SOIL-Erosion Studies Develop Information of High Practical Value
It has become a matter of common knowledge that the uncontrolled action of wind and water has done serious damage to great areas of some of the best agricultural lands of the United States. The installation of a series of erosion-control experiment stations was begun late in 1929 to study in a systematic way the character and control of the natural forces at work under a wide variety of soil and climatic conditions. So far 10 stations have been set up in various parts of the country by the Department. They have been established in cooperation with State experiment stations and other local agencies.
At some of the stations much leading information already has become available on several phases of the subject which should facilitate the task of planning a land-use program for denuded and semidenuded acres. This information is proving useful as a basis for establishing general control measures against current and future losses of soil and water.
The development of this phase of the work has been particularly timely in connection with the national program of conservation. Many influences have been brought to bear upon this subject, and more control work has been started during the past year than ever before. Programs of work have been intensively fostered in this field not only by the regular Extension Service of the Department and by the E. C. W. camps of the Civilian Conservation Corps, under the direction of the Department of Agriculture, but also by the recently created Soil Conservation Service in the Department of Agriculture. Intensive efforts are being made by the latter to develop impressive control demonstrations, based upon the data furnished by the investigational work of this Department’s erosion experiment stations. This work is under way on more than 20 watersheds, most of them 100,000 to 200,000 acres in size, located in widely different sections where erosion is bad.
The terrific dust storms that prevailed throughout the Middle West during the past year have developed public concern regarding the erosion problem. The more violent of these storms traveled eastward to the Atlantic seaboard and passed out to sea carrying thousands of tons of choice soil materials swirling in mid-air to heights of 2 or 3 miles. In many ways such disturbances are comparable to the "black storms" of Russia. Following a violent storm of this type in the Ukraine on April 25-26, 1928, 700 widely distributed measurements showed that a total of 15,400,000,000 tons of soil had been swept up into the air and deposited in other parts of the country as well as in Poland and Rumania.
This type of soil denudation, just as in the case of sheet and gully erosion by water, is the usual consequence of injudicious land use in these semiarid sections of the country. The illustrations in figure 63 show in a general way the extraordinary conditions that prevail during such storms and those that follow. Control of soil losses by wind may be promoted by the use of judiciously spaced windbreaks and protective covers of close-growing vegetation, as well as by the adoption of proper methods of cultivation, especially during critical seasons of the year.
The aggregate area served by the present series of erosion stations is approximately 225,000,000 acres. Each station already has contributed constructive information for direct application in the field and for extension activities. This information has dealt with the relative rates of soil and water losses from various soils under definite conditions of slope, with climatic relations, and with surface exposure and other treatments, and has included suggestions for erosion control under working conditions.
Rather definite physical relations exist between established soil types and erosional behavior. Type relations and comparisons are being studied especially from the standpoint of infiltration rates. Important results are accumulating which are of basic value in an accurate evaluation and study of soil erosion. A definite knowledge of the sum of the basin capacity, in inches of rainfall, of the surface conformation of a soil developed by a given type of cultivation or treatment, and of the rate of infiltration of water into that soil under those conditions, is a factor of considerable importance in run-off and erosion control. The difference between this value and the total rainfall must represent the amount that will run off the surface, be lost to plant growth, and cause erosion unless the soil is protected. The effect of the incorporation of organic matter, and of such cultural practices as careful contouring or the use of the hole-digging machine on the infiltration rate, makes these cultural practices of primary importance in erosion control. They may also have a secondary effect through the direct improvement which they exercise over plant growth.
The dominant role of vegetation, whether it be grass, close-growing cover crops, shrub, or forest cover, as a controlling factor in soil and watery losses, has come to stand out in an exceedingly important way. Highly effective control measures involving vegetation are much in use where gully control is a major aim. The effectiveness of vegetation in protecting against gully encroachments is well shown in figures 64 and 65 taken at the Bethany (Mo.) station where a considerable amount of work along this line is in progress. The role of vegetation in holding the soil in place is, of course, not all new information. Were it not for this natural force, which has been continually at work throughout the ages, soils never would have developed as we now find them under virgin conditions, even on comparatively slight slopes. Its effectiveness is well shown by the simple comparisons of table 11 which represents soil and water losses from control plots on a wide variety of soils in widely different sections of the country under definite conditions of slope and surface exposure. According to the results presented as soil and water losses it is apparent that close-growing vegetation such as grass, alfalfa, etc., slows down water losses, and decreases soil losses hundreds and even thousands of times when compared with uncontrolled plots.
|Area, soil type, and rainfall (inches)||Plot treatment*||Soil loss|
|Upper Mississippi Valley, La Crosse, Wis., Clinton silt loam, 16 percent slope (1933 only). 29.11||Bare Soil, uncultivated||51.5||15.90|
|Missouri-Iowa, Bethany, Mo., Shelby silt loam, slope 8 percent (average 3 years, 1931-33). Average annual rainfall, 33.53.||Bare Soil, uncultivated||112.48||25.98|
|Continuous bluegrass and timothy||.36||7.72|
|Red Plains, Guthrie, Okla., Vernon fine sandy loam, slope 7.7 percent (average 4 years, 1930-33). Average annual rainfall, 32.92||Bare Soil, uncultivated||14.59||26.04|
|Texas-Arkansas-Lousiana, sandy lands region, Tyler, Tex., Kirvin fine sandy loam, slope 8.75 percent (average 3 years, 1931-33). Average annual rainfall 42.31||Bare Soil, uncultivated||12.20||18.20|
|Central piedmont, Statesville, N. C., Cecil sandy clay loam, slope 10 percent (average 3 years, 1931-33). Average annual rainfall, 42.9||Bare Soil, uncultivated||65.3||32.00|
One of the most important fields for study in the relation of plants or plant covers to erosion control, especially where cultivated crops are necessarily involved, is to be found in the adjustment of crop rotations for best results. Thus cotton planted continuously on a Vernon fine sandy loam is much more conducive to erosion and water losses than when used in a rotation of cotton, wheat, and sweetclover. Under the former condition it developed an average annual soil loss of 28.0 tons per acre, and an average annual water loss of 14.2 percent of the rainfall over a 4-year period, against a loss of 16.4 tons of soil and 11.7 percent of the rainfall where the crop appeared in the rotation referred to but under otherwise identical conditions. When the average for the entire rotation is considered the loss of soil is reduced to 6.3 tons per acre and that of rainfall to 11.7 percent. The unusual effect of the association of the other crops with cotton under the conditions of the rotation referred to in reducing these losses is readily apparent. The same relation has been found to hold for corn and other cultivated crops in this and other areas.
Under natural conditions of cultivation, strip cropping, or the alternation of close-growing crops such as alfalfa or sorghum with cultivated crops such as corn or cotton, in strips of definite width, depending on the degree of slope and other factors, shows highly interesting possibilities for erosion control. The procedure of course falls within the limitations of availability of desirable crops for a given soil, locality and type of farming as well as the seasonal exposure involved during the periods of seeding. The degree and uniformity of slope as well as the systematic protection of all depressions or natural waterways are also important factors requiring careful attention. Where the practice is to be adapted to an impervious soil, the strips should be placed somewhat off the contour or slightly graded down the slope toward the protected drainageways, in order to develop surface flow in that direction rather than down the slope.
On the impervious Shelby silt loam at the Bethany station in Missouri, strip cropping on the modified contour, with well-protected drainageways, is proving a very practical and efficient method of reducing erosion on slopes of moderate grade where severe gullying has not produced a rough topography. The arrangement of these strips in relation to the protected drainageways for a rotation involving corn, oats, and clover at the Bethany station is well shown in figures 66 and 67. Generally similar results have been obtained at the Temple and Tyler (Tex.), Clarinda (Iowa), and Guthrie (Okla.) stations.
The principal weakness in any attempt to use vegetation alone as a complete control for erosion, especially on steeper slopes, lies in the fact that under practically all farm conditions where erosion is a serious factor, such areas must be used for cultivated crops at some point in the rotation. The supporting effect of terraces thus becomes important. While terracing is not regarded as a complete control for sheet washing under conditions of exposed, cultivated surfaces on slopes conducive to the erosion of a given soil, the use of broad contoured channelways of this type across the face of erosive, sloping fields tends very effectively to reduce sheet erosion and to prevent the development of the more severe type of gullying.
Just as control of soil and water losses by vegetation requires the assistance of terracing or other forms of contour operations under certain conditions of soil, crop, and slope, so terracing requires the assistance of the plant as completely as this protection can be afforded. Erosion control increases with the extent that vegetation is used. This is due to the fact that cultivated slopes, even on terraced areas, are exposed to somesheet erosion. This protection is afforded by the use of more cover crops and the more frequent use of thick-growing crops in the crop rotations and by effecting certain combinations of strip cropping in which the strips are definitely arranged in relation to the terrace positions. Studies are under way at some of the stations involving the combination of strip cropping with a lower type of terrace than is ordinarily constructed especially under moderate conditions of rainfall. Combinations of mechanical means with vegetation used in a proper manner have interesting possibilities.