Soil Conservation

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Soil Conservation

Soil conservation refers to the control of soil erosion in order to maintain agricultural productivity. Soil conservation practices are based on an analysis of land use needs and the suitability of land for different uses.

Erosion is any process by which soil is transported from one place to another. At naturally occurring rates, land typically loses about one inch (2.5 cm) of topsoil in 100 to 250 years. A tolerable rate of soil erosion is considered to be 48 to 80 lb of soil per acre (55-91 kg per hectare) each year. Natural weathering processes that produce soil from rock can replace soil at about this rate. Cultivation, construction, and other human activities have greatly increased the rate of soil erosion in most regions. Some areas of North America are losing as much as 18 tons of soil per acre (40 tonnes per hectare) per year.

Soil erosion not only results in the loss of soil particles, but also organic matter and nutrients. The first 7 to 8 in (18-20 cm) of soil is the surface layer (topsoil) that provides most of the nutrients needed by plants. Because most erosion occurs from the surface of the soil, this vital layer is the most susceptible to being lost. The fertilizers and pesticides in some eroded soils may also pollute rivers and lakes. Eroded soil damages dams and culverts, fisheries, and reservoirs when it accumulates in those structures as sediment (this is known as sedimentation).

History

Human activities have caused increases of soil erosion since the beginning of agriculture more than 5,000 years ago. Plentiful land and a scarcity of labor in some countries encouraged farmers to intensively farm a piece of land, abandon it, and then move on to more fertile ground. This practice is still common in some developing countries, in the form of shifting cultivation or so-called slash and burn agriculture. In slash and burn agriculture, farmers burn part of a forest and plant crops among the ashes. After several years, the farmer moves to another area of forest and repeats the process. Although shifting cultivation is commonly considered to be a major cause of soil erosion, if sufficient time is allowed between clearings, soil fertility can maintain itself over the longer term.

Practices to protect the land from erosion have existed for several thousand years, particularly in the tropics and subtropics. For example, Chinese artifacts dating from about 4,500 years ago (2500 BC) depict terraces used to control erosion on cultivated slopes. Similarly, terraces have been used to grow rice in the Philippines for more than 1,000 years.

In the United States, abusive agricultural practices in combination with drought caused the great dust storms of 1934 and 1935, which carried large amounts of soil from the Great Plains to the Atlantic Ocean. Soil conservation became a practice of national importance as a result of those storms. President Franklin Roosevelt signed bills in 1935 that established the Soil Conservation Service, an agency responsible for implementing practices to control soil erosion. Individual states also passed laws establishing nearly 3,000 local soil conservation districts.

For the next several decades, U.S. farmers produced consistent surpluses of agricultural commodities. They had little incentive to push the land for higher yields. However, in the 1970s grain exports increased, especially to the Soviet Union. Farmers were encouraged to cultivate marginal lands to fill the export quotas. Those areas, amounting to almost two million acres (800,000 hectares), included land on slopes and wetter areas that are relatively vulnerable to erosion.

The concern of the environmental movement about water quality in the 1970s helped to return attention to the problem of soil erosion. Excessive amounts of phosphorus and nitrogen occurred in streams and lakes as a result of agricultural fertilization practices, and this added to public criticism of soil conservation programs. Congress passed the Soil and Water Resource Conservation Act to evaluate and conserve soil, water, and related resources on non-federal land. Erosion can also contribute to increased turbidity levels in streams, which inhibit the development of such endangered or threatened fish as salmon.

The 1985 Food Security Act encouraged land management practices that were intended to reduce soil erosion. The Act removed up to 45 million acres (18 million hectares) of erosion-prone land from intensive cultivation. It also prevented the conversion of rangelands into cultivated fields through its sodbuster provision. The Act withdrew some commodity (feed grain, wheat, rice, upland cotton, etc.) acreage from production, through multiyear acreage set-asides and conservation easements. It also required farmers to develop plans and apply management practices that would keep soil erosion on highly erodible lands within acceptable limits.

How soil erodes

Soil erosion is caused primarily by water and wind. There are several different types of water-caused erosion: sheet, rill, gully, and stream channel. In sheet erosion, the flow of water over the surface of the soil detaches and transports particles in thin layers. Concentrated flows of water form small channels or grooves (rills), and eventually develop larger gullies that carry away large amounts of soil. Sometimes, underground tunnels are formed by erosion of the subsoil. Eventually, the tunnel roof falls in to form deeper gullies. Stream channels erode when soil is removed from the fringing banks, or from within the channel of the stream itself.

Soil erosion is influenced by several variables, especially climate, soil type, density and types of plants and animals, and topography. Climatic factors include precipitation, evaporation, temperature, wind, humidity, and solar radiation. Frequent and extreme changes in these conditions, such as freezes and thaws and severe rainstorms, often increase the rate of erosion.

Soil conditions that affect erosion include detachability and transportability. Detachability is the tendency of soil particles to separate from each other. Detachability increases as the size of soil particles increases. Transportability is the ease with which soil is carried from one location to another. Transportability increases as the size of soil particles decreases.

Vegetation helps to reduce erosion by intercepting rainfall, decreasing the surface velocity of runoff, physically restraining soil movement, improving the porosity of the soil so that percolation is rapid, and by decreasing the amount of runoff, by evaporating water to the atmosphere through planttranspiration.

Soil topography features that influence soil erosion include the degree, shape, and length of the slope, and the size and shape of the watershed. Erosion increases rapidly with increasing steepness and length of slope.

Soil conservation methods

In addition to erosion control, comprehensive soil conservation includes the maintenance of organic matter and nutrients in soil. Soil conservation practices also prevent the buildup of toxic substances in the soil, such as salts and excessive amounts of pesticides. Soil conservation maintains or improves soil fertility, as well as its tilth, or structure. These all increase the capacity of the land to support the growth of plants on a sustainable basis.

There are two basic approaches to soil erosion control: barrier and cover. The barrier approach uses banks or walls such as earthen structures, grass strips, or hedgerows to check runoff, wind velocity, and soil movement. Barrier techniques are commonly used all over the world.

The cover approach maintains a soil cover of living and dead plant material. This cover lessens the impact and runoff of rain water, and decreases the amount of soil carried with it. This may be done through the use of cover crops, mulch, minimum tillstage, or agroforestry.

Barrier approaches

Terracing is the construction of earthen embankments that look like long stair-steps running across the slope of rolling land. A terrace consists of a channel with a ridge at its outer edge. The channel intercepts and diverts downhill runoff. Terraces help to prevent soil erosion by increasing the length of the slope, thereby reducing the speed of overland water flow to allow for greater infiltration. The channels redirect excess runoff to a controlled outlet. Terraces help prevent the formation of gullies and retain runoff water to allow sediment to settle.

Extensive systems of irrigated terraces have long been used in numerous countries, including Yemen, the central Andes, the southwestern United States, Ethiopia, Zimbabwe, and northern Cameroon. Soil terraces occur widely in Southeast and South Asia, New Guinea, East Africa, and Nigeria.

The construction of reservoirs, usually ponds, is another barrier method for intercepting the surface runoff of water and sediment. Reservoirs increase soil moisture, thereby improving the resistance of soil to erosion. Water stored in reservoirs is also available for use in irrigation.

Contouring is plowing, planting, cultivating, or harvesting across the slope of the land, instead of up and down the hillside. Contouring reduces the velocity of surface runoff by impounding water in small depressions.

Cover approaches

Strip or alley cropping grows alternate strips of different crops in the same field. For example, rows of annual cultivated crops such as corn or potatoes, which have the most potential to cause erosion because of frequent plowing, are rotated with small grains such as oats that allow less erosion, and also with dense perennial grasses and legumes such as lespedeza and clover, which provide the best erosion control because the soil is not disturbed very often.

A combination of contouring and strip cropping provides relatively efficient erosion control and water conservation. Both contour and strip crops can be planted with shrubs and trees, known as windbreaks or shelterbelts, that form perennial, physical barriers to control wind erosion. In addition, shrubs and trees produce litter that increases soil cover, while helping to accumulate soil upslope to eventually develop terraces, and stabilizing the soil with their root systems.

Protective cover cropping and conservation tillage are systems of reduced or no-tillage that leave crop debris covering at least 30% of the soil surface. Crop residues on the surface decompose more slowly than those that are plowed into the soil, and they release nitrogen more uniformly and allow plants to use it more efficiently. Crop residues also reduce wind velocity at the surface, trap eroding soil, and slow down surface and subsurface runoff of water. Residues also attract earthworms to the surface, whose burrows act as drains for the percolation of runoff water during heavy rains. Crop residues also provide insulation that lowers spring and summer soil temperatures, and increases soil moisture by reducing evaporation. In areas that are more productive under irrigation, conservation tillage reduces water requirements by one-third to one-half, compared with conventionally tilled areas.

KEY TERMS

Contouring —Plowing along a slope, rather than up and down it, to create furrows that catch soil and water runoff.

Fertility —The capacity of the soil to support plant productivity.

Minimum tillage —A farming method in which one or more planting operations is eliminated so as to reduce the exposure of the soil to erosion by wind and water.

Strip cropping —A farming method in which alternating bands of soil are planted in crops that are prone to soil erosion and others that prevent it.

Terracing —The creation of steplike basins on hilly ground in order to irrigate crops grown there.

Topsoil —The uppermost layer of soil, to a depth of approximately 7.1 to 7.9 in (18 to 20 cm), which is the primary feeding zone for agricultural plants.

Degrees of conservation tillage range from no-till, in which the soil is not plowed and seeds are planted by a drilling technique, to varying degrees of tillage. However, during tillage the soil is not completely turned, as it would be if a moldboard plow was used. Weeds and pest insects are controlled using herbicides and insecticides, respectively. Conservation tillage eliminates the need to let fields lie fallow (unplanted) for a year to “rest.” Fallow acreage is somewhat prone to soil erosion and to becoming dominated by intruding vegetation.

Another cover approach can provide temporary erosion control, such as that needed at construction sites. When certain chemical substances known as polymers are added to the soil, they form aggregates with the soil particles. These additives have no toxic effect, but stabilize the soil to provide temporary erosion control until a longer-lived plant cover can be established.