Erosion on Cultivated Land

B. D. Blakely, J. J. Coyle, and J. G. Steele.

To protect soil from erosion and to hold as much of the rain as possible in a place where crops can use it are a big part of modern soil management.

We cannot avoid all risks of erosion when we lay a soil bare by cultivating it. Neither can we hold all the rain where it falls in humid or subhumid areas. But we need to know the risks and control them the best we can.

Erosion is slow wherever the soils are covered by trees or grass. Near Zanesville, Ohio, scarcely any loss of soil could be measured in 9 years from a woodland watershed of 2 acres. A nearby pasture lost soil during the same period at an average rate of one-tenth ton an acre a year, or about 1 inch in 1,500 years. A similar watershed cropped to a 3-year rotation of corn, wheat, and hay lost an inch of soil in the 9 years.

The rate of erosion in any storm depends on the force with which raindrops stir up soil and the amount and speed of the runoff water. Other factors affecting erosion include kind and amount of cover, kind of soil, and steepness and length of slope.

To judge the erosion hazards in a particular situation, we need to look at more than one rain and consider the pattern of rainfall for a whole year or for several years.

We need to study the location by looking uphill to see how much water is likely to cross the field and by looking downhill to see where the runoff water and the soil it carries are likely to go.

We need to look at the cover that the cropping system provides each season, especially if a crop must be planted at a time when hard storms are likely to occur.

We need to study the whole soil profile to find out the effect of layers that lie beneath the surface.

We need to know what recent tillage practices have done to the structure of the surface soil and how much protection we can get from mulches.

Having appraised these hazards, we need to plan the cropping system and supporting conservation practices for the field to offset them.

Short-time changes other than erosion also are important in the management of soil and water. Nutrients may be depleted through leaching and removal of crops. Changes in soil structure can have great effect on the supply of water in the root zone available to plants. The nutrients can be replaced by fertilizers, but changes in soil structure are harder to correct.

Structure of surface soil affects intake of water and air. Whenever the surface layer becomes puddled by a hard rain or compacted by heavy machinery, the danger of runoff and erosion increases and the intake and storage of water decrease. Structure is likely to break down as organic matter becomes oxidized.

Tillage pans, also called plowpans or traffic pans, are formed in some soils as they are cultivated. A tillage pan is a thin, dense layer that develops just under the plow layer. It restricts root growth and can seriously reduce crop yields. Many silt loam and loam soils are subject to formation of tillage pans.

Structure of subsoil is important because we need the full capacity of a soil to furnish plant nutrients, water, and air. Many farmers have found that their soils drained well at first but needed more drainage after they had been farmed a few years and the crevices and root channels in the subsoil began to be filled.

We need good management of water for crops to get the full benefits of controlling erosion, improving fertility, and maintaining soil structure. Most soils contain either too little or too much water during at least part of each year.

Although drainage of wet lands and irrigation of and land are major operations, they affect only part of the total cropland, probably less than one-third. Irrigation is increasing in humid areas but will continue to be limited by water shortages, high costs, and other factors.

Farmers who can reduce runoff from upland fields and who can increase the amount of water taken in and stored for use by plants during the growing season have a good chance of increasing their crop yields. Such management of water on most of our cultivated land where neither drainage nor irrigation is involved may produce greater benefits than the more spectacular measures.

A cover of vegetation is the first defense against erosion and runoff.

A soil protected by the right kind and amount of sod or forest litter is not likely to erode, no matter how hard or how long it rains. A soil so protected readily absorbs the rainfall. Its good structure permits the water to move freely through it, and heavy runoff seldom occurs unless rock or impervious layers near the surface block the downward movement of water.

Occasionally there are exceptions. Concentrated water may start a gully, or a cloudburst can saturate an entire slope and unleash a landslide. Long rains may overrun the water-storage in the soil and cause streams to rise.

The protection cultivated crops give is intermediate between none at all, as on a newly plowed field, and the nearly complete protection of thick sod or forest litter.

Some hay crops are almost as good as pasture sod. Others, like alfalfa and sweetclover, can allow much washing if the spaces between plants are not filled in with grasses or mulched by dead stems and leaves. Small grains offer only partial protection, and that only after they have made considerable growth. Intertilled crops are little better than bare fallow unless the rows are on the contour. Crops that must be dug, like potatoes and peanuts, leave the soil ready for erosion by the next hard rain.

To judge the risk of erosion in a cropping system, we need to know the chances of erosive rains whenever the soil is cultivated or cover is thin. In a 4-year rotation of corn, grain, and 2 years of hay, the soil is plowed twice and is cultivated two or three times after the corn is planted. The risk of erosion-producing rains at those times is high in most corn-growing areas.

As many farmers know, a rotation of fallow, wheat, and kafir leaves soil exposed to erosion for most of a year and also part of the spring in which kafir is planted. A rotation of wheat 1 year and alfalfa mixed with bromegrass for 3 years gives protection most of the time and allows both crops to be planted when the risk of erosion is low.

Much can be done to reduce erosion by matching the crops grown to the erosion hazards of each field. Certain crops also can be used to keep or to restore fertility and good structure.

Green manure crops, which return large amounts of organic matter to the soil, are needed in humid and sub-humid areas to keep good filth in the topsoil. In drier farming areas, crops that leave heavy mulches on the surface may serve better.

Deep-rooted legumes, such as sweet-clover, improve subsoil structure. We need legumes in cropping systems on many soils to keep the subsoils porous enough so that water, roots, and air can get through them.

TILLAGE METHODS, as well as the crops grown, affect soil conditions and therefore runoff and erosion.

Corn, cotton, and soybeans for many years have been considered the most soil-depleting crops. Much of the damage resulted from overtillage in preparing seedbeds, controlling weeds, and attempting to get more nitrogen from decomposing residues.

New cultural practices and tillage implements now available will overcome some of the hazards of clean tillage. Agronomists are studying mulch tillage, rough seedbeds, sod seedbeds, and many other practices to determine their effects on crop yields, erosion, and control of weeds and crop diseases.

Ten years of study in the Southeast showed that mulch tillage permitted less runoff and erosion and resulted in higher corn yields than clean tillage.

Rough seedbeds prepared with a field cultivator in the Midwest lost about half as much soil as plowed fields.

Contour listing of corn in western Iowa reduced water losses about 50 percent and soil losses 73 percent from those where the crop was surface planted on the contour.

Minimum tillage that keeps crop residues on or near the soil surface, used with nitrogen fertilizers and living mulches, has proved successful in many places. More research is needed to adapt these methods to all sections.

A farmer usually can choose different combinations of soil-saving and water-saving practices to use with the crops he wants to grow.

The land capability classification is a useful guide in matching conservation practices with the cropping system or type of cover on each field.

On a soil of a certain capability, for example, the cover needs to be hay or pasture two-thirds of the time if no extra practices (such as stripcropping or terracing) are used to support the protection given by the rotation. If the field is stripcropped, hay or pasture is needed half the time. Sod is needed only one-third of the time if it is terraced.

This is only one example of the many choices that can be made. In general, whenever sloping soil is to be cultivated and exposed to erosive rains, the protection offered by sod or close-growing crops in the rotation needs to be supported by practices that will slow the runoff water and thus reduce the soil it can carry.

The most important of these supporting practices for cropland are waterways, contour tillage, stripcropping, terracing, and diversion terraces.

WATERWAYS (as used in soil and water conservation work) are natural or manmade depressions on sloping land. Waterways can carry specified amounts of water without erosion and serve as outlets for terraces, diversions, and contour rows. They are needed often as passageways for water that enters a farm from other land.

Waterways are the most important single item in the control of runoff water from cultivated land. If they fail, all other parts of the system may fail.

The best locations for waterways usually are the natural drainage-ways of the landscape. Natural depressions require a minimum of shaping to be able to carry the expected runoff. Soil and moisture are favorable for good growth of protective vegetation. The topography usually allows free discharge of water into natural drainages from terraces, diversions, and rows.