaction by farm machinery, cattle hoofs, and the impact of raindrops on denuded soil surfaces. Each pound of steak produced causes about 35 pounds of eroded soil. CONSERVATION TILLAGE Soil conservation involves reducing soil erosion, preventing depletion of soil nutrients, and restoring nutrients already lost by erosion, leaching, and overcropping. Most methods used to control soil erosion involve keeping the soil covered with vegetation. In conventional-tillage farming the land is plowed and the soil is broken up and smoothed to make a planting surface. To lower labor costs, save energy, and reduce erosion, many U.S. farmers are trying conservation-tillage farming (no-till farming). The idea is to disturb the soil as little as possible in planting crops. With minimum tillage special tillers break up and loosen the subsurface soil without turning over the topsoil. Conservation tillage prevents soil erosion, saves fuel, cuts costs, holds more water, keeps the soil from compacting, and allows more crops to be grown during a season. Soil erosion can be reduced 30-50% on gently sloping land by contour farming: plowing and planting crops in rows across instead of up and down. Terracing can be used on steeper slopes. In strip cropping, a row crop like corn alternates in strips with a soil-saving cover crop, such as a grass or a grass-legume mixture, which completely covers the soil and thus reduces erosion. The strips of cover crop trap soil that washes from the row crop, catch and reduce water runoff, and help keep pests and plant diseases from spreading from one strip to another. Erosion can also be reduced by alley cropping, or agroforestry, a form of intercropping in which crops are planted between hedges, trees or shrubs that provide fruit or fuel. Wind erosion can be reduced by windbreaks or shelterbelts: long rows of trees planted to partially block the wind. Windbreaks also provide habitats for birds, pest-eating and pollinating insects and other animals. The 1985 Farm Act grants subsidies for farmers who take highly eroded cropland out of production and put it into a conservation reserve, and requires farmers to use soil-conserving techniques to reduce erosion on land still being cultivated. Fertilizers partially restore plant nutrients lost by erosion, leaching, and crop harvesting. Three basic types of organic fertilizer are animal manure, green manure, and compost. Animal manure includes the dung and urine of cattle, horses and poultry. Animal manure improves soil structure, adds organic nitrogen, and stimulates beneficial soil bacteria and fungi. Green manure is fresh or growing green vegetation plowed into the soil to increase the organic matter and humus available to the next crop. Compost is a rich natural fertilizer and soil conditioner made up of organic matter such as grass clippings, leaves, kitchen scraps and animal manure. Today, especially in the United States and other industrialized countries, farmers rely on commercial inorganic fertilizers, which contain nitrogen, phosphorus, and potassium. Commercial inorganic fertilizers however, do not add humus to the soil. The widespread use of commercial inorganic fertilizers, especially on sloped land near streams and lakes, causes water pollution as well. Where the resulting eutrophication causes algae blooms that use up dissolved oxygen and kill fish. Drinking water drawn from wells containing high levels of nitrate ions can be toxic, especially for infants. Another method for conserving soil nutrients is crop rotation. Corn, tobacco, and cotton can deplete the topsoil of nutrients (especially nitrogen) if planted on the same land several years in a row. Farmers using crop rotation plant areas or strips with corn, tobacco, and cotton one year. The next year they plant the same areas with legumes, whose root nodules add nitrogen to the soil, or with crops such as oats, barley or rye. This method helps restore soil nutrients and reduces erosion by keeping the soil covered with vegetation. Varying the types of crops planted from year to year also reduces infestation by insects, weeds, and plant diseases. Concern about soil erosion should not be limited to farmers. At least 40% of soil erosion in the United States is caused by timber cutting, overgrazing, mining, and urban development carried out without proper regard for soil conservation. SALINIZATION Irrigation water contains dissolved salts. In dry climates much of the water evaporates, leaving its salts, such as NaCl, behind in the topsoil. The accumulation of these salts, called salinization stunts crop growth, lowers yields, and eventually kills crop plants. When irrigation water is repeatedly withdrawn from and returned to a stream the salinity of the water steadily increases. Using this saltier water for irrigation accelerates salt buildup in the soil. For example, by the time the Colorado River makes its way from the Rocky Mountains to Mexico, its salt concentration has increased 20-fold, and it cannot be used for crop irrigation. This problem is at the root of a long-standing dispute between Mexico and the United States that may be partially resolved by the recent opening of the Yuma Desalting Plant in Arizona. Salts can be flushed out of soil by applying much more irrigation water than is needed for crop growth, but this practice increases pumping and crop-production costs, and wastes enormous amounts of water. Heavily salinized water can also be renewed by taking the land out of production for two to five years. WATERLOGGING Another disadvantage of irrigation is waterlogging. Farmers often apply heavy amounts of irrigation water to leach salts deeper into the soil. Without adequate drainage, however, water accumulates underground, gradually raising the water table. Saline water then envelopes the roots of plants and kills them.
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