BULLETIN 245 NOVEMBER 1936 Sheet Erosion Studies on Cecil Clay By E. G. DISEKER and R. E. YODER AGRICULTURAL EXPERIMENT STATION OF THE ALABAMA POLYTECHNIC INSTITUTE M. J. FUNCHESS, Director AUBURN, ALABAMA AGRICULTURAL EXPERIMENT STATION STAFFt President Luther Noble Duncan, M. J. M.S., LL.D. Funches, M.S., Director of Extieriment Station W. I. Weidenbach, B.S., Executive Secretary P. 0. Davis. B.S., Agricultural Editor Mary E. Martin, Librarian Sara Willeford, B.S., Agricultural Librarian Agronomy and Soils: Head, Agronomy and Soils J. W. Tidmore, Ph.D.Associate Sail Chenist Anna I. Sommer, Ph.D. Associtate Soil Chemtist G. D. Scarseth, Ph.D.__ Associate Soil Cheniust N. J. Volk, Ph.D. _-Assistant Soil Chemist J. A. Naftel, Ph.D. __Associate Plant Bretler H. 13. Tisdale, M.S. Associate Agroiomist .IlS. j. T. Willianson, Assistant Agrinomist H. R. Albrecht. 'h.D Agronomist (Coop. U. S. B. A.) Associate ------------J. B. Dick, B.S. Associate Agronomist . U. Siurkie, l'h.D. Assistant Agronomist E. L. Mayton, M.S. _. Assistant in Agrotiomy B1.S. (Brewton) J. W. Richardson, .. Assistant in Agronony -J. R. Taylor, M.S. Gratuate Assistant T. H. Ro"ers. 11.8. Animal Husbandry, Dairying, and Poultry: Head, Animal Husbandry, Dairying, and Poultry J. C. (rims, M.S. Animal Nutrititnist W. D. salinon, 11. . Xssociate Animal Nutritionist C. J. [Koehn, Jr., Ph.D. Associate Animal Nutrititnist _ -- _ C. 0. Prickett, B.. \.__ Associate Animal Nutrittorist G. A. Schrader, Ih.). Associate Animal Nutritionist W. C. Sherman. I'h . Assistant Animal Husbailttin W. E. Sewell, M.S. Associate Poultry Husandman D. F. King, M.S. Assistant Poultry Husbanimtn C. D. Gordm, M.S. _-____------- .-Assistant in Poultry Husbatndry M.. X Cotir. G. . Botany and Plant Pathology: Head. Iotany and Plant Pathology h. ). .1. L. Seal, and Plant Paitholis Associate Ilotnist F. V. Smith, M.S. Assistant in liotany atd Plait Pathology J. R. Jackson,, Ph.D. Assi stant Plant Pathologist (Coop. State I)ett. M.S. (Foirhope) H. M. Darling, Agrl. and Ala. Extension Service) Agricultural Economics: Head, Agricultural Economics I. F. Alvordt, M.S______Associate Agricultural Economist -C. M. Clark, MS. Assistant Agricultural Economist ii. T. Inman, M.S Assistant Agriculturl Economist R. E. Melcher, MS.--------_ Statistical Assistant Edith M. Slights Agricultural Engineering: Head, Agricultural Engineering "M. L. Nichouls, M.S.,_____------------Acting Head, Agricultural Engineering A. Crnes, M.S. Agricultural Engineer (Coop. U. S. D. A.) J. W. Randolph, M.S. Assistaut Agricultural Enginer F. G. Diseker, IS. __Assistant Agricultiral Ergitteer R. E. Yoler. Ph.D. ricultural Engineering (Coop. U. S. t). A.) -- Assistant in Ag I. F. Reel, MS. Graduate Assistant --Fred [Kummer. l.S. Graduate Assistant B. C. Small, B.S. Entomology; __-. -Head. Entomology and Zoology ________. J. M. Robinson, MA.-Associate Enttmologist H. S. Swingle, MS. Associate Entomologist L. L. English. I'h.D. (Spring Hill) Assistant Entomologist F. S. Arant. M.S ate Biologist (Coop. U. S. D. A. anti State Associ------------Associ H. S. Peters, M.S. Department of Conservation) Special Investigations: esearch Professor of Special Investigations K.-----. --J. F. Duggar, M.S. Horticulture and Forestry: ----Head, Horticulture and Forestry .----------. M. Ware MS. Horticulturist C. L. Ishell, Ph.D. Assistant Horticulturist _ -_ E. W. McElwee, MS.---------------------------Assistant Plant Ireeder _ ---------------Ilarrons, M.S. Keith . Assistant Iorticulturist R. W. Taylor, M.S. Assistant Forester Donald J. Weddell, M.S. __.Assistant in Horticulture _______ . Hubert Harris, B.S. Substations; Fred Stesart, Bt.S. R. C. Christopher, B.S. J. P. Wilson. B.S.. K. G. Baker, I.S. -Otto Brown, M.S. Harold Yates, B.S. " On leave f Staff as of November 1, 1936 Sup t. Tennessee Valley Substation, Belle Mina Supt. Sand Muuntain Substation, Crossville Supt. Wiregrass Substation, Headland pt. Black Belt Substation. Marion Junction - - So-----------Supt. Gulf Coast Substatioti, Fairhope Gulf Coast Substation, Fairhupe Ac ---------.-----ting Supt. ._ Sheet Erosion Studies on Cecil Clay By ELLIS G. DISEKER Assistant Agricultural Engineer and ROBERT E. YODER Assistant Agricultural Engineer BULLETIN 245 NOVEMBER 1936 Contents INTRODUCTION EXPERIMENTAL METHODS AND PROCEDURE 3----------5 Description of Plots Soil Type and Uniformity Precipitation Measurements and the Control of Artificial Rainfall Measurement of Runoff and Eroded Materials EXPERIMENTAL RESULTS 5 5 7 8 10 The Relation of Soil Moisture Content and Ab10 sorption to Erosion Losses 14 Influence of Intensity of Rainfall on Erosion 19 Influence of Quantity of Rainfall on Erosion Influence of Pulverization, Structure and 20 Shape of Surface on Erosion The Physical Nature of Erosion Losses and Certain Factors Affecting the Nat23 ure of Erosion Losses 36 Influence of Vegetative Protection on Erosion 37 Influence of Strip Cropping on Erosion Erosion Losses from Contour and from Slope-Planted Crops 46 47 Relation of Slope to Erosion 49 Effect of Erosion on Yields SUMMARY LITERATURE CITED ACKNWLEDMENT 50 51 52 Sheet Erosion Studies on Cecil Clay ROSION control has become a serious agricultural problem since the advent of intensive agriculture. Excessive soil erosion has resulted from certain activities of man which have disturbed the natural equilibrium between the processes of erosion and of soil formation. Since the soil is agriculture's greatest natural resource, its conservation is a fundamental problem. Experiments have shown that man-accelerated erosion may result in an annual loss of plant nutrients several times as great as that required to produce a normal crop of cotton or corn (1)1. The natural productive capacity of an enormous acreage of once fertile land has been depleted by sheet erosion which has washed away varying portions of the surface soil (2, 3). In addition, gullying, a more apparent type of erosion, has damaged extensive areas of once agricultural soil beyond the stage of immediate recovery (3). The basic agriculture of the Cotton Belt is built around a system of open-cultivated crops. Even the average farmer is aware that erosion is a serious hazard under such conditions. Terracing has been practiced for more than two generations. This practice is almost universally accepted by Alabama farmers as a necessity. It is an accepted fact that the construction and maintenance of an adequate system of terraces is the first step in any sound program of erosion control under the above described system of agriculture. Likewise, contour planting and cultivation are widely accepted and practiced by nearly all progressive farmers. Further control measures by necessity must be modifications of or supplemental practices built into and around this basic system. It is apparent that sheet erosion control must be considered as a "between-terrace" problem since the land between terraces constitutes the unit areas from which runoff occurs. Three groups of supplemental practices offer possibilities from which practical sheet erosion control measures may be developed. They are as follows: (a) increased and improved use of vegetation, (b) improved methods of tillage and mechanical manipulation of the soil, and (c) a wiser selection and use of land for the production of clean-cultivated crops. The last mentioned is beyond the scope of this publication but results of work of the type herein reported should be of value in serving as a guide to a sound land use program. Much has been written concerning the erosion process. However, a large portion of these writings are opinions based on 1 Numbers in parentheses refer to literature cited. 3 SEROSION LOSSES SSOILAN°FERTILITY LOSSES LANDVALUES AND DECREASEDPRODUCTIVITY, PROBLEMS FARMINCOME;SEDIMENTATION WATER LOSSES FLOODS. DROUGHT,WATER SUPPLY AND NAVIGATION PROBLEMS PUBLIC WELFARE FIGURE 1. Interrelationship of factors involved in the soil erosion problem. .5 empirical observation. Notable exceptions are the work of the Missouri Agricultural Experiment Station (6, 10) and that of the recently established Federal Erosion Experiment Stations (8, 9). The extent of erosion losses is dependent on a large number of complexly related variables which in turn are dependent upon factors of climate, topography, vegetation and soil. An attempt has been made in Figure 1 to show diagramatically the interrelationship of these factors. Topography influences vegetation, climate, and soil characteristics. Climate, vegetation, and soil conditions mutually exert pronounced influences on each other. As a result soils with different physical, chemical, and biological properties have been formed. Nature has integrated these environmental forces with the result that different soils possessing characteristic structure have developed. Structure, along with precipitation, temperature, plant growth, and organic residues, determines the tendency of a given soil to erode-the erodibility of the soil. The climatic and vegetative factors as well as the slope factors of topography are important in determining the nature and amount of runoff. It is the purpose of this publication to (a) present methods and procedure by which some of the basic principles involved in sheet erosion control may be analyzed, and (b) to report the results of several years experimentation on the measurement and control of sheet erosion on Cecil clay. EXPERIMENTAL METHODS AND PROCEDURE Description of Plots.-The experimental area consisted of ten controlled plots, each enclosed by concrete walls. Each plot was 1/58 acre; the slope length was 50 feet and the width was 15 feet. The slope length was chosen to correspond approximately with the horizontal distances between terraces on critical slopes (4). The areas were sufficiently large to approximate field conditions and still permit the control of variable factors involved in the sheet erosion process. A concrete cistern 3 feet wide, 15 feet long and 5 feet deep, located at the lower end of each plot, was used to facilitate the measurement of both runoff and soil losses. These cisterns were constructed so that they might be drained by gravity. A general view of the plot layout is shown in Figure 2. Soil Type and Uniformity.-The soil used in these experiments was a Cecil clay. Detailed data concerning the physical and chemical properties of the Cecil series as represented in In order to insure Alabama have been reported by Davis (5). uniformity of soil on the ten plots, the surface soil and six inches FIGUREii' 2.-G hi i\ %ie\ of erion~ Plot la 1 mean values and the standard deviation from the mean values of the subsoil were removed separately and each thoroughly mixed. Small quantities of the subsoil were added and tamped until the required six-inch layer was replaced. The same procedure was followed in replacing the surface soil. Mechanical analyses of the soil by the pipette method (12) showed that texture was uniform from plot to plot. A three-foot fill was required at the upper ends of three of the steeper plots in order to establish the required grades. Subsoil was used in making these small fills and an attempt was made to tamp it back to its normal volume weight before the surface soil was added. By 1933 the initial surface soil had become so severely TABLE 1.-Mechanical Analysis of the Surface Soil on the eroded, especially on the steepErosion Plots. er slopes, that it was removed and six inches of another typical Diameter in Quantity in per Cecil clay added. Mechanical analyses showed that the texture 0.9±0.4 2.0 -1.0 of this soil was uniform from 2.4±0.2 1.0 -0.5 plot to plot. Thus, only the 0.5 -0.25 5.40.4 0.25-0.10 0.10-0.05 13.4+0.5 9.2±0.5 16.30.5 52.4±1.2 of these analyses are given in Table 1. 0.05-0.005 < 0.005 Precipitation Measurements and the Control of Artificial Rainfall.-Two rain gauges were located at the plots. One was a standard rain gauge; the other was a standard recording gauge which recorded the amount, rate and duration of natural rainfall. Control of quantity, intensity and duration of rainfall is an essential part of experimental technique required in certain types of sheet erosion studies. A portable irrigation system of the "Skinner" type was designed to meet this need. The apparatus used to distribute the artificial rainfall consisted of 8 two 50-foot sections of 3/4-inch galvanized steel pipe equipped with "catfish" nozzles which were attached at 1-foot intervals along each pipe. These distribution pipes were located three feet above the soil and 31/2 feet from the side walls of the plots and were supported by means of open hangers inserted in the top of pipe posts which were driven vertically into the soil. Water was supplied by a three-inch main which led to one corner of the plot area. From this main a two-inch pipe was run along the upper ends of the plots; this line was equipped with valves and hose connections located at appropriate points. 'The fill was of appreciable area on the last plot of 20 per cent slope. Runoff from this plot has been low, indicating that it is undesirable to disturb the subsoil in the construction of plots for erosion studies. The distribution pipes were moved from plot to plot and were attached to the two-inch pipe line with a one-inch rubber hose. A gate valve and water meter were inserted in the three-inch main; these, together with a stop watch, were used to control the rate and quantity of artificial rainfall. The "catfish" nozzles were selected after testing several different types. These tests were made by placing a large number of small pans at intervals over the plots and measuring the amount of water caught in each pan. It was found necessary to adjust the openings in the nozzles in order to obtain a uniform distribution of water. During artificial rainfall applications, ten-foot portable canvas walls were used to surround the plots to prevent wind from blowing the water onto adjacent plots. These walls were supported from cables suspended between steel I-beam tracks located above and extending along the two sides of the plot area; the ends of the cables were attached to small cars mounted on the tracks. This equipment facilitated the movement of the canvas barriers from plot to plot during artificial rainfall experiments. Measurement of Runoff and Eroded Materials.-A trough located at the lower edge of each plot was used to divert runoff and eroded material through 6-inch sheet-metal pipes and into 32-gallon galvanized cans. These pipes extended to within a few inches of the bottom of the cans. Utilizing the principle of Stoke's law of settling velocity (14), the coarse sediments which could not be measured accurately by suspension samples, were retained in the can. After the can overflowed, the smaller sediments which were measured by suspension samples passed into the pit with the water. The water was slowly decanted and the cans were then lifted from the cisterns by a differential hoist mounted on a continuous metal track extending over the entire length of the cisterns. The coarse, wet material was weighed when it was lifted from the pit. This material was thoroughly mixed and a 500- to 600-gram representative sample was taken and its oven-dry weight determined. The amount of coarse, dry sediment was then calculated in pounds lost per acre. Runoff and the finer eroded materials in the cistern were thoroughly agitated by means of a perforated metal agitator. Three one-half gallon samples were quickly taken after each agitation from three uniformly spaced locations in the pit. These samples were thoroughly mixed and-a one-half gallon composite sample taken. The suspension solids were flocculated with an aluminum sulfate solution, filtered, oven dried, and weighed. The pits were calibrated and the depth of the runoff water was measured to the nearest one-hundredth of a foot. Calculations of the pounds of suspended material lost per acre were then made and the amount added to the dry weight of coarse material lost per acre, thus giving the total quantity of soil losses. In event the galvanized can did not hold all the coarse eroded material and some of it passed into the cistern, which was frequently the case during extremely heavy rains, a sample of the suspension was taken without agitation. The water was then slowly drained off and the remaining coarse material was shoveled into a can and weighed as previously described. A special gauge was designed to measure the rate of runoff produced by artificial rainfall of known intensity and duration. By means of this gauge, the rise of water (See Figure 3.) in the cistern was recorded on a calibrated chart at the desired time intervals. A permanent record was made by marking on the chart at the desired time as the pointer moved down the chart. L ... d.F=Float S= Scale Runoff and soil losses E=Fennel B Wave batle board depend upon a numbeafflecylinder B'. Wave D.Drain pipevalve ber of interrelated facT= Sheet metal trough C 3Oallon can for coarse sediments tors; these have already W= Counter-weight with scale pointer G Gage support with adjustable Clamps been listed. Under ordinary field conditions it is frequently impossible to evaluate the influence of a single variable because of its close association with or interdependence on one or more other factors. In order to isolate and study the influence of a single factor, it is essential to have an experimental set-up which permits the control or measurement of th e maximum number of interrelated variables FIGURE 3.-Sketch of a cistern showing while the single factor equipment for measuring soil is allowed to vary. losses and rate of runoff. = 'The term "soil losses" as used herein refers to the quantity of soil eroded from the Under field conditions with an adequate system of terraces, unplots into the cisterns. doubtedly an appreciable portion of the "so called" soil losses would be deposited in the terrace channels. 10 The plot layout and methods already described were developed in an attempt to obtain such a set-up. The factors which could be controlled or measured were as follows: Soil conditions Vegetation Climate Rate, amount and duration of artificial rainfall Topography Controllable variables Factors of soil type Type of plants Planting methods Length, degree and continuity of slope Measurable variables Soil moisture Organic matter Ground coverage Pulverization Amount of growth (Dynamic conditions) Rate, amount and duration of Microtopography natural rainfall (height of beds) Temperature Resulting variables Erosion losses-rate, amount and nature of runoff, soil movement and soil losses Natural rainfall is so fortuitous with respect to distribution and intensity that interpretation of resulting data concerning erosion losses is exceedingly difficult. (See Table 2.) Thus, the artificial rainfall system was not only necessary to control intensity, quantity, and continuity of rainfall, but it also speeded up experimental work. The high rates of rainfall application were chosen so that measurable erosion losses would be produced on all slopes under a wide range of conditions. The rates approach the maximum intensity of natural rainfall of the region as recently reported by Yarnell (15). The intensity and duration of a large number of natural rains, as recorded at the erosion plots during the course of these experimerits, are given in Table 3. EXPERIMENTAL RESULTS The following discussions are based for the most part on experimental work conducted on Cecil clay. Many of the principles involved are believed to be of rather general application to the sheet erosion process. However, it is realized that the magnitude of erosion losses is quite different from that which would be obtained under similar conditions with other soil types of markedly different physical and chemical properties. The Relation of Soil Moisture Content and Absorption to Erosion Losses.-JIt is obvious that the extent of saturation of a soil with respect to water determines the rate and extent of further absorption and hence influences the amount of runoff and soil losses. Both the immediate absorptive capacity and TABLE 2.-Rainfall Month January February March April May June July August September October November December Totals 1929 Inches 4.28 9.64 17.47 5.32 7.05 4.19 1.68 1.53 4.55 4.04 6.83 4.71 71.24 by Months at Auburn, Alabama Year 1931 Inches 2.91 3.38 2.97 4.95 2.82 0.57 4.63 6.37 0.48 0.95 1.50 8.54 40.07 1932 Inches 5.61 4.43 3.45 1.92 2.58 2.43 4.79 4.38 3.21 2.12 6.18 7.14 48.24 (1929-1936) 1933 with the 55 year Average. 55 yr. average 1930 Inches 4.78 3.05 6.36 3.19 2.84 2.09 4.97 4.56 6.17 2.59 7.14 2.12 49.86 1934 Inches 1.79 4.25 3.57 3.77 3.63 5.20 3.11 6.28 1.66 4.82 2.52 2.25 42.85 Inches 2.56 6.58 7.41 2.27 1.33 2.32 3.21 3.49 3.44 4.19 1.10 39.85 1935 Inches 1.82 3.90 7.17 3.85 3.00 3.78 5.48 6.01 2.44 1.49 2.86 46.61 1936 Inches 12.09 7.89 4.85 9.34 1.17 2.93 3.68 7.72 1881-1935 Inches 4.70 5.35 5.77 4.07 3.51 4.07 5.36 4.64 3.08 2.81 3.39 5.11 51.86 TABLE 3.-Duration and Amount of Continuous Portions of Intense Erosive Rains by Months at Auburn, Alabama. Year Month January February March April May June July August September October November December 1931 Inches Hours 0.50 0.40 1.60 0.40 2.00 1.35 1.65 1.10 1932 Inches Hours 1.70 1.15 0.95 0.45 0.55 0.40 1.35 0.55 0.50 0.60 0.50 1933 Inches Hours 0.75 4.2 1934 Inches Hours 0.70 1.70 1.05 1.45 0.60 0.60 1.50 0.95 _________ 1.0 0.8 6.0 1.3 1.5 0.7 8.5 3.0 7.3 4.7 1.7 0.5 0.8 0.5 1.3 0.7 0.2 1.5 1.7 1935 Inches Hours 0.35 1.5 3.00 0.95 0.50 1.10 1.35 0.65 0.60 1.15 2.00 0.50 0.55 0.80 1.20 0.50 0.40 0.30 0.2 0.2 0.5 1.0 1.5 0.2 0.8 2.8 9.0 4.0 1.8 1.0 0.2 2.0 5.0 18.0 3.0 0.7 2.3 5.3 4.3 1.2 3.1 14.7 1936 Inches Hours 1.00 1.0 0.85 0.1 0.90 0.1 0.50 0.2 0.60 0.1 0.65 0.3 2.15 11.0 0.50 0.3 _________ 12 are influenced by the moisture 3 content of the soil profile (11, 13). Moisture . also affects the .content dispersion or slaking of 20 the soil at the time rains'' fall occurs (16). In ' GFigure 4 is given a comparison of the moisture 3n content of the surface on a level plot with 2soil that of a 20-per-centslope plot over a period of 45 days. The time o 4s is 2o as 5 o s Time in Days and amount of rainfall FIGURE 4.-Relationship between time and are also shown graphiamount of rainfall and soil cally on the same figure. moisture content on different The average moisture . 0% slope; slopes; . .-----.20% slope, contents of the surface soil during the 45-day period were 24.6, 23.1, 22.0, 21.2, and 21.0 per cent on the 0, 5, 10, 15 and 20 per cent slopes, respectively. Moisture content was consistently lower as slope increased. This may be accounted for by the fact that the rate of runoff increases with increasing slope. Thus for any given rain the time interval during which the absorption and infiltration processes could function decreased with increased slope. Other data relevant to absorption are shown in Table 4. The moisture content of the soil profile exerts a pronounced influence on the quantity of erosion losses from a given rain. The influence of soil moisture on erosion losses is revealed by the following example. A one and one-half-inch artificial rain was applied in 25 minutes to a 5-per-cent-slope plot when the surface soil contained 10.8 per cent moisture. The runoff from the plot was 26 per cent and the soil eroded was 72 pounds per acre. On the same plot, when the surface soil was saturated, the runoff from an artificial rain of the same amount and rate was 69 per cent and the material eroded per acre was 3,555 pounds. Soil moisture content is sufficiently important to make it highly desirable to duplicate artificial rainfall experiments at low field moisture with runs immediately following when the surface horizon is still saturated with water. During the growing season of clean-cultivated crops and of winter cover crops, there are invariably one or more rains which produce extreme erosion losses. A large percentage of the seasonal losses result from such rains. Three rains were responsible for 89 to 100 per cent of the soil losses (depending upon slope) Spermeability 10 TABLE 4.-Percentage Runoff from Cecil Clay with Variations of Slope, rains - 1935). Vegetative Cover and Rainfall (Natural Amount of rainfall Inches 0.83 Duration of rainfall Hours 7 Estimated coverage vetch Per cent 4 F1 2 0 V1 F 5 Slope of land in per cent 10 V F V Runoff in per cent of rainfall 15 F V F 20 I V 36 2 4 37 35 40 34 45 37 43 IF = 2Plot 1.28 0.81 1.65 3.93 1.64 1.08 13 2 14 18 2 1.3 7 10 35 50 75 100 1 3 1 25 57 37 10 9 3 24 39 4 48 62 59 75 87 86 50 64 39 63 83 6 49 56 51 76 87 88 44 49 19 62 75 20 50 44 35 80 87 95 42 48 21 65 76 18 46 52 59 90 89 97 42 50 21 56 69 24 smooth fallow; V = vetch planted in 18-inch, contour rows. out of level, hence runoff values are abnormally low. 14 during the growing season of a crop of cotton. The three rains accounted for about 30 per cent of the seasonal rainfall. This principle is in agreement with the findings of Bartel (1). Erosive rains of this type are usually characterized by high percentages of runoff. Excessive runoff is caused by (a) the soil profile being highly saturated with water and hence possessing a low rate of infiltration or by, (b) hard rain falling at an intensity that greatly exceeds the rate of infiltration. Excessive soil losses sometimes occur when the amount of runoff is not extremely excessive. Such results have been found to occur from rains falling when the surface soil was extremely susceptible to erosion, i.e., a fine pulverized condition immediately fol(See Tables 5 and 6.) lowing cultivation or plowing. Influence of Intensity of Rainfall on Erosion.-Studies have shown that the intensity of rainfall is more important than the amount of rainfall in determining the extent of erosion losses. This is shown by the following example. During February, when the plots were partially covered with a small growth of vetch, a 1.0-inch rain occurred over a period of 76 hours and the soil losses varied from zero on the level plots to 4 pounds per acre on TABLE 5.-Runoff and Soil Losses from Different Amounts of Rainfall under Saturated and Non-saturated Conditions on Fallow, Plowed and Vetch Plots of Cecil Clay. Rainfall Applied in inches Duration of rainfall in minutes Soil 1 moisture Surface conditions Slope in per cent 0 5 10 15 20 0 5 10 15 20 0 0 4 5 10 0 0 127 217 1,794 1st. 2nd. 2nd. 1st. increment increment increment increment 1.25 1.25 1.25 1.25 11 11 11 11 Continuous 2.50 22 Continuous 2.50 22 F.M. Freshly plowed Sat'd Freshly plowed F.M. Smooth fallow Sat'd Smooth fallow F.M. Smooth fallow Sat'd Mature vetch Runoff in per cent of rainfall 34 66 62 65 72 147 1,277 3,743 19,402 39,981 27 70 79 81 81 52 58 86 90 86 58 84 87 91 97 40 51 51 53 59 8 6 26 48 521 Soil losses in pounds per acre 227 53 20 11,188 4,541 5,027 30,150 6,356 11,238 34,384 9,287 18,778 42,519 12,377 25,152 IF. M. = soil at low field moisture; Sat'd = surface soil saturated from 1st. increment of rain or by rainfall immediately preceding the run. 2nd. increment applied immediately following the 1st. increment. 15 the 20 per cent slope. About five days later a 1.1-inch intermittent rain fell in 6 hours. The losses varied from 113 pounds per acre on the level plots to 3,122 pounds on the 20 per cent slope. Excessive soil losses frequently occur before absorption is satisfied, provided the rate of rainfall exceeds the rate of infiltration. When the plots were planted to cotton, an 0.83-inch rain occurred in 20 minutes. The soil losses varied from zero on the level plot to 5,452 pounds on the 20 per cent slope. Later a 1.4-inch rain fell in 36 hours and the losses varied from zero on the level plot to 114 pounds per acre on the 20 per cent slope. These results are in accord with the findings at the Statesville Station (1). (See also Tables 7 and 8.) A comparison was made of the erosion from two artificial rains of constant amounts applied at different rates when the soil was broken five inches deep. In one case a 1.0-inch rain was applied in 8 minutes and in the other case 1.0 inch of rain was applied in 16 minutes. The moisture content of the soil at the beginning of each test was the same. The erosion resulting from the first application of rain ranged from 636 pounds per acre on the level plot to 19,000 pounds on the 20 per cent slope. When one inch of rain was applied in 16 minutes, the soil losses TABLE 6.-Soil and Water Losses from Cecil Clay with Different Surface Conditions of the Soil. VIVI IVI LVIVML I r lLL AV Slope of land Per cent Soil condition Freshly cultivated Compact and crusted Freshly cultivated Compact and crusted Freshly cultivated Compact and crusted Freshly cultivated Compact and crusted Freshly cultivated Compact and Vcrusted Soil moisture Per cent 10 12 Runoff Per cent 13 37 Soil losses1 Pounds per acre 117 83 sam2 the 0 5 9 10 52 60 62 67 64 70 72 662 706 21,377 19,151 36,486 20,325 50,358 10 8 11 15 9 11 10 11 20 from 2 inches of artificial rain applied in contour rows in all cases. 2 Values not determined. iLosses 18 minutes; cotton planted in 3-foot, TABLE 7.-Erosion Losses Produ4ced by Natural Rainfall on Cecil Clay during the Crops (Nov. 1933 - April 1934, inclusive). -Estimated tionAmount offground ran- coverranalrfall- rainrainfall fall age Time ________(Vetch Growing 15 Season of Winter Cover Slope of land in per cent 0 Vetch' ) 5 Rye' Vetch 10 Rye Vetch Rye Vetch 20 Rye Vetch Rye I Date 11/22/33 12/6/33 12/19/33 12/24/33 1/22/34 2/1/34 2/10/34 2/18/34 2/22/34 2/25/34 3/3/34 Inches 0.40 0.72 0.65 0.30 0.43 0.59 1.00 0.62 0.83 1.11 2.83 0.62 Hours 1/3 2 1 1/3 1/3 10 76 7 5 13 30 2 Per cent Nil Nil Nil 5 7 10 10 20 20 25 30 50 Soil losses in pounds per acre 136 Nil Nil 3 34 Nil Nil Nil Nil 113 352 43 55 65 153 5 Nil Nil 46 174 923 78 100 145 189 45 Nil 57 79 1,674 1,002 55 84 103 241 45 Nil 28 97 1,355 1,0002 146 181 194 295 53 Nil 46 176 1,811 1,168 74 117 114 283 35 Nil 53 163 1,183 2,0002 100 308 282 321 59 5 73 160 2,020 2,115 160 291 256 339 49 2 36 210 2,797 2,084 253 499 350 413 56 4 45 136 6,457 49 38 142 114 13 13,775 2,152 205 405 318 478 48 2 29 197 3,103 3/19/34 3/26/34 4/15/34 4/19/34 4/29/34 Totals 0.50 1.39 0.95 1.05 9 7 3 18 75 100 100 100 412 Nil Nil Nil Nil Nil 698 v- 950 Nil Nil 27 50 13 1,933 Ivv 2,101 43 10 64 22 1 5,531 Ivv 3,296 57 16 90 216 46 6,731 u II 3,326 36 23 85 34 10 7,416 CIVV 3,292 64 25 221 354 112 7,258 5,0002 56 16 95 143 13 10,651 4,686 50 27 274 427 181 11,900 3,12.2 6,110 45 35 303 407 170 14,007 13.99 184 TABLE 7.-Erosion Losses Produced by Natural Rainfall on Cecil Clay during the Crops (Nov. 1933 - April 1934, inclusive). (Continued). Growing 1,482 395 Season of Winter Cover Date 11/22/33 12/6/33 12/19/33 12/24/33 1/22/34 2/1/34 2/10/34 2/18/34 2/22/34 2/25/34 3/3/34 3/19/34 3/26/34 4/15/34 4/19/34 4/29/34 Totals Inches 0.40 0.72 0.65 0.30 Hours 1/3 2 1 1/3 1/3 10 76 0.43 0.59 1.00 0.62 0.83 1.11 Per cent Nil Nil Nil 5 7 10 10 ;I -i Runoff in cubic feet per acre 1 7 5 13 30 20 20 25 30 113.99 2.83 0.62 0.50 1.39 0.95 1.05 2 9 7 3 18 1 38 325 49 75 50 100 215 673 1,048 100 54 635 100 255 70 69 0184 6,424 (12,0621-18,509 50 427 5 40 38 212 69 102 293 85 1,210 3,598 26 39 156 780 272 357 496 502 89 113 292 234 1,992 5,330 1,287 331 440 721 748 400 102 1,287 378 1,340 531 610 644 819 568 1,495 3,445 7,120 378 112 500 1,861 3,785 7,760 440 664 730 742 430 126 610 1,650 2,800 6,120 209 1,152 335 638 682 633 1,293 341 802 1,398 466 109 470 994 875 889 523 165 731 475 1,168 892 1,320 358 881 735 712 384 779 683 390 119 392 925 616 149 673 1,448 1,575 2,860 6,170 449 1,622 2,776 6,230 382 92 920 1,786 154 720 684 181 1,850 1,575 148 856 1,052 1,334 2,794 5,780 325 229 2,023 1,732 2,805 5,725 318 299 1,472 131 392 1,130 2,795 5,435 276 839 182 20,872 18,.211 120,200 1,055 119,394 120,139 120,216 119,425 i i i 174 1,023 1,512 341 283 2,280 1,717 59~3 'Vetch planted in 18-inch drill rows on contour; rye planted in 10-inch drill rows on contour; 20.0 inches total rainfall during the period. 2 Data incomplete; values estimated to ohtain seasonal totals. TABLE 8.-Soil Losses and Runoff Resulting from Natural Rainfall on Cecil Clay during the Winter Season with and without Vetch as a Cover Crop (Nov. 1934-May 1935), Inclusive. Amount of rainfall' Inches 0.95_. 0.63 0.83 1.28 0.81 2.85 1.65 3.93 1.64 1.25 1.47 1.08 1.63 i Time of rainf all Date 11/21/34 11/29/34 12/19/34 12/28/34 1/7/35 2/14/35 2/26/35 3/6/35 3/12/35 3/28/35 4/8/35 4/11/35 5/7/35 Totals Date 11/21/34 11/29/34 Duration of rainfall Estimated ground 0 cover-FallowI Vetch2 age (Vetch) 13 11 Nil 12 48 106 14 167 369 53 Nil 117 17 927 \ I I Slope of land in per cent 5 10_15_20 Fallow I Vetch Fallow j Vetch Fallow Vetch Fallow Vetch Hours Per cent 2 5 3 5 4 7 6 13 10 2 30 72 37 14 50 18 65 2 31/2 75 90 1/ 100 7 100 14 i v 81 58 Nil 37 87 247 20 139 84 11 Nil 1 8 773 765 574 113 I I Soil losses in pounds per acre Ii 242 207 92 237 373 4,782 1,533 8,247 6,529 1,478 Nil 981 2.863 27,564 A 7) I 357 197 129 280 509 1,321 96 620 237 24 Nil 2 12 327 445 192 413 436 21,534 4,185 26,354 17,598 5,750 1,314 4,294 7,376 I I 465 522 180 529 934 2,529 54 843 265 23 Nil 33 15 6,392 20.00 Inches 0.95 0.63 0.83 164 II I I~~I 3,784 ~ Hours Per cent 2 5 5 7 Runoff 2,683 II 90,218 Nil 63 73 2,220 1,478 3 4 12/19/34 12/28/34 1,096 2,251 1,831 1,552 1,039 1/7/35 1.28 0.81 13 2 6 10 26 76 458 273 2,324 1,877 2/14/35 2/26/35 3/6/35 2.85 1.65 3.93 72 14 18 30 37 50 825 76 3,550 1,625 169 3,380 4,610 3,554 10,755 3,740 2,332 8,915 3/12/35 3/28/35 4/8/35 4/11/35 5/7/35 Totals 2 1.64 1.25 0.47 1.08 1.63 20.00 2 3'/2 112 7 14 164 _ 65 75 90 100 100 3,410 95 Nil 1,452 412 10,058 2,330 58 Nil 156 200 10,101 5,180 2,363 Nil 3,359 4,490 43,187 4,960 95 Nil 256 328 30,101 cubic feet per acre 2,354 2,552 1,511 1,500 1,337 1,012 2,310 2,037 1,285 1,435 2,940 4,805 2,098 1,120 11,335 8,850 5,215 4,500 2,318 425 658 Nil 3,715 772 700 4,261 43,202 27,843 4[3,076 2,872 1,395 1,195 2,297 1,643 4,440 3,064 1 0,860 5,225 2,253 151 3,464 4,217 -/ in I Ll ,,,, 355 509 280 922 863 35,007 5,184 42,915 25,261 6,686 2,411 6,732 8,054 135,179 884 . 448 242 728 676 7,156 992 2,574 986 28 Nil 33 20 14,767 2,587 1,583 1,123 1,964 1,414 3,140 1,284 9,220 4,530 352 26 722 810 28,755 946 694 295 1,320 1,158 56,160 6,682 54,233 31,090 8,456 3,487 8,475 9,666 182,662 I 1,844 868 322 641 1,116 7,306 1,046 4,527 1,772 106 Nil 107 20 19,675 -- 2,401 1,567 1,305 2,141 1,518 4,530 3,554 12,875 5,340 2,022 757 3,821 4,255 46,086 2,610 1,297 1,070 1,938 1,465 2,735 1,284 8,065 4,150 718 52 939 862 27,185 'Total 20.0 inches as rainfall producing runoff. rainfall for period = 24.5 Vetch planted in 18-inch drill rows on contour. inches; 19 varied from 336 pounds per acre on the level plot to 7,520 pounds per acre on the 20 per cent slope. In the above case it is evident that the rate of rainfall rather than the amount is the influencing factor which determines the quantity of losses. Numerous other examples showing the effect of the rate of rainfall on erosion may be cited from the erosion experiments. For example, two similar natural rains occurred when the plots contained a small growth of vetch two inches in height in 18inch contour rows. The first rain of 0.83 of an inch was distributed over a period of 7 hours; soil losses ranged from zero on the level plot to 322 pounds per acre on 20 per cent slope. Nineteen days later an 0.81-inch rain fell in 2 hours. Soil losses varied from 87 pounds per acre on the level plot to 1,116 pounds per acre on the 20 per cent slope. Similar results were obtained on compact fallow during natural rains. Detailed data may be found in Table 8. When the quantity of rainfall is constant, the rate of rainfall is the factor which determines the extent of erosion losses provided other conditions are comparable. Influence of Quantity of Rainfall on Erosion.-With a given intensity of rainfall, the quantity of rain has a marked influence on erosion. In Figure 5 is given a comparison of the losses from a 0.74-inch rain which fell in approximately four hours with those from a 1.60-inch rain which fell uniformly throughout a ten-hour period. The rates were approximately the same but the duration of the first was about one-half that of the second. In the case of the 0.74-inch rain, the amount of eroded material varied from 30 pounds per acre on the level plot to 765 pounds on the plot having a 20 per cent slope. With a rainfall of 1.60 inches, the eroded material increased from 706 pounds per acre on the level plot to 8,720 pounds on the 20 per cent slope. The greater part of the 0.74-inch rain was consumed in saturating the soil. This accounts for the small amount of runoff and eroded material. In the case of the 1.60-inch rain, approximate saturation was reached and consequently a greater quantity of runoff and erosion occurred during the latter part of the rain. This principle has been repeatedly verified under numerous natural and artificial rainfall conditions. Continuity or duration without intermission of rain is of vital importance in erosion control and is so closely related to intensity and quantity of rainfall that it will be discussed in brief at this point. The greater the duration of a rain of a given intensity the greater the soil and water losses. During the fall of 1934 when alternate plots were in smooth fallow, an 0.83-inch rain occurred in 7 hours. Several days later a 1.65-inch rain occurred in 14 hours. The amount of the second rain was approximately twice that of the first but the intensity was the same. The losses from the latter were 20 about 20 times as great as those from the former. It was found that with the same amount of rainfall erosion losses were greater when there were no breaks in the rainfall than when there were breaks or short lapses during the period of rainfall. This is clearly shown in Figure 6. The same principle is likewise substantiated by data reported in Table 5. Two 1.25-inch increments of artificial rain were applied during a period of 11 minutes per increment, with a slight interval between the two applications. The soil losses from the last rain were greater than those from the first. Later, when the soil conditions were comparable, 2.50 inches of rain were applied in 22 minutes without interruption. This was twice the amount of rain applied at the same rate. The losses from the 2.50-inch rain were considerably greater than the combined losses from the two 1.25-inch increments. Influence of Pulveri- 5 10 15 20 Slope of Land in Percent FIGURE 5.-Comparison of the amounts of soil eroded from various slopes by different quantities of rain falling at comparable intensi- ties. nti h a c c a 2 eLrj J'OZ liV) 4 6 8 /0 12 /4 16 Duration of Rainfall in Hours 0 prn 0/0 - s - 5in s 14 hours 5 5 J 1 5 15 I0 zo Slope of Land in Percent FIGURE 6.-Comparison of the amounts of soil eroded from different slones. by continuous and in- termittent rains of comparable zation, Structure and quantities. Shape of Surface on Erosion.-Runoff and erosion are greatly affected by the shape of surface and state of pulverization of the soil. Soil and water losses from freshly plowed and from firm fallow' plots 'The term '"'fallow" is used herein to describe a practice by which the soil was kept smooth, compact and free of weeds; weeds were removed by hoeing them off at the ground surface. 21 are shown in Table 5. In the case of the first 1.25-inch rain which was added when the surface soil moisture was low, the greatest losses in all instances occurred from the fallow plots. The rate of absorption was extremely high on the plowed soil as compared with that on fallow, hence runoff and soil losses from the plowed areas were extremely small. A second 1.25-inch rain was applied within a few minutes The surface horiafter the completion of the first application. zon of the soil was still approximately saturated with water. Soil losses from the second increment of rainfall were appreciably less on fallow than those from the first increment even though runoff from the second rain was greater than that from the first rain. This may be attributed to the fact that the first rain slaked a thin layer of soil loose from the surface of the fallow plots; such material was quickly eroded from the steeper slopes and left the soil in a relatively non-erodible condition. The second 1.25-inch of rainfall on the plowed plots caused erosion losses many times greater than those produced by the first rain; runoff was likewise greatly increased. A comparison of losses from fallow and plowed land during the second increment of rainfall shows that the soil losses from the plowed areas on steep slopes were markedly greater than those from fallow even though runoff was greater from the latter. This was not true on the more gentle slopes. The apparent inconsistency may have been due to the fact that the first rain did not nearly satisfy absorption on the plowed plots or to a failure of this first increment of rainfall to wash off all of the loose soil from the fallow plots. To determine the effect of cultivation on erosion losses while all plots were in cotton, they were given a 2-inch artificial rain in 18 minutes when the soil was compact and crusted from previous rainfall. The plots were allowed to dry until the moisture was the same as in the first case after which a shallow cultivation followed. Two inches of rain were again applied in 18 minutes. With the exception of the 5-per-cent-slope plots, soil losses were greater when the plots were freshly cultivated than they were when compact and crusted; the differences were most pronounced on the steeper slopes. (See Table 6.) The increased losses on the freshly cultivated plots were caused by absorption being exceeded when the soil was in a very erodible condition. The influence of cloddy structure and ridged effect is again brought out quite vividly in Table 9. These data are from interplanted corn and velvet beans and from firm fallow. Numerous clods were present in the cultivated area and the corn was planted flat, but as cultivation progressed ridges were gradually developed and were quite pronounced by the time of the last cultivation. Increased obstruction due to clods and ridges increased the amount of absorption and decreased the runoff velocity. Pro- TABLE 9.-Soil Losses and Runoff Resulting from Natural Rainfall during the Growing Season of an Interplanted Crop of Corn and Velvet Beans Compared to Smooth Fallow on Different Slopes. Duration of rainfall Estimated ground coverage on C. B. plots 5 8 10 15 20 25 40 75 85 85 85 Slope of land in per cent 0 F' C. B.' F 5 I C. B. F 10 C. B. F 15 20 C. B. F C. B. Time of rainf all Amount of rainfall' Inches 1.32 .95 .65 1.40 1.15 1.50 1.45 1.01 .75 1.20 2.00 13.39 Inches 1.32 .95 .65 1.40 1.15 1.50 1.45 1.01 .75 1.20 2.00 13.39 I Date 6/5/35 6/22/35 6/29/35 7/6/35 7/11/35 7/13/35 7/16/35 8/6/35 8/11/35 8/15/35 8/20/35 Totals Date 6/5/35 6/22/35 6/29/35 7/6/35 7/11/35 7/13/35 7/16/35 8/6/35 8/11/35 8/15/35 8/20/35 Totals Hours Per cent 6 4 48 31/2 34 412 Soil losses in pounds per acre 122 79 0 45 40 198 0 121 0 0 69 674 1,688 777 0 472 1,020 1,727 14 1,940 0 0 789 8,427 102 0 0 23 0 110 0 0 0 0 0 235 162 0 0 147 0 202 0 2,765 0 0 317 3,593 I I I 3 12 '/2 48 36 155 Hours Per cent 6 5 4 8 48 10 312 15 4/4 20 41/2 25 3 40 12 75 1/2 85 48 85 36 85 155 5,239 1,760 531 4,318 3,737 7,075 2,378 350' 1503 522 3,071 29,131 4,620 2,480 1,005 3455 3,325 4,081 2,840 3,000' 1,0003 2,347 5,162 33,315 632 7,893 57 5,047 0 1,141 684 12,945 431 11,231 4,510 16,868 2.981 7,344 2,898 14,291 708 6,991 48 2,495 198 5,827 13,147 3,403 92,073 73,784 Runoff in cubic feet per acre 148 4,431 1,030 3,814 0 2,442 98 2,559 0 1,130 0 1,090 760 3,422 1,595 3,241 0 3,162 185 3,174 2,432 4,078 2,486 3,985 1,735 2,810 1,945 2,400 2,225 3,5003 1,912 3,415 507 1,786 612 1,380 670 1,975 43 1,386 841 4,756 1,653 4,297 136 12 0 350 300 930 254 272 88 35 38 7,910 5,169 1,557 7,705 8,744 11,555 6,449 10,832 6,230 2,722 4,911 561 143 0 2,914 87 13,720 7,190 7,074 2,235 758 240 34,922 598 574 0 1,654 321 2,648 2,160 2,700 905 937 2.070 14,567 10,508 6,793 1,678 14,512 12,391 21,450 10,714 19,703 8,181 3,393 8,874 118,197 4,516 2,400 1,267 3,534 3,375 3,978 2.818 3,5003 1,662 2,033 4,862 33,945 which were .576 380 0 3,997 248 16,350 7,174 10,584 1,721 713 595 43.228 441 260 0 1,653 348 2,491 2,110 2,520 774 963 1,839 13,399 converted 1 9,318 33,492 11,559 30,741 'F= smooth, compact C. B. - corn and velvet beans interplanted at through cultivation into rough or cloddy contour beds. '1.45-inch erosive rain falling in 25 hours not reported. 'Data values estimated to obtain seasonal totals. fallow; 18-inch intervals in 4-foot, contour rows incomplete; 23 vided that the saturation capacity was not exceeded under such conditions, the erosion losses were decreased. When the soil was ridged and the intensity of the rain exceeded the rate of absorption to a point where the water "over-topped" the ridges, the soil losses were much greater than those on non-ridged soil. These extreme losses were probably due to a hydraulic head being released when the holding capacity of the ridges was exceeded. Under such conditions runoff started quicker on the non-ridged soil but was gradual throughout the duration of the rain. This resulted in less erosion on the smooth soil. The reverse was true when the capacity of the ridges was not exceeded. From the above experiments and others, it was concluded that tillage practices are effective in controlling erosion until the rate and amount of absorption is exceeded. After these have been exceeded, greater losses will occur on freshly plowed soil than on firm soil. The Physical Nature of Erosion Losses and Certain Factors Affecting the Nature of Erosion Losses.-Several basic facts concerning the sheet erosion process have been revealed by a detailed study of the physical nature of the soil material eroded from the controlled plots of Cecil clay located on the several slopes. The size distribution of water stable aggregates was determined on representative samples of the soil material eroded from plots under a wide range of soil conditions and vegetative coverage. The wet screening or sieve method of aggregate analysis developed by Yoder (16) was employed. Typical results of this phase of the work are summarized in Tables 10 to 13 inclusive. In all cases, the mechanical analysis of the soil is given along with the aggregate analyses of the The latter determinations were made on the eroded materials. wet samples immediately following the completion of the rain in question. The aggregate losses are expressed in percentage of total soil losses and also in pounds per acre in order to facilitate study of these data. Runoff data are likewise included. A comparison of the mechanical analysis of the soil and the aggregate analyses of eroded materials shows that the unit particles primarily involved in the erosion process, in the case of structural soils, are aggregates (compound particles) rather than textural separates (sands, silt and clay). Undue emphasis has been given the frequently encountered statement that sheet erosion losses are particularly detrimental because excessive amounts of the most valuable part of the soil--the colloidal fraction-are lost during the process. As a general statement, the above is not true. From the results reported in Tables 10, 11, and 12, it may be seen that hundreds of pounds of aggregates or compound particles having diameters greater than those of coarse sands are frequently eroded from all unprotected (fallow) slopes during natural rains. This fact alone is sufficient reason for TABLE 10.-Physical Nature of Erosion Losses from Cecil Clay when Fallow and when Protected by Vetch at 30 per cent Ground Coverage. Mechanical analysis of soil (Textural separates) m. m. Per cent Aggregate size class of eroded sediments m. m. 0 Vetch FallowVetch 0.4 0.8 3.7 4.8 8.1 12.8 69.4 100 0.4 0.8 3.9 5.1 8.6 13.6 73.5 106 825 0.2 0.9 2.1 2.2 5.2 6.2 83.2 100 0.5 2.2 5.2 5.4 12.7 15.3 205.4 247 1,625 Fallow 12.9 16.5 13.9 9.4 8.7 7.9 30.7 100 617 779 665 449 416 378 1,468 4,782 4,610 5 Slope of land in per cent 10 Vetch 2.5 3.9 6.9 5.7 6.9 5.9 68.2 100 Fallow 15 2 20 I Vetch Aggregate losses in per cent of total soil losses 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 0.9 2.4 5.4 13.4 9.2 68.7 16.3 >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total 9.7 11.7 18.4 15.4 14.4 14.4 16.0 100 losses in 3.6 3.7 8.6 8.0 9.6 7.8 58.7 100 pounds per 12.2 13.3 13.0 13.8 15.0 14.2 18.5 100 acre 2 5.9 8.6 10.1 8.2 8.8 9.8 48.6 100 422 615 723 587 630 701 3,478 7,156 3,140 12.5 15.9 14.8 12.7 13.8 13.9 16.4 100 7,020 8,930 8,315 7,135 7,750 7,810 9.210 56,160 4,530 6.3 9.3 12.9 12.9 15.1 15.9 27.6 100 460 678 942 942 1,104 1.162 2,018 7,306 2,735 <0.005 52.4 Aggregate >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 < 0.05 Total soil losses in lbs./acre Runoff in cu. ft./acre 'Vetch 2 33 52 91 75 91 78 901 1,321 3,740 2,088 2,520 3.964 3,316 3,100 3,100 3,446 21,534 4,440 91 94 217 202 243 197 1,485 2,529 2,940 4,270 4,655 4,550 4,830 5,250 4,970 6,482 35,007 4,805 planted in 18-inch drill rows on the contour. Losses from a 2.85-inch natural rain; 1.5 inches of rain falling in 2 hours producing most of the erosion losses. TABLE 11.-Physical Nature of Erosion Losses from Cecil Clay when Fallow and when Protected by Vetch at 75 per cent Ground Coverage. Mechanial analysis (Tx al separates) m. m. Per cent Aggregate size class of eroded sediments m. m. Slope of land in per cent 0 Fallow 2.2 2.3 3.0 3.3 8.2 5.1 75.9 100 8.2 8.6 11.1 12.1 30.1 18.8 279.9 369 3,410 Vetch1 0.7 1.3 1.9 3.5 4.3 7.5 80.8 100 0.6 1.1 1.6 2.9 3.6 6.4 67.9 84 2,330 Fallow 4.2 8.3 11.2 14.5 16.4 8.5 36.9 100 274 542 732 947 1,071 554 2,409 6,529 5,180 5 Vetch 1.3 1.2 2.2 3.2 5.9 3.2 83.0 100 Aggregate 10 Fallow 6.6 9.5 13.3 14.9 23.6 13.3 18.8 100 losses in 15 Vetch 2.2 2.2 4.2 3.5 6.0 5.7 76.3 100 pounds 20 Vetch 2 Fallow 7.7 11.9 16.0 15.6 16.0 14.7 18.1 100 per acre 2 Fallow 10.5 12.9 13.4 13.7 14.2 16.6 18.8 100 3,252 4,014 4,172 4,244 4,421 5,147 5,840 31,090 5,340 Vetch 3.8 5.7 7.3 8.5 12.8 11.6 50.3 100 67 101 130 150 226 206 892 1,772 4,150 Aggregate losses in per cent of total soil losses 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 0.9 2.4 5.4 13.4 9.2 68.7 16.3 >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 2.6 4.9 5.3 5.3 12.4 10.4 59.1 100 26 48 52 52 123 103 582 986 4,530 <0.005 52.4 3.2 2.8 5.3 7.5 14.0 7.5 196.8 237 4,960 1,172 1,672 2,334 2,627 4,152 2,340 3,301 17,598 5,225 5.9 5.8 11.2 9.2 15.8 15.1 202.4 265 4,500 1,942 3,017 4,039 3,943 4,036 3,714 4,570 25,261 5,215 Total soil losses in lbs./acre Runoff in cu. ft./acre 'Vetch planted in 18-inch drill rows on the contour. 2Losses from 1.64-inch natural rain falling in less than 2 hours. TABLE 12.-Physical Nature of Erosion Losses from Cecil Clay when Fallow and when Protected by Vetch at Complete Ground Coverage. Mechanical analysis of soil (Textural separates) m. m. 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 Aggregate size class of eroded sediments m. m. 0 Fallow 3.1 4.0 7.5 6.6 19.8 14.1 44.9 100 7 9 17 15 45 32 102 227 5,260 Vetch Nil Nil Nil Nil Nil Nil 100.0 100 Nil Nil Nil Nil Nil Nil 8.1 8 3,610 1 Slope of land in per cent 5 Fallow 3.0 4.3 18.0 19.1 16.3 13.0 26.3 100 331 485 2,009 2,139 1,828 1,455 2,941 11,181 7,600 Vetch Nil Nil Nil Nil Nil Nil 100.0 100 Nil Nil Nil Nil Nil Nil 6.4 6 4,660 Fallow 10.4 11.1 16.3 14.3 18.0 11.2 18.7 100 3,134 3,350 4,915 4,303 5,431 3,372 5,645 30,150 7,870 10 Vetch 1.1 2.7 5.0 4.2 10.3 6.9 69.8 100 0.3 0.7 1.3 1.1 2.7 1.8 18.2 26 4,610 Fallow 6.7 9.3 15.8 16.4 19.4 10.4 22.0 100 2 15 Vetch 1.5 3.9 6.0 5.6 9.7 7.7 65.6 100 0.7 1.9 2.9 2,7 4.7 3.7 31.6 48 4,830 Fallow 4.5 8.4 14.0 14.2 24.1 12.7 22.1 100 1,925 3,586 5,933 6,009 10,248 5,419 9,399 42,519 8,770 20 Vetch 1.6 3.5 8.5 12.7 11.4 8.8 53.5 100 8.1 18.3 44.4 66.4 59.6 45.6 279.0 521 5,320 Per cent 0.9 2.4 5.4 13.4 9.2 68.7 16.3 2 Aggregate losses in per cent of total soil losses >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals <0.005 52.4 Aggregate losses in pounds per acre >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total soil losses in lbs./acre Runoff in cu. ft./acre 1 Vetch 2 2,336 3,181 5,421 5,652 6,677 3,576 7,641 34,384 8,260 planted in 18-inch drill rows on the contour. Losses from 2.50 inches of artificial rainfall applied in 22 minutes when surface soil was at low field moisture. 27 concluding that the sheet erosion process, when uncontrolled, bodily removes the top-most part of the soil profile layer by layer. In addition, field observations also indicate that if the process is not controlled, the surface horizon of the soil is finally washed away. If any part of the surface material is left behind, it is only a gravel or rock blanket. However, there are certain conditions under which the relative loss of colloidal material may be excessive. The relative loss of colloidal material may be excessive during (a) erosion from soil protected by considerable vegetation (land planted in soil conserving crops), (b) erosion produced by soil conditions resulting in small amounts of runoff, (c) erosion from flat land and possibly from extremely gentle slopes, and (d) erosion produced by intermittent small showers of rain falling at slow rates. The above conditions are all characterized by small quantities of runoff or by runoff of low forward moving velocity or by both. Ample supporting data for the first three conditions may be found in Tables 12, 13, and 14. The resulting effects of the last mentioned condition have been repeatedly measured on the controlled plots. In brief, the relative amount of colloidal material lost is excessive only when the sheet erosion process is controlled, in a practical sense, or when the total quantity of soil lost is almost negligible. The above conclusions have the additional support of field observations. In the Southeast, the only conditions under which sandy surface horizons have developed on soil profiles having large clay contents are where topography is flat or where a thick, permanent, vegetative coverage has been allowed to persist on gentle slopes. During the course of the experiments, considerable information has been obtained concerning the basic principles involved in the use of vegetation to control sheet erosion. Aggregate analyses of eroded materials served as a basis for the analysis of the problem. It has been found that cover crops function in reducing sheet erosion losses by (a) filtering out the larger water stable aggregates, (b) decreasing the quantity of runoff, (c) decreasing the velocity of runoff, (d) minimizing the turbulence of runoff and hence lessening the abrasive or dispersive action of sediment loaded water, and (e) minimizing the mechanical dispersive action of beating rainfall. From a study of the data presented in Tables 10, 11, and 12, which permits a comparison of the nature of the materials eroded from fallow plots and from plots protected by vetch at different stages of growth, it may be seen that in every case the plants functioned by filtering out large quantities of the coarser aggregates. As the growth of the plants and hence the extent of ground coverage increased, the efficiency of the process increased. It was not uncommon to find two to three inches of soil piled above the upper side of contour rows of vetch which TABLE 13.-Physical Nature of Erosion Losses from Cecil Clay under Different Strip Cropping Practices. Mechanical analysis (Tof soil (Textural separates) m. m. Per cent Aggregate size class of d eroded sediments m. m. 12.5 ft. plowed 37.5 ft. vetch F. M. Nil Nil Nil Nil Nil Nil 100.0 100 Sat'd Nil Nil Nil Nil Nil Nil 100.0 100 Slope width of Strip 2 25 ft. plowed 37.5 ft. plowed 50 ft. plowed 25 ft. vetch 12.5 ft. vetch No vetch F. M. Nil Nil Nil Nil Nil Nil 100.0 100 Sat'd 0.2 0.2 0.4 0.3 0.8 0.4 97.7 100 F. M. Nil Nil Nil Nil Nil Nil 100.0 100 50 ft. fallow No Vetch F. M. 2.1 2.4 9.2 12.5 23.9 16.1 34.0 100 Sat'd 2.6 5.9 15.3 11.1 19.1 13.8 32.2 100 ISat'd 0.1 0.1 0.1 0.1 0.2 0.3 99.3 100 F. M. Nil Nil Nil Nil Nil Nil Nil 100 Sat'd 0.4 0.8 1.7 2.1 3.3 3.3 88.4 100 Aggregate losses in per cent of total soil losses on 5 percent slope 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 1.2 2.9 6.4 13.0 8.5 68.0 15.9 >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals <0.005 52.1 Aggregate losses in pounds per acre on 5 per cent slope >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total soil losses in lbs./acre Runoff in cu. ft./acre Nil Nil Nil Nil Nil Nil 12.4 12 960 Nil Nil Nil Nil Nil Nil 63.8 64 2,870 Nil Nil Nil Nil Nil Nil 37.8 38 1,071 0.3 0.4 0.7 0.6 1.6 0.6 172.7 177 2,839 Nil Nil Nil Nil Nil Nil 31.3 31 419 0.1 0.1 0.2 0.2 0.6 0.8 291.1 293 3,358 Nil Nil Nil Nil Nil Nil Nil Nil Nil 5 10 22 27 43 42 1,128 1,277 3,002 105 121 463 617 1,202 810 1,709 5,027 3.195 118 268 696 504 869 626 1,460 4,541 3,875 1 TABLE 13.-Physical Nature of Erosion Losses from Cecil Clay under Different Strip Cropping Practices . (Continued) m. m. Per cent 0.8 2.4 5.3 13.5 9.7 68.3 16.9 51.4 v v I m. m. 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 . 0.05-0.005 <0.005 uv --r v I >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 uTotals v >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Nil Nil Nil Nil Nil Nil 100.0 100 Nil Nil Nil Nil Nil Nil 14.2 14 i Aggregate losses in per cent of total soil losses on 10 per cent slope 2.8 1.3 4.4 0.1 0.1 0.1 Nil 0.3 6.4 1.3 8.1 0.1 0.2 0.1 Nil 0.7 17.2 3.9 10.2 0.2 0.7 Nil 0.3 1.2 12.6 3.2 7.3 0.2 0.6 0.9 Nil 1.1 23.7 7.3 0.4 11.5 0.6 1.4 2.2 Nil 9.5 13.5 8.0 1.7 0.5 2.0 Nil 1.4 27.8 65.3 54.7 94.7 97.3 97.8 93.1 100.0 100 0.2 0.5 0.8 0.7 1.5 0.9 61.9 67 3,190 I 4.0 7.0 18.4 13.7 18.4 10.2 28.3 100 252 444 1,173 873 1,167 646 1,801 6,356 3,899 100 100 100 100 100 100 100 313 724 1,930 1,411 2,662 1,071 3,127 11,238 3,572 -° Total soil losses in lbs./acre Runoff in cu. ft./acre 1 2 ggregate losses in pounds per acre on 10 per cent slope 164 0.3 1.6 0.4 0.1 Nil 1.7 302 0.5 0.4 0.2 Nil 4.9 381 0.8 0.7 0.9 Nil 277 0.9 4.1 1.5 1.1 Nil 14.6 272 1.5 1.6 1.6 Nil 298 17.2 2.3 6.4 1.2 Nil 2,050 366.8 83.1 249.4 111.1 27.8 3,743 127 117 377 28 255 664 3,110 854 3,410 180 2,811 1,535 Losses from 1.25 inches artificial rainfall in 11 minutes with vetch at full ground coverage in all cases. Slope width of all plots - 50 feet; strip crop below plowed area; plowing done day previous to test in all cases; F. M. -= soil at low field moisture; Sat'd. = surface soil saturated from first 1.25-inch rain as second 1.25-inch was applied immediately after the first run in each case. w TABLE 13.-Physical Nature of Erosion Losses from Cecil Clay under Different Strip Cropping Practices. (Continued) Mechanical analysis of soil (Textural separates) m. m. 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 <0.005 Per cent 0.9 2.4 5.0 13.8 9.4 68.5 16.2 52.3 Aggregate size class of eroded sediments m. m. 12.5 ft. plowed 37.5 ft. vetch F. M. Sat'd _______ - 25 ft. plowed 25 ft. vetch F. M. Sat'd J Slope width of 37.5 ft. plowed 12.5 ft. vetch F. M. I Sat'd strip' 50 ft. plowed No vetch F. M.I Sat'd 50 ft. fallow No vetch F. M J Sat'd ____ >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals 1.5 2.0 3.5 7.9 9.9 5.4 69.8 100 0.3 Aggregate losses in per cent of total soil losses on 15 per cent slope 8.7 0.5 0.4 3.8 0.1 3.8 Nil 0.1 4.8 11.6 0.7 0.3 3.4 0.1 Nil 0.2 2.1 14.3 0.7 0.1 11.7 Nil 5.8 0.3 2.2 13.0 0.7 0.4 7.0 13.1 Nil 0.2 3.3 0.6 10.1 20.1 1.9 8.7 0.6 Nil 2.8 12.9 2.5 Nil 2.4 18.6 15.0 0.5 30.8 88.4 100 96.3 49.9 93.6 32.9 98.1 100 100 0.6 Nil 4.8 7.0 15.5 13.4 14.1 11.1 34.1 1100 100o 0.3 100 0.5 100 0.6 100 8.3 100 1,694 100 710 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total soil losses in lbs./acre Runoff in cu. ft./acreL >2.0 _Aggregate losses in pounds per acre on 15 per centslope_ 442 0.4 0.7 1.6 2.0 1.1 14.1 20 1,540 0.9 2.7 2.8 4.2 3.6 113.1 128 3,500 Nil Nil Nil Nil Nil 29.0 29 857 0.7 1.1 0.9 2.4 1.9 411.6 419 3,359 0.4 1.0 1.0 2.7 3.4 133.3 142 967 0.7 0.9 2.7 3.7 15.6 608.2 632 3,468 10.3 12.6 15.1 21.9 40.3 108.2 217 240 2,253 2,783 2,520 1,678 2,502 5,972 -19,402 2,951 640 2,190 2,465 3,770 2,827 6,176 18,778 3,691 652 1,438 1,242 1310 1,032 3,171 9,287 4,097 TABLE 13.-Physical Nature of Erosion Losses from Cecil Clay under Different Strip Cropping Practices1 . (Continued) m. m. 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 Per cent 0.8 2.4 5.5 13.7 9.1 68.5 16.1 m. m. Aggregate losses in per cent of total soil losses on 20 per cent slope 1.1 1.6 4.9 3.5 15.2 11.4 62.3 100 1.8 2.4 5.3 5.7 9.2 6.4 69.2 100 3.2 4.4 9.6 10.4 16.6 11.5 125.4 181 3,360 0.3 0.4 1.2 1.4 1.8 2.9 92.0 100 0.5 0.7 1.9 2.2 2.8 4.6 143.7 156 1,129 0.2 0.4 1.2 0.8 2.7 2.3 92.4 100 1.5 2.5 7.3 5.0 17.1 14.2 577.4 625 2,924 0.2 0.2 0.6 1.2 2.2 3.6 90.0 100 0.4 0.4 1.4 2.8 5.2 8.4 212.1 231 885 0.1 0.2 0.4 0.8 1.9 4.9 91.7 100 1.9 2.9 5.4 11.3 25.8 66.6 1,249.2 1,363 3,115 1.9 2.0 5.9 4.8 17.2 20.2 48.0 100 34 36 105 87 308 362 862 1,794 450 7.7 10.6 14.2 12.1 17.7 13.8 23.9 100 3,080 4,245 5,680 4,825 7,080 5,510 9,560 39,980 3,495 8.6 13.4 15.9 11.7 13.0 11.1 26.3 100 2,154 3,372 3,990 2,933 3,271 2,794 6,638 25,152 3,684 6.8 7.8 12.0 13.3 11.8 10.1 38.2 100 841 959 1,488 1,648 1,455 1,253 4,733 12,377 3,902 >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals <0.005 52.4 Aggregate losses in pounds per acre on 20 per cent slope >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total soil losses in lbs./acre Runoff in cu. ft./acre 1 2 Losses 0.4 0.6 1.8 1.3 5.6 4.2 22.9 37 1,295 from 1.25 inches artificial rainfall in 11 minutes with vetch at full ground coverage in all cases. Slope width of all plots - 50 feet; strip crop below plowed area; plowing done day previous to test in all cases; F. M. soil at low field moisture; Sat'd. surface soil saturated from first 1.25-inch rain as second 1.25-inch was applied immediately after the first run in each case. TABLE 14.-Percentage Runoff from Cecil Clay with Various Strip Cropping Practices (Artificial Rainfall-1935). Slope width of strips2 Slope of land land 12.5 ft. plowed 37.5 ft. vetch F. M. I Sat'd 0 22 34 34 28 54 63 70 77 74 25 ft. plowed 25 ft. vetch F. M. Sat'd 0 24 15 19 25 54 63 69 74 64 Per cent 0 5 10 15 20 F. M. Runoff in per cent of rainfall 0 9 19 21 20 68 74 75 76 69 0 0 4 5 10 37.5 ft. plowed 12.5 ft. vetch F. M. Sat'd 50 ft. plowed No vetch Sat'd 34 66 62 65 77 50 ft. fallow No vetch F. M. 27 70 79 81 81 Sat'd 52 85 86 90 86 from 1.25-inches of artificial rainfall in 11 minutes in all cases. 2 Slope width of all plots= 50 feet; strip crop was vetch at maturity or maximum ground coverage; strip crop helow plowed area; plowing done immediately before tests in all cases; F. M. soil at low field moisture; Sat'd= surface soil saturated from first 1.25-inch rain. 'Runoff - had been~ plahnted onl smooth steep slop~es. The stairv-step (effect produtc ed by the filtering process maY be seen in Figurec 7. / y ,~. 911y 'N+sl.Y u.' . *00i i - L 1 6_ 4 rw hI G I R lt bot in rr, ii fie. n ru 1'lo h u d r tc oa u .r ifr lt. w u ys o t Jlp ini.o e tpn t time juin the!d sa epei ptrodnlc'eat grtet rin thea foilteriooreoil the ote par tice an etgh err p l . 34 It is common knowledge that the quantity of runoff is markedly decreased by vegetation. This fact is repeatedly verified by runoff data presented at various places in this publication. The reduction in quantity of runoff is caused primarily by (a) permitting increased absorption and infiltration of water through old root channels, and (b) by holding the water on the slope for a longer period of time during which these processes may (See Table 12.) In addition, plant resicontinue to function. dues and resulting organic matter exert a pronounced effect on the aggregation characteristics of soils. The effects of these characteristics on erosion have been observed in the field but have not been quantitatively measured. The data in Table 15 may serve to illustrate the influence of organic matter on aggretion. TABLE 15.-Aggregate Analysis of Hartsells Fine Sandy Loam and of Porters Sandy Loam. Soil type Hartsells fine sandy loam Porters sandy loam Aggregate size classes in millimeters 2.0-1.0 1.0-0.5 10.5-0.25 10.25-0.100.10-0.05 Aggregate separates in per cent of total 5.9 50.1 5.9 10.4 8.1 13.1 8.7 9.7 13.1 11.8 35.7 4.0 22.6 0.9 >2.0 <0.05 The above two soils were found to be very similar in mechanical analysis but the Porters soil contained 5.6 per cent organic matter while the Hartsells contained only 1.5 per cent organic matter. The Porters soil is strongly aggregated; field observations indicated that this soil possessed extreme resistance to erosion. The Hartsells soil is structureless (single grained) and is known to be very susceptible to erosion. The velocity of the film or layer of water during runoff is the factor which primarily determines the tendency of runoff to produce soil movement. The size of soil particle which water can transport is a function of its velocity. This velocity is difficult to measure directly. However, rate of runoff curves may be used to approximate slope velocity. Data from which such curves may be constructed have been obtained from a large number of artificial rainfall trials during which water was added at constant and known rates. A portion of a typical set of such curves is shown in Figure 8. From this figure it may be seen that between two and three minutes were required, after the addition of water had ceased, for runoff to stop on a 20 per cent fallow slope. With a plot slope-length of 50 feet, this indicates a slope velocity of about 20 feet per minute. In a like manner, it may be seen that for- 35 4 8 12 /6 20 24 28 32 36 Time in Minutes FIGURE 8.-Part of a set of typical runoff curves used to approximate slope 412.5 cubic feet per (Rate of rainfall velocities of runoff. acre per minute.) ward moving velocities of runoff were reduced to about 5 feet per minute and 7 feet per minute on 5 and 20 per cent slopes respectively, by a complete ground coverage of vetch. The approximate velocities of the runoff from flat plots during this trial were 21 2 and 5 feet per minute, respectively, for vetch and fallow. These curves may be taken to indicate that on the slopes studied vegetative coverage had more influence than slope on the velocity of runoff. The importance of vegetation in reducing the slope velocity of runoff can hardly be over emphasized. It is believed that this variable will have to be accurately measured before a quantitative relationship between runoff and soil losses can be established'. Nevertheless, it may interest the reader to compare the above approximated velocities with the magnitude of soil losses occurring during the same trial as reported in Table 5. 'Concurrent measurement of slope velocity of runoff and rate of soil losses are being These findings made under vegetative, tilled and fallow conditions on the different slopes. wlll be reported at a later date. 36 It was observed that the flow of runoff frequently became turbulent on steeper slopes denuded of vegetation. The presence of thick vegetative coverage tended to promote a non-turbulent type of flow thus holding the abrasive action of sedimentloaded water to a minimum. At the same time ample soil coverage by vegetation caused a water blanket to be formed which protected the soil from further mechanical dispersion by beating rainfall. The relative importance of these dispersive processes has not been evaluated. Influence of Vegetative Protection on Erosion.-Where nature has sufficiently covered the soil with vegetation, the runoff and soil losses due to erosion are not serious. Vegetation and vegetation residues contribute to retaining the soil in place in the following ways: (a) The vegetative cover breaks the falling velocity of rain which results in less soil being brought into suspension, (b) vegetative growth retards the forward moving velocity of runoff and allows coarse materials to be filtered out, (c) plant roots, organic matter, fungi and molds either increase absorption or bind the soil in place, and (d) vegetation intercepts a portion of the rainfall. The value of vegetation in controlling erosion is determined by the growth habits of the plant (7) and methods of planting rather than the number of pounds of green material per unit area. Prostrate plants with a wide lateral spread are most effective in controlling erosion. Contour row plantings are much more effective than slope plantings. A number of different crops have been tested to determine their value as soil saving crops. The winter cover crops grouped in order of their effectiveness in preventing soil losses from Cecil clay are: (a) vetch planted in 18-inch, contour rows, (b) rye planted in 10-inch, contour rows, and (c) oats planted in 10-inch, contour rows. The summer crops tested may be listed in order of their effectiveness in reducing soil losses as follows: (a) Alternate 12-foot, contour strips of soybeans planted in 18-inch rows and of cotton planted in 3-foot rows, (b) corn and velvet beans interplanted at 18-inch intervals in 4-foot, contour rows, (c) unchopped cotton planted in 3-foot, contour rows, and (d) chopped cotton planted in 3-foot contour rows. During the fall and winter months rye was slightly superior to vetch in reducing erosion losses. Beyond this period the vetch rapidly outgrew the rye to an extent that it finally covered the entire surface of the plots. A maximum coverage of approximately two-thirds of the area between the 10-inch rows was attained by the rye. Thus, the vetch was much more effective than the rye during the latter stages of growth. From a study of the total seasonal losses in Table 7 it was concluded that rye was practically as effective as vetch for decreasing erosion. 37 However, vetch is a superior crop due to its nitrogenous value and is the most practical from a soil fertility viewpoint. In this experiment both rye and vetch were planted at normal seeding rates. When the vetch and rye plants were at about one-half maturity a 1.25-inch artificial rain was added to each plot in 11 The soil was fairly well saturated from rainfall minutes. immediately preceding the trials. Under these conditions both runoff and soil losses were greater from the rye plots than from The nature of the eroded mathe vetch plots in all instances. terial was rather similar when the plants were at half maturity. After the plants were mature, erosion was (See Table 16.) practically controlled on the vetch plots; the very small quantities of soil lost consisted for the most part of material in suspension. However, in the case of rye, appreciable quantities of both coarse material and suspension solids were eroded. During the growing season of a crop of cotton, in which time the erosion producing rainfall was 12.50 inches, normal-spaced cotton plots lost appreciably more soil than plots of unthinned However, the low yields from un(See Table 17.) cotton. thinned cotton make it impractical to leave cotton unthinned. During a period from November to May, vetch was found to be about ten times more effective for controlling erosion than was fallow; the erosion producing rainfall for the season was 20 inches. Detailed data for the experiment may be found in Table 8. Summary Table 18 reveals that the combined seasonal erosion losses from vetch and cotton are about one-half as much as those from cotton and fallow over the same period; the fallow condition differed from farm practice in that weeds and stalks were removed. Influence of Strip Cropping on Erosion.-The practice of planting land to soil conserving crops in the form of contour strips of various widths between similar strips of clean-cultivated land is being recommended and used to some extent as an erosion control measure in some sections of the United States. A review of the literature fails to reveal experiments sufficiently comprehensive to serve as a basis for recommendation of such practices. During the summer of 1932 cotton was grown on low contour beds on one of two plots on each slope; the companion plots were planted in alternate 12.5-foot, contour strips of cotton and of soybeans. Rainfall during the growing season totaled 22.5 inches. Soil losses from the plots in cotton alone ranged from 9,455 pounds per acre on 5 per cent slope to 62,121 pounds per acre on 20 per cent slope. Corresponding seasonal soil losses from the strip cropped plots varied from 7,614 pounds per acre on 5 per cent slope to 18,412 pounds per acre on the 20 per cent slope. The results of this experiment are summarized in detail TABLE 16.-Physical Nature of Erosion Losses from Cecil Clay Protected by Vetch and Rye as Winter Cover Crops. Mechanical analysis of soil (Textural separates) m. m. Per cent Aggregate size class of eroded sediments m. m. Slope of land in per cent 0 Vetch Nil 7.5 6.6 5.8 22.6 17.0 40.5 100 Nil 2.2 1.9 1.7 6.6 4.9 11.8 29 1,772 1 5 Rye' Nil 3.6 4.1 3.4 15.6 15.5 57.8 100 Nil 4.0 5.0 4.0 19.0 18.8 70.0 121 3,305 Vetch 1.7 2.6 5.3 4.4 20.6 42.4 23.0 100 2.3 3.6 7.3 6.0 28.3 58.3 31.8 138 2,730 Rye 0.7 1.7 4.0 2.7 14.5 40.5 35.9 100 Aggregate 10 Vetch 1.0 2.6 7.3 5.9 18.7 37.6 26.9 100 losses in 15 Rye 2.8 8.0 15.5 9.4 15.0 15.9 33.4 100 70 203 394 238 378 401 843 2,527 3,127 Vetch 2.4 4.7 10.9 9.6 21.4 27.3 23.7 100 2 20 Rye 2 Vetch 2.7 3.5 8.4 6.7 22.3 25.9 30.5 100 46.0 60.5 144.8 114.8 383.8 446.7 525.1 1,722 3,311 Rye 5.3 7.9 12.6 8.3 17.4 14.0 34.5 100 271 404 644 427 891 717 1,763 5,117 3,908 Aggregate losses in per cent of total soil losses 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 0.05-0.005 0.9 2.4 5.4 13.4 9.2 68.7 16.3 >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Totals 3.1 8.3 20.6 9.6 17.6 17.2 23.6 100 125 332 826 385 704 692 947 4,011 3,037 <0.005 52.4 pounds per acre >2.0 2.0 -1.0 1.0 -0.5 0.5 -0.25 0.25-0.10 0.10-0.05 <0.05 Total soil losses in lbs./acre Runoff in cu. ft./acre 3.0 7.2 16.8 11.4 60.9 170.5 151.1 421 2,988 4.2 11.0 30.7 24.8 78.8 158.3 113.3 421 3,046 20.6 40.6 94.1 83.5 185.5 236.5 205.0 866 2,844 'Vetch planted in 18-inch drill rows on contour at about 50 per cent ground coverage; rye planted in 10-inch drill row on contour at about 35 2 per cent ground coverage. Soil losses from 1.25 inches artificial rainfall applied in 11 minutes; surface soil partially saturated from previous rainfall. TABLE 17.-Erosion Losses Time of rainfall Date from Chopped Cotton and from Unchopped Cotton-1934. 5 Slope of land in per cent 10 Unchoped Chopped pounds per acre Amount ra fall Inches of Duration 0 15 20 of rai fall Hours h d Un Un-Unoppechopped Chopped ch hopped Soil losses in Unchopped Chopped chopped 6/12/34 6/18/34 7/13/34 8/12/34 10/6/34 10/13/34 Totals 6/12/34 6/18/34 7/13/34 8/12/34 10/6/34 10/13/34 Totals 1.40 1.94 0.83 2.93 4.00 1.40 12.50 1.40 1.94 0.83 2.93 4.00 1.40 12.50 41/2 2 1/3 13 38 36 150 827 Nil 199 Nil Nil 1,176 440 1,551 Nil 191 Nil Nil 2,182 1,600 5,620 Nil 4,640 232 93 12,185 3,900 9,959 1,934 2,118 17 26 17,954 3,888 6,518 52 8,990 850 371 20,669 3,700 7,620 1,292 732 20 15 13,379 3,045 6,260 724 7,890 270 183 18,372 11,700 19,285 3,794 21,145 206 84 56,214 3,773 6,220 1,325 8,780 2,800 1,018 23,916 11,000 19,970 2,625 5,344 49 35 39,023 3,298 5,972 1,273 8,020 1,053 496 20,112 26,400 40,013 4,658 29,621 238 115 103,045 3,847 5,834 1,363 9,080 3,025 1,056 24.205 23,400 39,301 4,151 14,308 135 85 81,380 3,549 5,972 1,200 8,285 1,685 789 21,480 31,800 49,106 5,452 28,068 170 114 114,710 3,941 6,084 1,252 9,550 1,868 946 23,641 27,300 47,634 4,100 15,174 112 42 94,362 3,190 5,564 887 7,320 903 371 18,235 Runoff in cubic feet per acre 41/2 2 1/3 13 38 36 825 4,518 Nil 4,600 502 72 10,517 21.1 inches; 12.5 inches falling as rains which produced erosion losses; planted in 3-foot, contour rows; total rainfall for the season cotton chopped on "chopped" plots between rains of June 18th and July 13th. 2 Not determined. iCotton TABLE 18.-Yearly Erosion Losses from Continuous Cotton and from Cotton and Vetch Rotation on Cecil Clay . 1 Period of Year 0 Cotton 2 5 Cotton and Vetch Cotton Slope of land in per cent 10 15 Cotton Cotton Cotton and Cotton and Cotton and Vetch Vetch Vetch Soil losses in pounds per 20 Cotton Cotton and Vetch acre Cotton Season June-October inclusive Vetch Season November-May inclusive June, 1934 to June, 1935_____ 1,237 2,251 18,883 16,344 62,434 62,419 113,800 110,088 138,465 132,493 927 773 27,564 3,784 90,218 6,392 135,179 14,767 182,662 19,675 2,164 3,024 46,447 20,128 152,652 68,813 248,979 _________ 124,855 321,127 __________ 152,168 _________________________ Runoff in acre inches Cotton Season June-October, inclusive 3.53 4.05 6.85 6.55 7.73 7.53 7.75 11.90 7.71 7.92 15.63 7.83 12.69 7.48 Vetch Season November-May, inclusive June, 1934 to June, 1935 2.77 2.78 11.89 8.29 11.86 19.59 I I 7.67 7.48 6.30 6.83 18.74 I 14.84 I 15.20 19.65 20.52 14.96 cotton season, 21.1 inches; vetch season, 24.3 inches; vetch used as a winter legume planted in 18'Total rainfall for the year was 45.4 inch drill rows on contour. 2Plot out of level making losses abnormally low. inches; 41 in Table 19. This strip cropping practice was particularly effective in reducing soil movements on the steeper slopes. TABLE 19.-Seasonal Soil Losses from Cecil Clay Planted to Cotton and to Cotton-Soybean Strip Crops1. Period Rainfall inches Cropping system Slope of land in per cent 5 10 I 15 I20 Soil losses in pounds per acre Early summer (growing period of soybeans) 13.95 Cotton only Cotton-soybean strips 6,975 5,517 2,480 2,097 9,455 7,614 14,139 7,528 2,824 2,205 16,963 9,733 51,193 11,354 12,105 3,665 63,298 15,019 47,348 14,473 14,773 3,939 62,121 18,412 Cotton Late summer 8.52 only and fall Entire growing season Cotton-soybean stubble strips 22.47 Cotton only Cotton-soybean strips 150-foot slope width of cotton alone; alternate 12.5-foot strips of soybeans and cotton; soybeans in 18-inch, contour rows planted flat; cotton in 3-foot rows on low contour beds. Results of other experiments designed to determine the relative effectiveness of different widths of strip crops in controlling sheet erosion are summarized in Tables 20 and 21. Artificial rainfall of constant intensity and duration was applied in all instances while the ratio of strip crop to cultivated area was varied. The quantity of soil losses was greatly reduced by all widths of strip crops tested. Likewise, runoff as compared to that from fallow land was consistently reduced. These trials were made when the cover crops had reached maximum growth. At this stage the vetch in 18-inch rows completely covered the soil and the rye in 10-inch rows covered about two-thirds of the soil. A 12.5-foot strip of vetch or rye at maximum ground coverage on the lower end of a 50-foot plot practically controlled the movement of soil from all slopes studied when the upper 37.5 feet of each plot was freshly plowed and the soil was at low field moisture. One and one-fourth inches of water was applied to the plots in 11 minutes. When the soil was saturated and the above conditions held constant, a 12.5-foot strip of vetch or rye controlled erosion on a 5 per cent slope and reasonably controlled erosion on a 10 per cent slope. A 6.5-foot strip of vetch or rye controlled erosion on level plots when the rear part of the plot was freshly plowed. (Erosion was considered controlled when not more than 500 or 600 pounds of soil per acre was moved through the strips under the above conditions.) The way in which strip cropping functions in reducing sheet erosion losses is plainly revealed by a study of the nature of TABLE 20.-Soil and Water Losses from Cecil Clay under Different Strip Cropping Practices-1934. Slope width of strips 2 Slope of land 50 ft. Cover crop (at 1/2 coverage) Sat'd Vetch Rye 50 ft. Cover crop (at full coverage) F. M. Vetch Rye 37.5 ft. Cover crop 12.5 ft. Plowed F. M. Vetch Rye 25 ft. Cover crop 25 ft. Plowed F. M. Vetch Rye 12.5 ft. Cover crop 37.5 ft. Plowed F. M. Vetch Rye 12.5 ft. Cover crop 37.5 ft. Plowed Sat'd Vetch Rye No Cover crop 50 ft. plowed Sat'd Vetch Rye Per cent 0 5 10 15 20 0 5 10 15 20 29 137 421 866 1,722 1,772 2,730 3,046 2,844 3,311 121 421 2,527 4,011 5,117 3,305 2,988 3,127 3,037 3,908 6 17 103 223 170 82 1,157 1,534 2,058 1,646 35 169 1,199 1,870 1,927 2,632 2,759 2,702 2,680 2,625 0 23 47 88 120 319 1,592 2,377 2,520 2,491 Soil losses 29 128 832 972 585 Runoff in 1,461 2,268 2,811 2,899 2,191 in pounds per acre 0 0 26 45 54 58 504 71 40 204 100 643 183 519 201 cubic feet per acre 818 1,078 1,617 1,620 1,430 Nil 667 446 1,079 1,103 4 35 292 257 278 163 501 637 777 831 160 582 1,243 1,663 2,261 2,595 3,387 3,710 3,881 3,767 420 524 1,071 1,507 2,714 3,202 2,949 3,247 2,598 3,468 1,592 1,518 2,787 2,183 8,038 20,537 28,035 18,729 37,745 35,150 3,952 4,023 3,912 4,057 3,401 4,508 3,709 3,082 3,129 3,754 Nil 1,022 872 1,433 1,538 'Losses from 1.25 inches of artificial rainfall in all cases. 2Cover crop at maturity or stage of maximum ground coverage in all cases except as noted; plowing was done a day previous to trial; plowed area soil at above cover crop strip in all cases; vetch planted in 18-inch drill row on contour; rye planted in 9-inch drill rows on contour; F. M. low field moisture. Sat'd = surface soil partially saturated with water from rainfall immediately before trial. TABLE 21.-Soil and Water Losses from Cecil Clay under Different Strip Cropping Practices-19351. Slope 50 Ft. Fallow 50 Ft. Vetch of None Plowed None Plowed 37.5 Ft. Vetch 12.5 Ft.Plowed F. M. Sat'd Slope width of Strip 2 25 Ft. Vetch 12.5 Ft. Vetch 25 Ft. Plowed 37.5 Ft. Plowed F. M. Sat'd F. M. Sat'd No Vetch 50 Ft. Plowed F. M. Sat'd No Vetch 50 Ft. Fallow F. M. Sat'd land F. M. Per cent F. M. Soil losses in pounds per acre 0 5 10 15 20 0 5 10 15 20 227 11,188 30,150 34,384 42,519 5,260 7,600 7,870 8,260 8,770 8 6 26 48 521 3,610 4,660 4,610 4,830 5,320 Nil 12 14 20 37 Nil 960 1,535 1,540 1,295 Nil 64 67 128 181 2,470 2,870 3,190 3,500 3,360 Nil 38 28 29 156 Nil 1,071 664 857 1,129 26 177 255 419 625 2,440 2,839 3,110 3,359 2,924 Nil 31 117 142 231 Nil 419 854 967 885 101 293 377 632 1,363 3,067 3,358 3,410 3,468 3,115 Nil Nil 127 217 1,794 Nil Nil 180 240 450 147 1,277 3,743 19,402 39,981 1,542 3,002 2,811 2,951 3,495 20 5,027 11,238 18,778 25,152 1,207 3,195 3,572 3,691 3,684 53 4,541 6,356 9,287 12,377 2,376 3,875 3,899 4,097 3,902 Runoff in cubic feet per acre 11.25 inches artificial rainfall in 11 minutes inches failed to produce erosion on the vetch 2 Strip crop of vetch at full coverage below moisture; Sat'd - surface soil saturated from (The first 1.25 in all cases except columns 1 and 2 in which 2.50 inches were added in 22 minutes. plots.) plowed areas in all cases; plowing was done a day previous to each test; F. M. = soil at low field first 1.25-inch rain; second 1.25-inch rain applied immediately. 44 sediments eroded from firm fallow and freshly plowed slopes. Aggregate analyses of sediments eroded under the above conditions are presented in Table 13; runoff data are included. Runoff expressed in percentage of rainfall applied is given for this set of experiments in Table 14. A study of the data presented in Table 13 shows that, with the possible exception of the 12.5-foot strips on the steeper slopes, all widths of strip crop used almost completely filtered out the soil particles greater than 0.05 millimeters in diameter. A tremendous decrease of soil movement from the plots has resulted from the use of this control practice even in the case of relatively narrow filter strips. However large quantities of soil were sheet-eroded from the cultivated portions of the plots and deposited in the strip crop areas. The strip crop functioned similarly to a broad-based terrace in that the velocity of runoff decreased and the coarser sediments were deposited. It was assumed in these studies that strip cropping is not a substitute for but rather a supplement to terracing. A limited amount of field work on strip cropping was started in 1932 and has been conducted concurrently with the experimental work on the controlled plots. A 3.5-acre field which had been bench terraced and severely gullied was used for this work. Slope varied from 5 per cent to 35 per cent; the soil lacked uniformity but was predominantly a sandy loam. The old terraces were replaced with "Nichols" type terraces (4) and the area has been continuously strip cropped for nearly five years. Three systems of row direction were tried on the above field to determine the limiting slope on which equipment could be used efficiently and at what slope excessive erosion would result from different row directions. During the first year of the experiment, the key-terrace system was used (4). Rows were run parallel to the key terrace. All terraces above and below the key terrace were crossed with rows at different angles. The following year a 45 degree system was brought into effect; after careful study of a topographic map of this field it was seen that the majority of rows would incidentally be at an angle of 45 degrees to the slope. Equal width strips of cotton and soybeans as summer crops and oats and vetch as winter cover crops were rotated in both of the above instances. During the third year the contour system was brought into use and is still being used. The contoured cotton rows on the terraces were so spaced that the terrace channel was between two rows of cotton. The upper 2.2 acres of the field had slopes up to and including 20 per cent; this area was cropped in the following manner. In the fall the area that was in soybean stubble, consisting of approximately one-half the area between terraces and just below each terrace, was planted to either hairy vetch or 45 Austrian winter peas. The area that was in cotton on the terraces and extending above until joining the bean strip was planted to fall oats. Legumes were turned in the Spring and the land was planted to cotton. Oats were followed by soybeans and thus the rotation continued from year to year. On the lower portion of the field having 20 to 35 per cent slopes, the same rotation was followed except that Spanish runner peanuts were substituted in 2-foot rows for cotton as excessive erosion occurred when planted to cotton. (Cotton was planted on the tilled areas on all slopes in 1932 and 1933). By this system of rotation there was never more than approximately one-half the area between terraces freshly plowed at the same time. It was found that with any system used the steeper the grade of the rows the easier it was to control the machinery; however, more erosion occurred. The 45 degree system was the most efficient for the use of machinery and least efficient for erosion control. With an efficient operator, the machinery was operated fairly satisfactorily on contoured rows up to 20 per cent slope. Short rows were eliminated by any of the three systems. Even with the contour system the odd shaped areas were always in a legume or feed crop. Observations indicated that erosion losses were held to a minimum. Observations in the field and data from the controlled plots show that a wide strip crop is not necessary and that strip-width should be increased with the slope. Likewise the placement of the strip relative to the terrace is of major importance. The non-cultivated strip should extend above the terrace channel far enough to prevent the rapid movement of runoff and soil into the channel. The cotton belt of the Southeast is characterized to a large extent by small terraced fields located on rough or broken topography; climate and soil conditions are rather unfavorable for permanent pastures and extensive commercial production of small grains and hay. A consideration of the above conditions and of the limited experimental results indicate that there are certain advantages and disadvantages to strip cropping. The advantages may be listed as follows: (a) decreases the distance of soil movement on the field, (b) reduces the rate and amount of runoff, (c) decreases the amount of eroded material sedimented in terrace channels, thus decreasing terrace maintenance, (d) forces crop rotation and promotes the growing of small grains, hays and other roughages for local consumption, and (e) dispenses with "point" rows in the cultivated areas. The disadvantages of strip cropping may be listed as follows: (a) makes necessary a rotation practice on between-terrace areas, thus breaking the field into small patches, (b) tends to produce more hay, forage and grain than is necessary for home consumption in a region where low yields prohibit profitable commercial production, 46 (c) makes weed and insect control more difficult, and (d) involves the use of a larger variety of adaptable, soil conserving crops than is available at present for summer use on the vegetation strips. It is evident that a considerable amount of experimentation must be conducted to answer the several practical problems arising from strip cropping before such an erosion control measure can be logically recommended as a general field practice in the Southeast. Erosion Losses from Contour and from Slope-Planted Crops. -In general, the runoff from land planted on the contour is less than that from land planted with the slope. Likewise, soil losses During the growing usually increase as the runoff increases. period of a cotton crop, planted in 36-inch rows, seasonal soil losses from contoured and from slope-planted plots were determined. The seasonal losses from plots with contour-rows ranged from 4,178 pounds per acre on 5 per cent slope to 67,338 pounds on the 20 per cent slope. The losses from plots with slope-planted rows varied from 11,412 pounds per acre on the 5 per cent slope to 121,046 pounds on the 20 per cent slope. The rainfall during the period was 13 inches; about 7.5 inches of the total fell as erosive rains. Data for this experiment are presented in Table 22. On an average for all slopes, about twice as much soil was lost from the slope-planted cotton plots as was lost from the plots where the cotton was planted on the contour. TABLE 22.-Seasonal Soil Losses from Cotton Planted on the Contour and with the Slope on Cecil Clay 1 Direction Slope of land in per cent of rows With slope On contour 5 11,412 4,178 10 58,580 29,696 I 15 86,160 47,212 - 20 121,046 67,338 Soil losses in pounds per acre 'Season April 1st to August 1st, 1931; total rainfall for period of rainfall producing all of the erosion losses. 12.97 inches; 7.36 inches Experiments were conducted when vetch was planted in 18inch, contoured rows on one plot on each slope and when the companion plots were planted to vetch in 18-inch rows running with the slope. Artificial rains were applied to the vetch when it reached maturity. After this trial was completed, the vetch was removed at the soil surface with a hand knife without disturbing the soil. Tests were then made to determine the erosion control value of the stubble. Later, all plots were plowed and the same quantity and rate of rain was applied. Results of these experiments are reported in Table 23. 47 TABLE 23.-Soil Losses from Cecil Clay with Rows on the Contour and with the Slope1. Slope of land inn per cent Direction f rows Slope 0 Contour Slope Contour Slope Contour Slope Contour Slope Contour Mature vetch 63 95 81 65 90 82 569 268 604 284 Surface condition Vetch stubble 2 458 191 1,093 194 1,516 239 6,733 2,393 9,256 5,823 Freshly plowed Soil losses in pounds per acre 611 2,059 2,371 7,903 19,150 5 10 15 20 1 Losses from 1 inch of artificial rainfall applied in 8.5 minutes in all cases; surface soil partially saturated by rainfall immediately preceding the runs. 2 Vetch removed by cutting near the ground with a knife without disturbing the soil. The soil losses from mature vetch in both contoured and slope-planted rows were comparatively small in all cases. However, on the steeper slopes soil losses were about twice as much from slope-planted plots as from the contour-planted plots. After the vetch had been removed, the remaining stubble was much more effective on the contoured rows than the stubble on the slope-planted rows. Relation of Slope to, Erosion.-Runoff and soil losses, as previously stated, depend upon many interrelated variables. The severity of the losses increased with the slope of the land. This is in accordance with the findings of Bartel (1). The slope of land which can be tilled in practice without excessive erosion losses (the so called critical slope) varies quite widely with soil conditions, rainfall and farming practices. Seasonal soil losses from different slopes under varying amounts of vegetative protection and rainfall with variations in surface shape are reported in Table 24. A part of these data is shown in Figure 9. A study of these data reveals the following: (a) The seasonal soil losses from vetch, rye and alternate strips of cotton and soybeans were comparatively small on all slopes. The critical slope was probably never reached on any slope studied. This was due to vegetative protection of the cover crops and to the combined effect of surface shape, soil structure, and plant coverage on the strip-cropped plots. (b) Losses from interplanted corn and velvet beans and from cotton were much greater than those from the crops mentioned above. The rate of soil movement increased rapidly between 10 and 15 per cent slopes and the soil losses were excessive in 00 TABLE 24.-Seasonal Soil Losses Produced by Natural Rainfall on Cecil Clay when Fallow and when Planted to Various Crops'. Amount Amount Duration = rainfall Hours 184 184 164 164 155 155 91 91 307 698 1,933 773 927 235 674 2,182 1,176 1,618 Slope of land in per cent 0 Crop surface Smooth Smooth Smooth Smobth Bedded Bedded Bedded Smooth Bedded Bedded Smooth Soil rainfall Inches 13.99 13.99 20.00 20.00 13.95 13.95 13.39 13.39 12.50 12.50 35.15 erosive of0 5 5 5,531 6,731 3,784 27,564 5,517 6,975 3,403 29,131 13,379 17,954 9,898 10 7,416 7,258 6,394 90,218 7,528 14,139 13,147 73,784 39,032 56,214 23,075 15 10,651 11,900 14,767 135,179 11,354 51,193 34,922 92,073 81,380 103,045 52,894 20 13,775 14,007 19,675 182,662 14,473 47,348 43,228 118,197 94,362 114,710 60,136 Seasonal soil losses in pounds per acre Vetch Rye Vetch Fallow 2 Cotton-soybeans Cotton Corn-velvet beans' Fallow 4 Cotton Cotton Oats 2 'All crops planted on contour. Alternate contour strips. 3Interplanted in the same row. 4 Cotton not chopped. 49 Slope of Land in Percent FIGURE 9.-Seasonal soil losses from different slopes under different vegetative coverage during growing seasons with comparable total rainfall. all cases above the 10 per cent slope. A study of the data indicates that with these cropping practices the critical slope was between 10 and 15 per cent. (c) Erosion losses from smooth fallow were comparatively high at 5 per cent and the rate of losses increased rather uniformly above this point. The critical slope was possiblyreached at about 5 per cent. (d) Soil losses are a function of slope under any given set of conditions. However, vegetation, surface shape and state of soil pulverization frequently exert a masking effect upon the slope factor. These factors are more important than slope in determining soil and water losses. Effect of Erosion on Yields.-It is a generally accepted belief that yields are materially reduced by erosion. The North Carolina Station (1)has reported that the annual loss of plant food from sheet erosion may be seven times as great as that removed by a crop of cotton and four times as great as that removed by a crop of corn. Chemical analyses made at the Missouri Station (6) show that the amount of nitrogen, phos- 50 phorus, calcium and sulphur in the eroded material from corn or wheat land may be equal to or exceed the amounts taken off by these crops. Sheet erosion has so depleted large areas that crop production can no longer be conducted profitably; large areas of formerly productive lands have been gullied almost beyond redemption (3). Little reliance can be placed upon yields on the erosion plots at Auburn due to the fact that the experiment was not designed to study yields. SUMMARY Results of six years' experimentation on the measurement and control of the sheet erosion process on Cecil clay are reported. A set of ten controlled plots, each 15 feet by 50 feet was used; two plots were located on each of a 0, 5, 10, 15, and 20 per cent slope. Suitable equipment and methods were used to (a) measure the amount, rate and nature of soil material eroded from the plots, (b) measure the rate and amount of runoff, (c) supply uniformly distributed artificial rainfall at any desired rate and amount, and (d) record the intensity, quantity and duration of natural rainfall. Experiments were conducted under wide variations of soil condition and vegetative cover. The moisture content of the soil influenced the rate and extent of absorption and hence influenced runoff and soil movement during any given rain. A large portion of seasonal erosion losses invariably resulted from a few heavy rains which occurred when the soil was approximately saturated. Rainfall intensity was more important than the quantity of rainfall in determining the amount of erosion when other conditions were held constant and when the rate of rainfall exceeded the rate of infiltration to an extent that appreciable runoff occurred. With a given intensity of rainfall, the duration had a marked effect upon erosion-the greater the duration, the greater the soil losses. Losses from intermittent rains of a given quantity were decidedly less than those from rains of continued duration, provided the rate of rainfall exceeded the rate of absorption. This was due to the inability of the soil to absorb the quantity of water during a given time. The exception to this was on smooth fallow, in which case the soil losses per unit of runoff decreased because the loose or slaked soil was readily carried off by the first part of the rain. Pulverization and tillage practices which increased the rate and amount of absorption were very effective in controlling sheet erosion provided the rate and amount of rainfall did not exceed the rate and amount of absorption. When the rate of rainfall greatly exceeded the rate of infiltration, excessive soil losses usually resulted from such tillage practices. 51 Aggregate analysis of sediments eroded from Cecil clay under a wide variety of conditions showed that the unit particles involved in the sheet erosion process, in the case of structural soils, were aggregates rather than textural separates. In general, soil material is moved layer by layer in the sheet erosion process. The relative loss of colloidal material may be excessive under a condition or combination of conditions which results in small quantities of runoff or in runoff of low velocity or both. Winter cover crops and other vegetative control measures functioned in reducing sheet erosion losses and soil movements by (a) filtering out the large soil particles and water stable aggregates, (b) decreasing the quantity of runoff, (c) decreasing the velocity of runoff, (d) minimizing the turbulence of runoff and hence lessening the abrasive or dispersive action of sediment-loaded water, and (e) by decreasing the mechanical dispersive action of beating rainfall. Annual soil losses from land continuously in cotton were reduced to about one-half by the use of vetch as a winter cover crop. Rye, used as a winter cover crop, was nearly as effective as vetch in reducing erosion losses. Various width strips of soil conserving crops were effective in reducing erosion and in decreasing the distance of soil movement on between-terrace slopes. It seems that if strip cropping is practiced, it should be used as a supplement to terraces rather than as a substitute for terraces. Contoured, row-crop plantings had a pronounced soil and water saving ability when compared to slope plantings. The amount of soil eroded from slope-planted cotton was about twice as much as that from contour-planted cotton. Erosion losses increased with increased slope under all conditions studied. The so called "critical slope" or point above which a given soil cannot be cropped without excessive erosion losses is more dependent upon such factors as plant coverage and tillage practices than on topography itself. LITERATURE CITED (1) (2) (3) (4) (5) Bartel, F. O. Mimeograph, Progress Report of North Carolina Experiment Station, Raleigh, N. C. 1928. Baver, L. D. Soil erosion in Missouri. Mo. Agri. Expt. Sta. Bul. 349. 1935. Bennett, H. H., and Champline, W. R. Soil erosion a national menace. U.S.D.A. Cir. 33. 1928. Carnes, A., and Wilson, J. B. Terracing in Alabama. Ala. Ext. Cir. 148. 1934. Davis, F. L. A study of the uniformity of soil types and of the fundamental differences between different soil series. Ala. Expt. Sta. Bul. 244. 1936. 52 (6) Duley, F. L., and Miller, M. F. Erosion and surface runoff under different soil conditions. Mo. Agri. Expt. Sta. Bul. 63. 1923. (7) Meginnis, H. G. Using soil-binding plants to reclaim gullies in the South. U.S.D.A. Farmers' Bul. 1697. 1933. (8) Middleton, H. E., Slater, C. S. and Byers, H.G. Physical and chemical characteristics of the soils from the Erosion 1932. Experiment Stations. U.S.D.A. Tech. Bul. 316. Physical and chemical characteristics of the soils (9) from the Erosion Experiment Stations; Second report. U.S.D.A. Tech. Bul. 430. 1934. (10) Miller, M. F., and Krusekopf, H. H. The influence of systems of cropping and methods of culture on surface runoff and soil erosion. Mo. Agri. Expt. Sta. Res. Bul. 177. 1932. (11) Musgrave, G. W. The infiltration capacity of soils in relation to the control of surface runoff and erosion. Jour. 1935. Amer. Soc. Agron. 27:336-345. (12) Olmstead, L. B., Alexander, Lyle T., and Middleton, H. E. A pipette method of mechanical analysis of soils based on improved dispersion procedure. U.S.D.A. Tech. Bul. 170. 1930. (13) Slater, C. S., and Byers, H. S. A laboratory study of the field percolation rates of soils. U.S.D.A. Tech. Bul. 232. 1931. (14) Stokes, G. C. Camb. Phil. Soc. Trans. 8:287. 1845. (15) Yarnell, D. L. Rainfall intensity-frequency data. U.S. D.A. Misc. Pub. 204. 1935. (16) Yoder, R. E. A direct method of aggregate analysis of soils and a study of the physical nature of erosion losses. Jour. Amer. Soc. Agron. 28:337-351. 1936. ACKNOWLEDGMENTS The authors wish to express their sincere appreciation for the many constructive criticisms and helpful suggestions of Professor M. L. Nichols, under whose guidance these experiments were planned and conducted. The writers are also grateful to Professor A. Carnes for suggestions offered during the course of the work. Appreciation is also expressed to Messrs. T. N. Jones, Hugh Sexton, R. S. Kimbrough, B. C. Small and W. J. R. Browder who, as graduate students, contributed to the routine work.