Soil erosion

Soil erosion plays an important role in shaping the landscape. It is a process that plays out over an extended period. Humans, however, can greatly accelerate soil erosion when they remove vegetation. The control of soil erosion is essential to maintaining the world’s food supply.

Shaping the Landscape

Soil erosion is a natural process where rock and soil are broken loose from the earth’s surface and move to a different location. Erosion transforms land by filling in valleys, wearing down mountains, and making rivers appear and disappear. Generally, this process takes thousands or even millions of years.

The active forces of erosion are water and wind. Water erosion involves the movement of soil by rainwater and melted snow moving rapidly over exposed land surfaces. The principal types of water erosion are splash erosion and gully erosion.

Splash Erosion

Splash erosion is the removal of thin layers, from land surfaces. Fine-grained soil such as silt loams, fragile sandy soil, and all soils deficient in organic matter are especially vulnerable. Most troublesome are surface areas that tend to slope, are subjected to heavy rainfall, and ground composed of shallow surface soils overlying dense clay subsoil.

When rainwater is absorbed into the ground, small sieve-like openings are made in the soil. Eventually, fine particles carried by the water plug the openings, which causes the rainwater to flow off the land. As the soil is softened by the rainfall, clods, lumps, and granules break down and form a pasty mass, which resists penetration by rainwater. As a result, the runoff increases and forms a relatively impervious, skin-like film sometimes referred to as puddled soil. As rain continues, the abrasive force of the rain results in cutting, which penetrates the skin-like layer and starts trenching.

When splash erosion reaches an advanced stage, it is referred to as rill erosion. Rill erosion can be defined as localized small washes in channels that can usually be eliminated by ordinary plowing. Rilling is the most common form of erosion on soft, freshly plowed soils that are high in silt content where the slopes are deeper than 4 or 5 percent.

There are several specialized forms of splash erosion. Pedestal erosion occurs when a stone or tree root protects easily eroded soil from splash erosion. Consequently, isolated pedestals containing resistant material are left standing. This type of erosion occurs after several years, primarily on bare patches of grazing land, and often can be seen on a small scale on pebbly ground after a rain. The erosion patterns formed in highly erodible soils are called pinnacle erosion. This erosion is usually found in deep vertical rills in the sides of gullies. When these gullies cut back and join, they form pinnacles. When pinnacle erosion is present, reclamation is difficult. Another specialized form of splash erosion is piping, which is associated with the formation of continuous pipes or channels underground. Piping usually occurs in soil types that are subject to pinnacle erosion. It involves water penetrating the soil surfaces and moving downward until it reaches a less permeable layer. The fine particles of the more porous soil may be washed out if the water flows over the less permeable layer through an outlet. The more rapid lateral flow increases the sideways erosion, causing the entire surface flow to disappear down a vertical pipe. The water then flows underground until it reappears in the side of a gully. Once pipe erosion starts, it cannot be controlled; this has been a factor in some dam collapses. Fortunately, pipe erosion is restricted to the “bad lands.” The last type of splash erosion is slump erosion. It is prominent in areas of high rainfall with deep soils. Slumping can become the chief agent in the development of gullies, probably as a result of flood flow in channels. Riverbank collapse and coastal erosion are the other main cases of slumping.

Gully Erosion

The second principal type of water erosion is gully erosion. This type of erosion occurs in places where runoff from a slope is sufficient in volume and velocity to cut deep trenches. It also takes place in areas where concentrated water continues cutting in the same groove long enough to form deep rills. Gullies usually begin in slight depressions in or below fields where water concentrates, in ruts left by farm machinery, in livestock trails, or along furrows between crop rows. Gullies ordinarily carry water only during or following rains or melting snows. Most gullies cannot be removed by normal plowing because of their size; in fact, some of them take the form of huge chasms 15–30 meters deep. A field gully is a channel at least 45 centimeters wide and between 25 and 30 centimeters deep; a woodland channel, by comparison, is considered a channel deep enough to expose the main lateral roots of trees. Woodland gullies develop in wooded areas that received water from cultivated slopes immediately above or on closely cut, severely burned-over woodland.

There are two main types of gullies. The first are V-shaped gullies, which have sloping sides with narrow bottoms, are the most common. Second are Gullies that have straight, almost vertical sides with broad bottoms. These gullies tend to be U-shaped. These are less winding than the V-shaped gullies because the soft materials at the base of the sides tend to give way to the impact of currents, and irregularities in the channel walls are planed away. Typically, the presence of gullies means that the land has been overused or abused. U-shaped gullies are usually the more serious because the soft, unstable materials commonly found in their lower depths, such as sand, loose gravel, or soft rocks, are easily cut out by floodwaters. This cutting near the bottom causes the banks to split off from above in great vertical blocks. When the caved-in material is washed out of the gully, the trench is left box-shaped. Most gullies tend to branch out as they grow, but the tendency is more serious in the U-shaped gullies. They are the most difficult to control because of the instability of the understrata.

Waterfall erosion is an important form of gully erosion because it does so much damage. It is caused by water cascading over the heads and sides of gullies, over dams, and over terraces whose channels have been filled with the debris of erosion. This type of erosion is commonly with flooding.

Wind Erosion

The second active force of soil erosion is wind erosion. Wind erosion can occur in places where water erosion is also active, but it reaches its most serious proportions on both level and sloping areas during dry times. Like water erosion, wind erosion usually proceeds very slowly; however, wind erosion is likely to increase rapidly in relatively flat and gently undulating treeless regions, like the Great Plains. When grass is plowed up, the cultivated soil becomes much less cohesive. Organic material that normally collects under a cover of grass and that serves as a bonding agent when grass is present disappears by decay and oxidation when the grass is gone. After periods of drought, the soil turns into a dry powdery mass, which is easily swept up by the wind and lifted into the pathways of high air currents, which carry it hundreds and, at times, thousands of kilometers. Coarser, heavier particles, known as ground drift, are blown along near the surface of the ground and pile up in drifts about houses, fences, farm implements, and clumps of vegetation. The fine materials that are blown away are dust; the coarser materials that are left behind are sand. The susceptibility of soils to wind erosion depends on the size of the particles and on the content of the organic matter. Coarse sands are more likely than heavy clays to blow away immediately after plowing. Ironically, the finer-textured soils, especially those of granular structure, show the greatest resistance. In fact, they sometimes remain undisturbed through years of cultivation until their organic material is disrupted.

The massive removal of soil particles through the action of the wind takes the form of dust storms. Early in 1934, a dust storm originating in the Texas-Oklahoma Panhandle covered a vast territory extending eastward from the Rocky Mountains to several hundred kilometers over the Atlantic. This one storm deprived the Great Plains states of 200 to 300 million tons of soil.

There are essentially five different types of wind erosion, although there is some overlapping, and several of the processes occur simultaneously. Detrusion is the wearing away of rocks and soil formations by fine particles carried away in suspension. This process often carves large rocks in deserts into grotesque shapes or streamlined hills, called yardangs. Abrasion occurs close to the ground where the moving particles are larger and bound over the surface. The removal of very fine particles, carried off in suspension, is efflation, and the rolling away of large particles is extrusion. The removal of particles of intermediate size bouncing downwind is known as effluxion, and the bouncing process is called saltation. The latter has nothing to do with salt but is derived from the Latin word for “leap.”

Severity of Erosion

The severity of erosion is determined by a great many factors; one of the least controllable factors is climate. The United States has more erosion problems than England because it lacks England’s gentle rains and mists. Many areas in the United States have rainstorms of sufficient strength and intensity to erode many centimeters of soil from a field not protected by vegetation within minutes. For example, rain-induced erosion may be severe in the Corn Belt (roughly, the states of Indiana, Illinois, Missouri, Nebraska, and Kansas) because rains frequently occur in these states in June when the plants have not matured enough to protect the soil; in arid areas of the western United States, serious water erosion may exist because there is not enough rain to establish a protective ground cover for the infrequent rains.

Soil type and topography also influence the severity of erosion. Soils in the United States range from poorly drained and saturated to very dry, from sandy to clayish, from acid to alkaline, and from shallow to deep. They also vary in slope characteristics, porosity, organic content, temperature, and the capacity to supply nutrients. Land is better for crops if it is nearly level, with just enough slope for good drainage. About 45 percent of the cropland in the United States falls into this category.

A farmer’s cropping practices can also greatly influence the severity of erosion. Erosion is likely to occur when fields are plowed in the conventional way: in straight rows regardless of the topography, with all plant cover and crop residue removed. Erosion can be retarded if the farmer uses the best land for crops and puts any other land to different uses. After that, any number of conservation practices, such as strip cropping, contour planting, crop rotation, terracing, and various conservation tillage practices, can be introduced.

Measuring Erosion

Soil losses per acre are measured by either the Universal Soil Loss Equation (USLE) or the Wind Erosion Equation (WEE). These formulas have been developed from field experiments in various parts of the country. Both equations measure the tons of each soil type that are lost annually through the action of climate, cropping systems, management practices, and topography.

The use of these equations involves a number of limitations. Although both the USLE and the WEE measure the movement of the soil, they do not reveal the distance the soil has traveled or where the soil was deposited. Thus, the USLE could overestimate the severity of the erosion. Another drawback to the USLE is its failure to measure losses resulting from snowmelt. A revised version of the USLE, the RUSLE, has been developed to overcome these limitations.

The soil losses measured by the RUSLE or WEE are average figures taken from measurements over an extended period. They are usually reported in tons per acre. The computed losses are often connected with soil-loss tolerances, or T-values. These figures are the maximum soil losses that can be sustained without adversely affecting productivity. Some erosion is natural even without human disturbance. As long as soil evolution keeps pace with erosion, the soil loss is sustainable. Excess soil erosion is frequently defined as amounts greater than T-values. The U.S. Department of Agriculture has assigned T-values that usually range from 1 to 5 tons per acre, depending on the properties of the soil. Unfortunately, the validity of these numbers in representing maximum sustainable soil losses is doubtful. A soil loss of 5 tons per acre per year translates into a net loss of 2.54 centimeters (1 inch) of soil every thirty years. Even in fairly steep topography, natural long-term erosion rates are only a few millimeters in thirty years. T-values have been set too high on some soils to assure long-term maintenance of the soil. T-values are also limited in their value because they do not reflect the impact of technology on crop yields.

Erosion can also be studied according to how it will be affected by different types of rain and how it will vary for different types of soil. Therefore, the amount of erosion that occurs depends on a combination of the ability of the soil to withstand rain and the power of the rain to cause erosion. This relationship between factors can be expressed in mathematical terms. Erosion is a function of the erosivity (of the rain) and the erodibility (of the soil); that is, erosion equals erosivity multiplied by erodibility.

For given soil conditions, one rainstorm can be compared quantitatively with another, and a numerical scale of values of erosivity can be created. For given rainfall conditions, one soil condition can be compared quantitatively with another, and a numerical scale of values of erodibility can be created. Erodibility of the soil can be subdivided into two parts: the inherent characteristics of the soil (that is, mechanical, chemical, and physical composition) and the way the soil is managed. Management may, in turn, be subdivided into land management and crop management.

Costs and Benefits of Erosion

Soil erosion can be both beneficial and harmful. Erosion benefits people by contributing to the formation of soil by breaking up rocks. Erosion also creates rich, fertile areas as it deposits soil at the mouths of rivers and on the floors of valleys. The Nile River valley in Egypt has been sustained for thousands of years by sediments eroded from farther upstream. Erosion is also important from the standpoint of aesthetics. The Grand Canyon, for example, was created over millions of years through the eroding action of the Colorado River.

Yet soil erosion is one of the leading threats to the food supply, as it robs farmland of productive topsoil. Soil scientists estimate that it takes nature between three hundred and one thousand years to produce about 2.5 centimeters of topsoil, although humans can replace topsoil at a much faster rate. Still, considering that about 70 percent of the United States is subject to erosion, the prospect of building productive soil becomes overwhelming. Once the topsoil has been removed, a heavy layer of clay often remains, which may not contain enough porosity to support a good crop. According to a 2021 study, if the rates of degradation continued in a consistent manner, the world's topsoil would be eliminated within sixty years.

Crops can also be damaged by the loss of nutrients to erosion. Most serious are losses of nitrogen, sulfur, and phosphorus. In addition to the natural nutrients taken from the soil, synthetic fertilizers and chemicals are often washed from fields and into lakes and rivers, thereby contributing to pollution.

Finally, erosion that forms deep gullies presents problems to farmers. Gullies caused by flowing water are often too deep for farm machinery to cross. Unable to accommodate tractors and other farm equipment, fields riddled with gullies face ruin.

Principal Terms

granules: small grains or pellets

porosity: the ability to admit the passage of gas or liquid through pores

rills: small rivulets in channels

suspension: a condition in which particles are dispersed through a supporting medium

understrata: material lying beneath the surface of the soil

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