EROSION on CULTIVATED LAND 291 Water and the Soil It Carries Are Likely We Need Good Management of Wa- to Go
290 YEARBOOK OF AGRICULTURE 1957 what must occur in the root zone of the growing crop. Unless the soil is well drained, the application of irrigation water in con- siderable excess over that required for Erosion on the crop and for leaching can be as detrimental from the standpoint of salinity control as underirrigation. Cultivated Land Overirrigation increases the amount of water that the drainage system must B. D. Blakely, J. J. Coyle, convey; if the capacity of the system is and J. G. Steele exceeded, the water table will rise to an unsafe level. It is apparent therefore To protect soil from erosion and that a proper relation between irriga- tion, leaching, and drainage is of ut- to hold as much of the rain as most importance in preventing soils possible in a place where crops from becoming salt affected. The amount of water applied should be can use it are a big part of sufficient to supply the crop and satisfy modern soil management. the leaching requirement but not enough to overload the system. We cannot avoid all risks of erosion when we lay a soil bare by cultivating EXCESSIVE LOSS of irrigation water it. Neither can we hold all the rain from canals constructed in permeable where it falls in humid or subhumid soil is a major cause of high water areas. But we need to know the risks tables and salt accumulation. Seepage and control them the best we can. losses can be reduced by lining canals Erosion is slow wherever the soils are with cement, buried asphalt mem- covered by trees or grass. Near Zanes- branes, or more commonly with earth ville, Ohio, scarcely any loss of soil of low permeability. The maintenance could be measured in 9 years from a of drainage systems is also important woodland watershed of 2 acres. A and usually involves nothing more than nearby pasture lost soil during the keeping tile lines in repair or open same period at an average rate of one- ditches clean and excavated to grade. tenth ton an acre a year, or about i A gradual decrease in soil permea- inch in 1,500 years. A similar water- bility is a common cause of declining shed cropped to a 3-year rotation of productivity in land under irrigation. corn, wheat, and hay lost an inch of Without satisfactory soil permeability, soil in the 9 years. crops cannot be kept adequately sup- The rate of erosion in any storm de- plied with water and the leaching of pends on the force with which rain- salts is not accomplished. Soil treat- drops stir up soil and the amount and ments for the maintenance of permea- speed of the runoff water. Other fac- bility are the same as those we dis- tors affecting erosion include kind and cussed for improving soil structure. amount of cover, kind of soil, and To meet the demand for agricultural steepness and length of slope. products, it will be necessary to utilize To judge the erosion hazards in a salt-affected soils and irrigation waters particular situation, we need to look of inferior quality more and more fully. at more than one rain and consider Thus it can be assum.ed that the im- the pattern of rainfall for a whole year provement of salt-afifected soils and the or for several years. management of productive soils so as We need to study the location by to prevent the excessive accumulation looking uphill to see how much water of soluble salts and adsorbed sodium is likely to cross the field and by look- will grow in importance. ing downhill to see where the runoff EROSION ON CULTIVATED LAND 291 water and the soil it carries are likely We need good management of wa- to go. ter for crops to get the full benefits of We need to look at the cover that controlling erosion, improving fertil- the cropping system provides each ity, and maintaining soil structure. season, especially if a crop must be Most soils contain either too little or planted at a time when hard storms too much water during at least part are likely to occur. of each year. We need to study the whole soil pro- Although drainage of wet lands and file to find out the effect of layers that irrigation of arid land are major oper- lie beneath the surface. ations, they affect only part of the We need to know what recent tillage total cropland, probably less than one- practices have done to the structure of third. Irrigation is increasing in hu- the surface soil and how much protec- mid areas but will continue to be lim- tion we can get from mulches. ited by water shortages, high costs, Having appraised these hazards, we and other factors. need to plan the cropping system and Farmers who can reduce runoff from supporting conservation practices for upland fields and who can increase the field to offset them. the amount of water taken in and Short-time changes other than ero- stored for use by plants during the sion also are important in the manage- growing season have a good chance of ment of soil and water. Nutrients may increasing their crop yields. Such man- be depleted through leaching and re- agement of water on most of our culti- moval of crops. Changes in soil struc- vated land where neither drainage nor ture can have great effect on the sup- irrigation is involved may produce ply of water in the root zone available greater benefits than the more spec- to plants. The nutrients can be re- tacular measures. placed by fertilizers, but changes in soil A cover of vegetation is the first de- structure are harder to correct. fense against erosion and runoff. Structure of surface soil affects in- A soil protected by the right kind take of water and air. Whenever the and amount of sod or forest litter is surface layer becomes puddled by a not likely to erode, no matter how hard hard rain or compacted by heavy ma- or how long it rains. A soil so protected chinery, the danger of runoff and ero- readily absorbs the rainfall. Its good sion increases and the intake and stor- structure permits the water to move age of water decrease. Structure is freely through it, and heavy runoff likely to break down as organic matter seldom occurs unless rock or impervi- becomes oxidized. ous layers near the surface block the Tillage pans, also called plowpans or downward movement of water. trafile pans, are formed in some soils Occasionally there are exceptions. as they are cultivated. A tillage pan is Concentrated water may start a gully, a thin, dense layer that develops just or a cloudburst can saturate an entire under the plow layer. It restricts root slope and unleash a landslide. Long growth and can seriously reduce crop rains may overrun the water-storage yields. Many silt loam and loam soils in the soil and cause streams to rise. are subject to formation of tillage pans. The protection cultivated crops give Structure of subsoil is important be- is intermediate between none at all, as cause we need the full capacity of a soil on a newly plowed field, and the nearly to furnish plant nutrients, water, and complete protection of thick sod or air. Many farmers have found that forest litter. their soils drained well at first but Some hay crops are almost as good needed more drainage after they had as pasture sod. Others, like alfalfa and been farmed a few years and the crev- sweetclover, can allow much washing ices and root channels in the subsoil if the spaces between plants are not began to be filled. filled in with grasses or mulched by 292 YEARBOOK OF AGRICULTURE 1957 dead stems and leaves. Small grains paring seedbeds, controlling weeds, offer only partial protection, and that and attempting to get more nitrogen only after they have made considerable from decomposing residues. growth. Intertilled crops are little bet- New cultural practices and tillage ter than bare fallow unless the rows are implements now available will over- on the contour. Crops that must be come some of the hazards of clean till- dug, like potatoes and peanuts, leave age. Agronomists are studying mulch the soil ready for erosion by the next tillage, rough seedbeds, sod seedbeds, hard rain. and many other practices to determine To judge the risk of erosion in a crop- their effects on crop yields, erosion, ping system, we need to know the and control of weeds and crop diseases. chances of erosive rains whenever the Ten years of study in the Southeast soil is cuhivated or cover is thin. In a showed that mulch tillage permitted 4-year rotation of corn, grain, and 2 less runoff and erosion and resulted in years of hay, the soil is plowed twice higher corn yields than clean tillage. and is cultivated two or three times Rough seedbeds prepared with a field after the corn is planted. The risk of cultivator in the Midwest lost about erosion-producing rains at those times half as much soil as plowed fields. is high in most corn-growing areas. Contour listing of corn in western As many farmers know, a rotation of Iowa reduced water losses about 50 fallow, wheat, and kafir leaves soil ex- percent and soil losses 73 percent from posed to erosion for most of a year and those where the crop was surface also part of the spring in which kafir planted on the contour. is planted. A rotation of wheat i year Minimum tillage that keeps crop and alfalfa mixed with bromegrass for residues on or near the soil surface, 3 years gives protection most of the used with nitrogen fertilizers and living time and allows both crops to be mulches, has proved successful in many planted when the risk of erosion is low. places. More research is needed to Much can be done to reduce erosion adapt these methods to all sections. by matching the crops grown to the A farmer usually can choose differ- erosion hazards of each field. Certain ent combinations of soil-saving and crops also can be used to keep or to water-saving practices to use with the restore fertility and good structure. crops he wants to grow. Green manure crops, which return The land capability classification is large amounts of organic matter to the a useful guide in matching conserva- soil, are needed in humid and sub- tion practices with the cropping system humid areas to keep good tilth in the or type of cover on each field. topsoil. In drier farming areas, crops On a soil of a certain capability, for that leave heavy mulches on the sur- example, the cover needs to be hay or face may serve better. pasture two-thirds of the time if no Deep-rooted legumes, such as sweet- extra practices (such as stripcropping clover, improve subsoil structure. We or terracing) are used to support the need legumes in cropping systems on protection given by the rotation. If the many soils to keep the subsoils porous field is stripcropped, hay or pasture is enough so that water, roots, and air needed half the time. Sod is needed can get through them. only one-third of the time if it is terraced. TILLAGE METHODS, as well as the This is only one example of the many crops grown, affect soil conditions and choices that can be made. In general, therefore runoff and erosion. whenever sloping soil is to be culti- Corn, cotton, and soybeans for many vated and exposed to erosive rains, the years have been considered the most protection offered by sod or close- soil-depleting crops. Much of the dam- growing crops in the rotation needs to age resulted from overtillage in pre- be supported by practices that will EROSION ON CULTIVATED LAND 293 slow the runoff water and thus reduce parabolic, shape is best. A V-shape is the soil it can carry. satisfactory if the sides of the V arc laid The most important of these support- back so that the top width is at least ing practices for cropland are water- 10 times the depth. ways, contour tillage, stripcropping, It is desirable to establish a dense terracing, and diversion terraces. vegetative cover as soon as possible. Since the waterway will carry rather WATERWAYS (as used in soil and heavy flows at times, the seeding and water conservation work) are natural fertilizing rates normally used for pas- or manmade depressions on sloping ture are not sufliicient. A good rule is land. Waterways can carry specified to double these rates—or more—when amounts of water without erosion and seeding a waterway. In sodding, the serve as outlets for terraces, diversions, sod pieces should be closely spaced and and contour rows. They are needed should be well fertilized. often as passageways for water that Grasses that form a dense turf are enters a farm from other land. best for waterway protection. Where Waterways are the most important they are not adapted, bunch grasses single item in the control of runoff like the bluestems and gramas, or les- water from cultivated land. If they fail, pedeza sericea and many other plants all other parts of the system may fail. do a satisfactory job if the depth of The best locations for waterways flow is kept shallow and the slope is usually are the natural drainageways not steep. Kudzu is good in places of the landscape. Natural depressions where it is adapted. If kudzu is used, require a minimum of shaping to be the size of the waterway must allow able to carry the expected runoff. Soil for the bulky growth of the plant. and moisture are favorable for good Reed canarygrass is good in wet places. growth of protective vegetation. The topography usually allows free dis- CONTOUR TILLAGE is one of the sim- charge of water into natural drainages pler practices for reducing soil and from terraces, diversions, and rows. water losses. It is effective and in- Manmade channels near field bound- expensive in the right places. Its effec- aries or fence lines can be made to tiveness depends on the ridges, made work, but they are never so satisfactory by tillage implements, to retard the as the natural locations. flow of water. Contour cultivation Waterways need to be large enough alone protects the soil on the flatter to carry the runoff from the water- slopes during the less intense storms sheds they serve without overflowing but is of little benefit at times when during heavy storms. Usually this the rains are intense. means designing them to carry the Contour tillage is most effective on runoff from the heaviest rainfall to be 2- to 8-percent slopes not more than expected once in lo years. 300 feet long. Here the practice re- The waterway needs to be shaped duces soil loss to about half that with and smoothed to remove irregularities tillage up and down the hill. On slopes that would cause turbulence in flowing greater than 8 percent, the ridges re- water. If the site is in a natural depres- tard the flow less, and soil loss is re- sion, this usually can be done with or- duced only 20 to 40 percent. dinary farm equipment. If there is a On a 7-percent slope near Guthrie, small wash or gully down the center Okla., contour tillage reduced losses of of the depression, it should be plowed soil about 50 percent and losses of wa- in and the soil compacted by repeated ter 12 percent (as compared to farm- trips over the filled area with a tractor. ing up-and-down the hill) on land The shape of the waterway should planted to cotton with wheat as a win- keep water from concentrating to a ter cover. In Missouri, on a soil having great depth at any point. A dished, or a rather tight subsoil, contouring re- 294 YEARBOOK OF AGRICULTURE 1957
duced soil losses 52 percent and runoíT lines, using an inexpensive hand level. 20 percent on land planted to corn. First, determine which part of your Contouring alone is not sufficient helper's body is level with your eyes protection for soils that are severely (e. g., hair, face, or shoulder) when eroded or have hard clay subsoils, for you are standing together on level the excess runoff may break through ground. Then, when you sight through the rows. Other practices such as ter- the hand level and see the crosshair racing are needed when those condi- against this part of his body, you are tions exist. both standing at the same elevation. Contouring can increase yields of Next, decide where the first key con- row crops as much as 50 percent; in- tour should be. Set a stake at any point creases of 5 to 10 percent are common. on the proposed line. Stand at this The effect is greatest in years when stake while your helper walks about rainfall is scant during the growing 100 feet (or not so far on sharp curves) season. around the slope as nearly on the level To be able to plow and plant on the as he can. When he stops, sight through contour, a farmer needs one or more the hand level and signal for him to key contour lines marked on each field. move up or down the hill as necessary These are level lines across the slope until the crosshair again strikes the to be followed in tillage operations. part of his body previously determined. On short, uniform slopes, one key When you signal that he is on the level contour line about halfway down the with you, he drives a stake. hill is enough. If the slope is long or ir- Move to the new stake while your regular, several lines are necessary. It helper moves on, and repeat the proc- is important to have enough lines to ess. When you have crossed the field in guide farming operations, taking into this manner, you will have a line of consideration a convenient width of stakes all on the same level. The line land for plowing or planting. can now be marked with a plow. Anyone with a little experience and When the entire field is contoured a helper can stake out the key contour and planted to a single crop, odd-
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TURNSTRIP 4 EROStON ON CULTIVATED LAND 295 shaped areas may cause some difficulty problems, strips can be laid out with a in plowing. The sketch shows how to grade of I to 2 percent to lead the avoid turning on loose plowed ground. water into grassed waterways. Use each contour line as a backfur- Steepness of slope, kind of soil, the row and plow around areas i and 2. usual amount and intensity of rainfall, Plow area 3 next until the remaining and the size of the farm equipment are land is just wide enough to turn the factors to consider in determining the tractor. Then plow the turnstrip (4). width of the strips. In many localities, strips are made 100 feet wide on slopes IN STRIPCROPPING, close-sown crops of less than 6 percent, 80 feet on slopes alternating with strips of other crops of 6 to 10 percent, and 50 feet on slopes slow down the flow of water. of 11 to 16 percent. Stripcropping combined with con- When the system is laid out so that tour tillage is more effective than con- both the upper and lower edges of the touring alone. On experimental areas strips are on the true contour, all strips in Missouri, Ohio, and Wisconsin, are irregular in width, as shown in the contour stripcropping (compared to sketch below. up-and-down farming) reduced soil Strips are narrow where the slope is losses about three-fourths on slopes steep and wide where it is gentle. As a that were less than 12 percent and one- result, some rows in uneven-width half on steeper slopes. strips do not go all the way through There are three types of stripcrop- the field. Since these short, or point, ping: Contour stripcropping, field rows occur on the more gentle slopes stripcropping, and buffer stripcrop- where the soil is usually best, most ping. The type to use depends on the farmers prefer to plant these areas to kind of soil, the crops grown, and the the regular crops in rotation, despite topography. the difficulty of cultivating them. Grassed waterways are essential in a Others, however, prefer to plant them stripcropped field wherever excess run- to perennial hay crops or to close- off water accumulates. growing annual crops to eliminate the Contour stripcropping is adapted to point rows. well-drained cultivated soils on sloping Another common method is to lay land where rainfall causes erosion. This out two or more even-width strips from type of stripcropping is effective on 2- one key contour line, as shown at the to 12-percent slopes that are not longer top of page 296. When the topography than 400 feet. of the field changes, a new contour line Where both erosion and wetness are should be laid out as a guide for addi- 296 YEARBOOK OF AGRICULTURE 1957
tional strips of even width. Tliis results often are planted to perennial grasses. in an irregular correction strip be- Field stripcropping is used when tween the two sets of even-width strips. slopes are so irregular that contour A third method provides for even- strips would be hard to till. Field strips width strips alternating with irregular are uniform in width and are laid out strips. From the first contour line, an across the general slope instead of on even-width strip is measured. A second the true contour. This method is less contour line is staked two strip widths effective than contour stripcropping in above or below the first, and an even- reducing soil and water losses. A good width strip is measured from it. This crop rotation and cultural treatments process is repeated until the entire field are necessary therefore to provide the has been laid out. major protection to the soil. If the fields are large—i6o acres or Buñ"er stripcropping consists of nar- more—contour maps are helpful in row protective strips alternating with laying out the system. Uniform-width wide cultivated strips. strips separated by irregular-width The location of the protective strips strips can be located on the map in re- is determined largely by the width and lation to the contour lines. Minor vari- arrangement of adjoining strips to be ations can often be made so that a cropped in the rotation and by the minimum of land is devoted to the location of steep, severely eroded areas irregular strips. These irregular strips on slopes. Buffer strips usually occupy ft'^^^^^AlC
WATERWAY EROSION ON CULTIVATED LAND 297 the correction areas on sloping land. ness of slope changes every 100 feet or They are seeded to perennial grasses so. A tillable row pattern cannot be and legumes. This type of stripcrop- worked out on them. ping is not so effective as contour strip- The difficulties of constructing, cul- cropping but will serve as a temporary tivating, and maintaining terraces in- measure until a more adequate system creases with the steepness of slope. Sat- is established. isfactory waterways are also harder to Some extra precautions are necessary establish on the steeper slopes. For in farming the stripcropped fields. For slopes of more than 8 to Ií2! percent ease in handling farm equipment, we (even less for some of the tighter soils), need to have two types of crops in each we might better consider land uses or group of strips: A row crop may alter- cropping systems that will give ade- nate with a sod crop or a grain crop quate protection without terraces. with a sod crop. The first problem to be solved in ter- Plowing should be varied to prevent racing a field is how to dispose of the ridges at the edges and deep dead- water that cannot be taken into the furrows in the centers of strips. Two- soil. If graded terraces are used, or if way plows are often used to prevent the ends of level terraces cannot be formation of ridges and dead furrows. blocked, a waterway must be provided. When planting uniform-width strips, Level terraces with ends blocked pre- the farmer can start at the top or bot- sent no problem of water disposal. tom edge and continue across the en- An area covered with a dense sod of tire strip. If the strips are irregular in permanent grasses is a safe place on width, many farmers prefer to plant which to spill the water from terraces. the row crops from both sides. Then A good pasture next to the field pro- the greatest number of rows are on the vides a readymade solution to the contour, and all the point rows are in problem if the terraces can be run the center of the strip. onto it. If no such area exists, one must be developed or a waterway must TERRACING has long been used to pro- be established. If the terraces will tect cultivated land against erosion and bring more water to the location than to make best use of available water. had been coming there before, the We now usually use the word 'ter- vegetation needs to be established be- race" to mean a ridge, or a combina- fore the terraces are built. tion of ridge and channel, built across * Level terraces are used mainly in the slope on a controlled grade or on areas where the average annual rain- the level, depending on its purpose. fall is less than 30 inches. If the soil is Terraces intercept the flow of water able to take in water at a rate of at down the slope before it attains enough least one-half inch an hour, the ends velocity to damage the land. Graded of the terraces may be blocked, and terraces lead this water off the field at the runofi'from all but the most intense nonerosive velocities. Level terraces rains can be made to enter the soil. If hold back the water until it can soak the rate is much less than that, the into the soil. ends are left open or only partially Not all soils and slopes can be ter- blocked. Otherwise, the impounded raced successfully. It is nearly impos- water may damage small grain by sible to maintain terraces on deep causing uneven ripening or difficulty sands. The cost of construction and the in harvesting. For row crops, such as difficulty of maintenance make them grain sorghum, slower water intake is unsatisfactory on stony soils and on not always objectionable. shallow soils over rock or over tough, Where the average annual rainfall heavy subsoil. exceeds 30 inches, it is generally best Terraces are impractical on moundy to give terraces grade or fall to allow fields or fields where direction or steep- the surplus water to drain from the 298 YEARBOOK OF AGRICULTURE 1957 field. The grade must be gentle enough that the water will flow at low speed. It should not exceed 3 inches per 100 feet on soils that are easily eroded. On others it may be as much as 6 inches or more per 100 feet. The spacing of terraces usually va- ries, inversely with steepness of the field slope—that is, as the steepness in- creases, the distance between terraces decreases. Two basic spacing formulas are used : In the Northern States, V. /.= S in the Southern States, V. I.= A terraced field with terraces extended across de- pressions and ridges to reach waterways along field S boundaries. Shaded portion shows area in point rows. + 2. V. I. stands for vertical interval Equipment must be turned in the planted area. 4 in feet, and S for slope of the land m feet per lOO feet. The vertical interval is converted to horizontal distance by multiplymg by —ç— The way the soil is managed is about as important as steepness of slope in determining the correct spacing of ter- races. If good soil-management prac- tices are used, the spacing indicated by the formulas can be increased as much as 50 percent or more on some of the deep, permeable soils. If little attention is given to soil management, The same field with waterways in natural depres- the spacing should be narrowed. sions and terraces graded each wayjrom ridges. This permits adjustment in alinement at ridges. Sharp The more nearly two terraces paral- bends at depressions are eliminated. Note reduction lel each other, the easier it is to farm in point rows. Similar results could be achieved by the space between them. Irregular cutting and filling but cost would be more, wet areas spaces mean that some rows do not would exist above fills, and subsoil would be exposed. run the full length of the terraces, so that equipment has to be turned in the planted part of the field when row crops are grown, and fields of odd shapes have to be drilled and har- vested when drilled crops are grown.
IRREGULARITIES in the space between two terraces result from changes in land slope along the terrace line, the extension of terraces across natural depressions, or the grading of terraces so they drain across ridges or hogbacks. These irregularities can be reduced by grading the terraces to drain each The same field shown in first two sketches. By way from all ridges and to discharge careful planning, several sets of parallel terraces were into the first depression reached. On worked out. Note the small area in point rows. EROSION ON CULTIVATED LAND 299 some fields this may call for several the small adjustment area between the waterways, and on first thought the two sets of terraces. Some fields may farmer may object to them. Experi- require several sets of parallel terraces. ence has proved, however, that the in- A comparison of the bottom sketch convenience of raising tillage tools to with the other two illustrates the ad- cross waterways is less bothersome than vantages of this system. farming the additional.short rows that Two requirements govern the dimen- result when terraces are built across sions of the individual terrace. them. First, the terrace must be large The sketches on the preceding page enough (either in ridge size or channel illustrate the advantages of using the capacity) that it will seldom be over- waterways, in natural locations. topped. The usual recommendation is that a terrace be built to carry the run- THE INCONVENIENCE of farming ter- ofif resulting from the heaviest rain races can be reduced by using parallel normally expected to occur on an av- terraces. Actually, it is seldom possible erage of once in 10 years. to have all terraces in a field parallel, Second, the terrace should be shaped but the ideal can be approached by so that mechanized farming equipment care in laying out the system according can be used without undue difficulty. to the following suggestions: First, locate all ridges and depres- Two SKETCHES show dimensions gen- sions in the field. All terraces will be erally recommended for the two types graded to drain from the ridges toward of terraces most commonly used. the natural depressions. Adequate terraces can be constructed Next, stake a key terrace on the with almost any type of equipment grade recommended for the soil; ad- that will move soil. just the staked line to ease sharp curves as much as possible. All stakes moved Each side of terrace should be wide in making the adjustment must be re- enough to allow at least one trip checked with the level to be sure that with widest piece of equipment that must be used on it. Minimum the grade from ridge to depression is Effective Cross Sectional Area of "ot less than 7'. continuous and not excessive. Channel 8 to 16 Square Feet Stake a trial line above or below the key terrace and parallel to it. That is done by having one man walk along Channel-type Terrace
the line of the key terrace carrying one Generally used in humid and sub-humid areas where removal of excess end of a tape. A second man, holding water is the problem. Usually g-lven a grade. the tape at a point marking the proper The typical shape and dimensions of channel-type distance between terraces, keeps pace terrace. with the first man, setting stakes to mark the trial line. This line is then Total Width of Ridge 14 io 30 or checked with the instrument. The lo- More Feet Depending Upon Size of cation is accepted if the grade is not Farming Equipment To Be Used on excessive or does not reverse anywhere Terrace along the line. Repeat the procedure for each new line.
When the grade of the new line be- Height of Ridge Above Natural Ground 10 to 18 Inches Depending comes excessive or reverses, lay out a Upon Slope of Field and Length of new key terrace a distance of two or Terrace. more terrace intervals from the last Ridge-type Terrace
acceptable line. Generally used In low rainfall areas to impound water until it can be This gives two sets of terraces, all absorbed by the soil. Usually level. those in each set being parallel to each The typical shape and dimensions of ridge-type other. The only short rows will be in terrace. Starting point of terrace STAGGERED CUT METHOD Continue O O Method of progressing Sanie Sequence down length of terrace
Fig. 2
c~Tr\ Fig. 3 JSl Field Slope Fig. 2 -^~-S. 4 . .. Fig. 4 CONSTRUCTION PROCEDURE 3 2 1 3 2 1 A. Scarify, chisel, rip, or plow a 20-foot strip 8 or 10 inches deep on channel excavation I and 2 X'^ST..- ■ 5":" '\ STRAIGHT CUT METHOD ..:^, c„n,i,iue area. This step must not be omitted, ^. Same Sequence Loose soil is required to build an adequate Fig. 5 terrace section in two trips as shown.
B. Method of placing earth in cross section of terrace is shown above, figures 1 and 2. A I and ; X 3 A 5 \ C. Sequence of construction down the terrace is shown at right, figures 2 through 6. Fig. 6
D. Back slope of terrace must be buill while All additional trips are a bulldozer is moving forward down length repetition of trips 5 and 6. of terrace, as shown at right in figure 7. A motor patrol grader can be used to widen the back slope if the bulldozer operator is not skilled in keeping a smooth path of forward tratel, BACK SLOPING E. Resulting terrace, figure 8 at right, aver- Fig. 7 ages 15.6 square feet in ridge, 20 to 22 Cutting down, widening, and smoothing feet in base width, and 14 square feet ex- the lower slope of the terrace fidge. cavation in channel.
F. In easy moving soils a gap equal to half the width of the dozer blade may be left be- tween trips (1 and 2) and 3, and the same gap left between 3 and 5. This results in Fig. 8 faster construction. FINAL CROSS SECTION COMPLETED CROSS SECTION BY OTHER METHOD
Terrace construction with a bulldozer. EROSION ON CULTIVATED LAND 301
Trip 2 Trip )
Cross Section After Settlement and Cultivation Progressive steps in construcling a channel terrace with a 10-foot blade terracer. The terrace is constructed from the upper side. Terrace construction with a motor grader.
Terrace construction with 4-Joot disk tiller. The number oj rounds, or trips, required to build a ter- Four sketches show construction race depends on the size oj the terrace desired, the methods applicable to some types of effectiveness oJ the equipment, and the condition oj the soil. equipment. Keeping the terraces built up to their original size is important. This requires attention to maintenance every year. The job consists of removing silt bars that may have formed above the ter- race ; inspecting terrace ends to see that U Plowing—7 Trips they are fully open (if they are sup- posed to be open) or properly closed 3il Plowing—6 Trips (if that is the plan) ; building up low sections in the terrace ridge; and plow- 4th Plowing—5 Trips ing to maintain adequate size of ridge or channel. Having terraces on a field often calls for changes in farming operations. The problem is simple where most of the 7tli Plowing—2 Trips terraces are parallel. If that is impos- After setting channel stakes, set a second row of stakes at 50-foot intervals sible, one of the systems shown in the (closer on sharp curves) 11 feet below channel stakes. This row marks first plowing. sketches on the next page will be Drawing is based on use of a two-bottom 14- or 16-inch plow. Use sharp shares. Maintain enough speed to turn furrow slice completely over. helpful. Remove crop residues and disk heavy sod well before plowing. Construction oj channel-type terrace with two- A DIVERSION TERRACE (also Called way plow. This plow permits building terraces diversion ditch) is like a field terrace jrom one side—an advantage in terracing steep land. in shape. The main difference is that It is a valuable tool jor carrying out maintenance it is designed to handle larger flows of operations. It turns all the jurrows down hill. It is also used on the flatter lands, when the jurrows can water. Diversion terraces usually are be turned up hill. considerably larger than field terraces. 302 YEARBOOK OF AGRICULTURE 1957 Some of the important uses of diver- sion terraces or ditches are to protect fields from hillside runoff; divert water from a gully to assist in controlling it; Final Round increase or decrease the amount of run- off water entering a farm pond ; divert Rounds should be spaced so that width of plowed area below Icrrace is '/3 more tliati width of plowed aroa above lerracc. This calls for overlapping water from points of concentration to the rounds above the terrace. other nearby areas where it can be A method of maintaining channel-type terraces spread and used; protect terrace sys- with disk plow. This method tends to clean out the tems from runoff that originates out- channel and move the soil up onto the ridge. This side the terraced area; break up the method is not recommended for continuous use. It may be used occasionally in rotation with a motor concentration of water on long, gentle grader. slopes; and intercept and divert shal- low, or perched, subsurface water that would interfere with farming opera- C Back Furrow Channel Starting Point tions and plant growth. Above Channel Roünds_654¿2jll2345 Because diversions are used to inter- ^——»^"^^^ I v^ This series of rounds started cept the runoff from drainage areas of Oil back slope of terrace 2 to a few acres up to several hundred acres, 4 feet from to^), 17'-^i H each must be designed to fit the site. Direction of Dirt Movement The grade and dimensions should be First Step determined by a technician. The need for adequately protected waterways and outlet areas is just as. ■ Vary the width of this series important for diversions as for field ter- of rounds each time terraces are maintained so back races. Special structures of concrete or ilrrow will not fall in same •place.— other permanent materials often are Direction of *Dlrt Movement required to protect the outlet ends of Second Step diversions that intercept the runoff •Rounds from large drainage areas.
^Area between this point and^^S-^ . lower edge o'' next terrace —J 4' L— ONE OF THE GOALS in conservation is plowed as separate land. , farming is to keep soil losses down to Direction of Dirt Movemciit Tiiird Step something like the natural rate in the undisturbed landscape. It is often im- A method of maintaining channel-type terraces with moldboard plow. By starting with a back fur- possible and impractical to use farm- row at the terrace ridge^ as shown in step /, and ing systems that will completely pre- ending with a dead furrow in the channel {step j), vent all soil losses. the farmer can keep the terrace at the desired capacity. But the farmer and the conservation- ist need to be aware of the rate of loss
Start plow so that furrow slices their practices permit and to plan to barely meet. If Ihey overlap the keep erosion within allowable limits. ridge will be too "peaked". r This section presents a method of estimating soil loss as a guide to con- servation planning in the Corn Belt. The principles could be used in other sections where experimental data are Vary width of land each plowing so daad furrow will not fall k same place available to give reliable values for the repeatedly. Plow area between terriices as a separate land. various factors involved. NOTE! If terrace shoulj become peaked, start back furrow on back slopei one plow width from top of ridge. The first question to be answered is: What is "allowable soil loss"—that is, A method of maintaining ridge-type terraces with disk plow. This method is adapted for the gentler how much soil can be removed annu- slopes where level terraces are used in areas of low ally per acre, on the average, without rainfall and the conservation of water is vital. damaging the land or causing excessive EROSION ON CULTIVATED LAND 303
Start planting near top of ridge Start planting near top of ridge continue to top of slope or boundary. continue to top of slope or boundary.
Slarl planting near the top of ridge of second terrace and plant up Continue planting by starling on backslope of top terrace and plant the slope over channel, until a land 8 rows wide at narrowest portion down about one third of distance between 1st and 2d terraces. is left un planted. '
Land 8 rows wide at "narrowest portion.
Start planting the top of ridge of second terrace and plant up the Plant "shorter" rows on wide portion of turnland slope over channel until a land 8 rows at narrowest portion is left until 8 row turnland extends length of field. unplanted,
narrowest portion.'
Start near top of backslope of top Plant "shorter" rows on wide portion of turnland terrace and plant down 8 rows. until 8 row turnland extends length of field.
The 8 row wide turnland is planted last and extends through entire field length. Start planting near top of ridge continue to top of slope or boundary.
Continue planting by starting on bacl(sfope of top terrace and plant down about one half of distance between 1st and 2d terrace.
Area where point rows would occur Is established and retained \n sod crops or seeded to grain when rest of area is in row crops,
l_
Start planting near the top of second terrace and plant up the slope over channel, to previous planted rows leaving point rows in between terraces.
If point rows are not used leave tho uneven area in meadow or small grain,
Four methods oj planting row crops on terraced land. silting on lower lying fields or in streams Corn Belt should keep average annual and reservoirs? soil losses below 4 to 5 tons an acre on More eñ'ective erosion-control prac- deep soils and 2 to 3 tons an acre on tices are needed on shallow soils than shallow ones. Soil washing, gully for- on deep ones on similar slopes. Con- mation, and silting are not excessive servation practices for cropland in the at those rates. 304 YEARBOOK OF AGRICULTURE 1957 Harvest row crops in reverse of the way the field was planted. The illustra- erosion during a year. At Arnot, N. Y., tions below show how to harvest corn where the turnland was planted be- tween the terraces, 21 rainstorms out of 177 caused 65 per- cent of the erosion from 1935 to 1943. . Pick out turnland first At La Grosse, Wis., four storms a year from 1940 to 1943 caused 95 percent of the total soil loss and 84 percent of the runoff" from cornland. Length of slope is an important fac-
Pick "shorter" rows on wide portion oF turnland tor affecting the rate of erosion. Length is measured as the distance from a ridge or crest where runoff can begin to the point where water enters a chan- nel that cannot be filled and altered by the tillage operations or to a definite change in steepness where water is Pick corn land between turnland and second terrace ridge checked and deposition of eroded soil begins. Usually the length of slope, so considered, is less than the distance from the top to the bottom of the hill. The longer the slope, the more soil and water are lost when cultivated. If the length of slope is doubled, the soil Pick all corn above turnland to field'boundary. loss generally is increased about 1.5 times. Terraces and diversions help control erosion by reducing the length of slope over which runoff* water has a Harvesting corn on terraced land. chance to pick up speed and erosive power. An ''allowable soil loss" within these Steepness of slope is another factor broad limits has been assigned for each that affects the speed of runoff water important soil in the Midwestern and and amount of erosion. As the slope Northeastern States to serve as a goal gets steeper, the water flows faster and for judging the adequacy of erosion- carries more soil with it. If the per- control practices. centage of slope is doubled, soil losses Soil conservation experiment sta- are increased 2.5 times. This means tions since 1929 have been studying that soil losses on a 16-percent slope the factors that influence the amount are 2.5 times those on an 8-percent of soil and water lost during rain- slope, other things being equal. storms. The principal ones are the Relative erodibility of soils vary amount, intensity, and distribution of widely. One field may lose 10 tons of the rain; length and steepness of slope; soil an acre while another loses 15 tons kind of soil; kind and amount of plant under the same conditions. These dif- cover; and the tillage and conservation ferences in erodibility are due largely practices used. to the ability of the soil to absorb With the exception of the rainfall, all water and the tendency of the soil par- of these factors can be influenced to ticles to stick together. some extent by what the land operator To reflect the comparative rates at does. It is important therefore that he which different soils will erode, soil consider all factors when he develops scientists have assigned erodibility fac- a conservation plan for his farm. tors based on measurements at con- Usually once each year a rain will servation experiment stations. A fac- fall faster and in such amount that the tor of 1.0 is given to deep Prairie soils soil will be unable to absorb it. Usu- such as Marshall, Parr, and Tama. ally a few hard rains cause most of the Other soils, such as those developed EROSION ON CULTIVATED LAND 305 On land that slopes less than about 6% torraces are built wide enough to let crops. Most of the damage associated you cross them with machinery at almost any angle you choose. Thus you can cut such fields by going 'round and 'round thorn just as you always do. On with them was due to overtillage in steeper slopes, where terraces can't be built wide, it is probably belter to hafvest the grain with the terraces. The three drawings below showing a attempting to prepare fine seedbeds, windrower in operation illustrate how this may be done. control weeds, and get more nitrogen Stepi from decomposing organic matter. Im- Open land on top of second terrace proved cultural practices and new till- Terrace number three age implements have helped overcome some of the hazards of clean tillage. In the Midwest, seedbeds that were prepared with a field cultivator lost Step 2 about half as much soil as plowed ones. Cut land until terraces one and three are approached. _♦ j Terrace number three Contour listing in western Iowa re- duced water losses about 50 percent and soil losses 73 percent from those on
Windrows from cuts of step No, 1 fields planted on the contour. Leaving the crop residues, such as Step 3 cornstalks or grain straw, on the sur- Cut around the area left above, and including, terrace number one. face of the soil will reduce soil losses as much as 50 percent. The kind and amount of plant cover have a major influence on the rate of Harvesting grain on terraced land. erosion. On cropland this varies with different crops and their sequence in under a forest cover, have a factor of the rotation. Measurements at experi- 1.25, meaning that they erode 25 per- ment stations have shown the relative cent faster than those having a factor amounts of soil losses from different of i.o. For slowly permeable soils and crops in various rotations. These are some sandy soils, the factor is 1.5. Soils shown below in comparison to a stand- with tight, nearly impermeable sub- ard 3-year rotation of row crop (R), soils have a factor of i .75. spring grain (O), and hay (H)—that is, Conservation practices like terracing, R-O-H has the index number of 100. contour stripcropping, and contouring Percentage of materially reduce losses of soil and R-O-H water. Each has its limitations and Row crop, spring grain, hay (R-O- H) 100 needs to be used in its proper place. Continuous row crop (R) 422 For example, terraces are not gener- Row crop, spring grain catch crop ally constructed on steep slopes, con- (R-OJ 242 tour stripcropping is most effective on Row crop, row crop, spring grain, hay, hay (R-R-O-H-H) 125 slopes that are not more than 400 feet Row crop, winter grain, hay (R-W- long, and contouring alone is most H) 86 effective on 2 to 6 percent slopes that Row crop, spring grain, hay, hay (R- are not more than 200 feet long. O-H-H). . 65 Row crop, spring grain, hay, hay, hay Measurements show that contouring (R-O-H-H-H) 52 reduces soil losses about 50 percent from Winter grain, spring grain, hay, hay those occurring with up-and-down-hill (W-O-H-H) 27 cultivation, contour stripcropping re- The relative values of individual crops in duces losses about 75 percent and ter- different sequences have also been found. racing even more, depending on length Conservation workers in the Corn and steepness of the slopes involved. Belt and the Northeastern States have Tillage and crop-management prac- used tables of soil-loss factors reflecting tices modify the erosion-control effec- these variables to evaluate conserva- tiveness of crops they are used with. tion cropping systems. The tables serve Corn, cotton, and soybeans long as guides to help farmers select com- have been considered soil-depleting binations of rotations and practices for 406157»-—57- -21 3o6 YEARBOOK OF AGRICULTURE 1957 Combined Soil-Loss Factors for Different S ^ OD CO 01 0 r-- LO Rotations and Soils ^.. C< 01 CO O o ^ Soil Factor Rotation i.o i.2j 1.50 i. y^ O f^ J^ r^ CD í^ LO O R-Oj 2. 42 I. 85 3. 6 4. 24 1 HH t-i 01 Tt^ CD CO o CO R-R-O-H-H. 1.25 1.55 1.87 2.19 R-O-H 1.00 1.25 1.50 1.75 R-W-H 86 1.07 1.30 1.50 o V Ci Th CO ^ r^ lO y^ 0 R-O-H-H 65 .81 .97 I. 14 .^ ^ f. 0 J, w TiH m r^ cr> J R-O-H-H-H, .52 .66 .80 .93 o W-O-H-H. . . .27 .33 .40 .47 t_ 1 o ^ i The above values are based on good management. 0< o 01 LO lO For a high management level, multiply the J actor s 1 d CÔ 4^ «D á> — by o.y. o CO their soils that will reduce soil losses ctf LO CO CO Tf^ CO T^ LO x>. ■g to within the allowable limits. bx) o o J, 01 CO Ti- LO CD >c: The table opposite is based on data reí from soil and water conservation exper- 31 z: 0 CO ^ LO ^ co O O iment stations in that region. It shows 1 "§
res 1-1 Oí TjH CD '8 ^ CD CO 4^ ce responding slopes and practices with > »-I different rotations on different soils. reí For example, to estimate the soil loss 2 ^ -^ CO 1^ CO 0 0 o c o O from an R-R-O-H-H rotation on a
E: K o r^ CO r^ 0 0 o 0 long that is contoured, first find the o ^ 1— ^s CO m c^ CO O^ lO 01 (J iá Of CO TT basic soil loss figure in the first table for the slope and practice specified. It CO ^ is 4.7 tons per acre. This figure multi- CO rh t£> CO 01 LO LO 0 c CO i4 -^ o rí 4 x^ lO O CD 01 plied by the combined soil loss factor (N hH Ci ot CO from the second table (1.55) gives 7.3 o CO tons an acre as the expected average Jt^ CO CO o o LO LO 0 15 O annual soil loss under the conditions ►-I CO LO CO Tí^ 00 CO - Of described. <: 1 If the allowable annual soil loss for o ffi Í50 this soil is 4 tons an acre, this rotation ctS Ö with contouring is not an adequate <> ?. conservation system. Either a more effective rotation must be used or the 1 land must be stripcropped or terraced I to bring the expected soil loss down to i the allowable figure. Other calcula- tions with the two tables will indicate which combinations of crops and prac- o Ci Tt^ CD rjH t-t i tices will do that. (M C£) CO SLIGHT OR NONE MODERATE 25 to 75 percent of topsoil lost, may have some gullies. SEVERE More than 75 percent of topsoil lost, may have numerous or deep gullies. Includes severe geological erosion in parts of low rainfall areas. Many small areas could not be shown at this scale.
Generalized soil erosion. O