HOW SOILS DEVELOP UNDER GRASS 55 time productivity of the soil—prac- the individual soil types. Then too, we tices must be used to maintain within need to discover practices for grow- the soil conditions similar to those in ing good grass efficiently on those soils the natural Chernozem. This means for which we have as yet no satisfac- abundant plant nutrients and good tory methods. structure for considerable depth. Such a deep layer of fertile soil with good THE AUTHOR«-^- Charles E. Kel- structure often needs to be made by logg is chief of the Division of Soil the farmer from the natural soil. An Survey in the Bureau of Plant Indus- individual set of practices to this end try, Soils, and Agricultural Engineer- will be required in each individual ing. Before joining the Department in landscape. 1934, he taught soil science and did With the proper practices, grasses research in that subject at Michigan can be grown efficiently in most parts State College, the University of Wis- of the world, but not everywhere. One consin, and the North Dakota Agri- of the great problems of agricultural cultural College. He is a graduate of science is to learn how to make these Michigan State College, Dr. Kellogg practices more efficient and especially has written and lectured widely on how to adapt them more precisely to soil science and general amculture.

HOW SOILS DEVELOP UNDER GRASS

JAAIES THORP

RELATIONSHIPS between grass- and eastern forest belts, and extending land soils and their dominantly grassy from Canada on the north to Mexico cover are close and complex. The soils on the south, is the great grassland owe many of their properties to the area, broken only by river courses and kinds of vegetation they support; the occasional buttes or low mountains. kinds and qualities of grasses and as- "Grasslands characterize areas in sociated plants depend considerably which trees have failed to develop, upon the characteristics of the soil. either because of unfavorable soil con- Native dense stands of tall grasses ditions, poor drainage and aeration, in the United States usually have pro- intense cold and wind, deficient mois- duced thick, dark-colored soils, high in ture supply, or repeated fires. Grasses fertility and suitable for growing many of one kind or another are admirably economic crop plants, especially mem- suited to withstand conditions of excess bers of the grass and legume families. moisture, excess drought, and fires Less luxuriant grasses in drier areas w^hich would destroy tree growth." have produced lighter colored soils Important smaller areas of grassland with less organic matter and nitrogen, in this country occur also on the but frequently with more mineral plant Coastal Plain of Alabama and Missis- nutrients than those of tall-grass re- sippi, in north-central Oregon and gions. The variety of adapted crops on southeastern Washington, in the Cen- these soils is narrower, and yields are tral Valley of California, and in hun- limited by the lower rainfall, except dreds of narrow strips along the moun- where crops are irrigated. tains and basins from the eastern limit In the Atlas of American Agricul- of the Rocky Mountains to the Pacific ture, 1936, H. L, Shantz said this about coast. the location and conditions in the The type of grass and the density of United States under which grasslands cover—both important to soil forma- develop: "Lying between the western tion—are determined by the interac- 56 YEARBOOK OF AGRICULTURF: 19 48 tion of many factors. For example, rank soil materials, the invasion of these water-loving grasses and sedges and materials by grass, and the accumula- other grasslikc plants grow in poorly tion of organic matter and develop- drained areas and contribute enormous ment of soil structure. quantities of organic matter to soils of Mineral soil materials accumulate marshy and semimarshy lands. Wiesen- through the direct chemical and physi- boden ^ (wet meadow soils) and Half cal w^eathering of rocks and through Bog soils develop in these situations. the deposition of broken and chemi- The well-drained black Prairie and cally weathered rock fragments (sand, Chernozem soils have thick covers of silt, clay, and gravel) by streams, lakes, sod-forming tall-grass associations. glaciers, wind, and down-slope gravi- Heavy, dark, grayish-brown clay soils tational movement. A very rough esti- like the Pierre clay developed from mate is that one-third to one-fourth of Pierre shale of South Dakota and the soil materials of the natural grass- northwestern Nebraska seem to pro- lands in the United States are the vide conditions where western wheat- products of direct w^eathering of rocks grass is best able to survive as the domi- of greatly varying composition and nant grass. Shantz's map of the grass- hardness. The remaining materials lands of the United States, in the have been deposited by streams and Atlas of American Agriculiure^ shows the other agencies I have listed. an area of western wheatgrass coex- The larger areas of grassland soil tensive with a large area of Pierre clay materials, developed directly through soil. On very sandy soils, as in the the weathering of rock, came from soft sand hills of Nebraska, arc associations rocks that either were weathered easib^ of sand sage and sand rcedgrass with to form soil or were already soft bluestem grass. Farther south, as in enough in their original state to be sandy soils of Texas, a low scrubby penetrated easily by the ñbrous roots growth of shinncry oak is associated of grasses. The great area of Pierre with bunchgrasses. shale in South Dakota, Colorado, and Blue grama appears to be the domi- Nebraska, and the still larger areas of nant grass of medium-textured soils tertiary shales, silt-stones, and soft in the zone of Brown soils from Colo- sandstones of Montana, North Dakota, rado to the Canadian line, while Wyoming, Colorado, and Utah are mixed grama and bufîalograss are good examples. Vv'idely distributed in the dry parts of The extensive loess (wind-blown the Chestnut and Reddish Chestnut dust) deposits of the Great Plains cast and the Reddish-Brown soils zones. of the Rockies and of the Palouse re- Red Desert soils and some areas of gion of Oregon and Washington arc Reddish-Brown soils in New^ Mexico ideal for the growth of grass. Loess is and western Texas are dominated by a uniform unstratified mixture of silt, black grama grass, associated with very fine sand, and clay. It varies in other grasses and desert shrub. thickness from a few inches to more The formation of grassland soils in- than 100 feet in the United States. volves the accumulation of mineral Probably loess is the most extensive ^ Throughout the text of this paper ref- single kind of parent material of grass- erences are made to great soil groups, as land soils in the world as a whole. Prairie, Chernozem, Wiesenboden, Chestnut Glacial deposits composed of clay, soils. Brown soils, etc. These are described sand, silt, and stone fragments of many by C. F. Marbut in the Atlas of American Agriculture^ 1936, and redefined and (in kinds are parent materials of thou- part) renamed in the chapter on soil classi- sands of square miles of grassland fication in the Yearbook of Agriculture soils of the northern Great Plains of 1938. Prairie soils, for example, do not in- the United States and of the Great clude all grassland soils but are restricted in the United States largely to the Corn Plains of Canada. Possibly wind-laid Belt. and water-laid sands are third in im- HOW SOILS DEVELOP UNDER GRASS 57 portance. For example, the grassy Even noW', the opposing processes sand hills of Nebraska arc only slightly of erosion and deposition are w^orking smaller in area than all of the culti- in the grasslands of the United States. vated land of Japan, and very large In the Badlands of South Dakota, areas of aeolian sands occur in Kansas, Wyoming, and Montana, for example, Colorado, Wyoming, Oklahoma, Texas, catastrophic erosion is rapidly eating Oregon, and Washington. away the cliffs of soft bedrock and Stream alluvium and lake sediments dumping sediments indiscriminately of medium to clayey textures also are on the lowlands and along the stream important, especially along the large courses. After the first big wave of dep- river valleys and mountain fronts. osition has subsided, grasses and other Where climatic conditions are fa- plants take root quickly on smooth vorable, grass invades areas of freshly land below the Badland cliffs and a deposited soil materials very soon after new cycle of soil formation begins on they are first exposed. If accumulation the fresh deposits. High grasslands be- of sediments or weathered rock mate- hind the cliffs, meanwhile, are under- rials is slow, grasses become well estab- mined and destroyed. lished and begin immediately the work When grasses and other herbaceous of soil building. Roots spread through plants secure a foothold, the soil ma- the soil and sooner or later die, provid- terials are held in place by a netw^ork ing organic, humus-forming waste. of roots. Old roots die and new ones Indeed, where the accumulation is take their places. Dead roots are at- slow, soil formation can almost keep tacked by bacteria, microscopic ani- pace Vvdth weathering and deposition, mals, and fungi, and mineral and or- and upper soil layers rapidly take on ganic nutrients arc made available to a dark color. Where accumulation of living plants. Under the climatic and soil material is rapid, as is true of some biotic conditions of grasslands, part of sedimentary deposits, vegetation has the root material, as well as part of the relatively little eflect on soil develop- annual crop of grass tops, remain to ment until accumulation slow^s dow^n form dark-colored, finely divided or- or stops. Plants are covered before they ganic matter (humus) in the soil, and can contribute much to the soil. a larger part is absorbed by other Abundant geologic evidence in large plants or dissipated into the air as gas areas of grassland in the United States or into the soil water and carried away. shows not only that the stream sedi- Organic material in grassland soils, ments and wind-laid dust (loess) and then, includes living and dead plant sand are extensive, but also that many parts, living and dead organisms, and of them accumulated so rapidly that humus. The decay of grass roots and soil formation did not advance appre- leaves and the formation of humus so ciably until the deposits WXTC almost improve the fertility, physical condi- complete. Furthermore, buried rem- tion, and moisture-holding capacity of nants of soils, in some places one above the soil that morc> luxuriant grasses another, show that the history of soil will survive as time goes on. Light- formation has been repeated many colored soils, shallow^ to the underlying times. Waves of catastrophic erosion parent soil material, which are char- have advanced across the land, de- acteristic of the fixrst stages of develop- stroying strongly developed soils and ment, are thickened and darkened concurrently building up new deposits gradually wdth the improvement of of alluvium, loess, or sand. Local ac- the grassy cover. The end point of this cumulation of sediments has been so ¡Drocess varies with the mioisture supply. rapid that soil formation could not As soon as vegetation is established, keep pace, and soil development has many kinds of animals take up their been delayed until the rapid erosion abode in the soil to take advantage of and deposition have abated. the food provided by plants or to prey 58 YEARBOOK OF AGRICULTURE 194Í

Roots of different grassland plants draw their moisture from different soil layers. Some native plants extend their roots to depths of 20 or more feet. Drawing made at Hays, Kans., before the great drought, by J. E. Weaver and F. W. Albertson (Ecol. Monographs, volume 13, p. 100). Al, narrow-leafed 4-o'clock, Allionia linearis; Kg, prairie false boneset, Kuhnia gidtinosa; Bg, blue grama, Bouteloua gracilis; Mc, globemallow, Malvastrum coccineum; Pt, a legume, Psoralea tennifiora; Ss, Sideranthus spinulosis; Bd, buffalograss, Buchloe dactyloides; Ap, western ragweed. Ambrosia psilosíachya; and Lj, skeleton weed, Lygodesmia júncea. HOW SOILS DEVELOP UNDER GRASS 59 on one another. Insects (especially der natural conditions for a few thou- ants), millipedes, earthworms^ go- sand years. phers, prairie dogs, and many other During periods of greater-than-nor- animals live interdependent lives in the mal humidity, the grasses are more soil. Their burrowing operations stir luxuriant and contribute more organic the soil, mix it with fresh minerals, material to the soil; leaching goes to a kill some roots, and hasten the process greater depth than in dry years, and of humification. The soil becomes a some of the soluble compounds are car- community of teeming activity. ried out in subsoil water. In most grass- Organic matter in grassland soils lands, both wind and water erosion are acts as a sponge to absorb rain water, reduced during years of abundant as a home and as food for the micro- rainfall because the luxuriant vegeta- scopic plants and animals that pre- tion can better hold the soil in place. pare the soil for the use of higher During dry periods, grass cover may plants, and as a water reservoir to sup- be largely destroyed if the droughts are ply plants with needed moisture and prolonged, and contribution of or- to cushion them against drought. ganic matter to the soil may be stopped Much of the all-essential nitrogen is temporarily. Erosion by wind and stored in the organic material waiting water becomes active during or im- to be made available by micro-organ- mediately following these dry periods, isms for the use of higher plants. and organic matter is lost that took Field studies of grassland soils that years and years for accumulation. range in age from a few years to a Grasses have a profound effect on probable age of many thousand years the structure of soil. Organic matter indicate that organic matter accumu- from the decomposition of grassroots lates somewhat slowly during the first and leaves helps to hold clay, silt, and few years until a good relationship for sand particles together to form struc- the formation of organic matter is ture particles or aggregates of various reached among the microscopic plants shapes, sizes, and degrees of firmness. and animals, the higher plants, and Generally the upper layers of grass- the higher animals that live in the soil. land soils, where roots are most abun- Following the establishment of this dant, attain an extraordinary degree of favorable relationship, the rate of ac- granulation in which the structure par- cumulation increases rapidly for many ticles take the form of firm granules or years and more slowly after that. In porous crumbs ranging from less than medium-textured soil materials of one-thirty-second of an inch to a half well-balanced mineral composition, it inch in diameter. Some of these aggre- is thought that the maximum content gates, especially those containing more of organic matter will be attained in than about 30 percent clay, are firm several hundred years. and hold their size and shape even Ultimately some rough balance is Vv^ith rough handling; others, especially reached between the rate of accumu- those containing much sand and silt lation and destruction of humus and and little clay, are soft, porous, and between types of grasses and forbs pres- unstable. The development of granu- ent and the degree of their luxuriance. lar structure makes the clayey soils of The level at which this rough balance grasslands crumbly and friable and is struck depends on the texture, po- almost as easily tilled as sandy soils. rosity, and plant-nutrient content of In all but the very sandy soils, the the soil material and on the amount structure particles of subsoil layers of easily available moisture received, (roughly, the layers lying betw^een 10 absorbed, and retained by the soil for and 30 inches below the surface) usu- use of plants. This balance is a dy- ally are coarser, more angular, and namic one, subject to minor fluctua- firmer than those of the upper soil lay- tions, and probably is maintained un- ers, partly because many of them are 6o YEARBOOK OF AGRICULTURE 19 4 8 inore clayey and partly because they slowly with the aging process. Plowing contain less organic matter. and cultivating the soil may either in- Some of these structure particles are crease the rate of im^provement or angular and blocky, others arc more or hasten deterioration. Good husbandry less rounded and nut-shaped, and improves the land for man's use and still others take the form of small raises the level of fertility and produc- prisms. They have less organic matter tivity. On the other hand, because the than the surface layers and usually are nutrient requirements of cultivated held together by thin films composed plants are different from those of the of jelly-like clay mixed with organic native grasses, nutrient deficiencies matter. Probably the shape^ size, and may be expected to develop. In Illi- arrangement of the subsoil structure nois, Iowa, and eastern Nebraska and particles are related partly to shrink- Kansas, for example, farmers are al- ing and swelling caused by changes in ready beginning to obtain benefit from moisture conditions, and partly to the the use of superphosphate on some of wedging action of roots that penetrate the older soils. deep into the soil in search of moisture. A large proportion of the Cher- Once the pattern of soil structure is nozem, Chestnut, Reddish Chestnut, established, many of the grass roots P)rown, and Reddish-Brown soils of the work their way into minute crevices Great Plains and of the Pacific North- where they expand with growth and west probably had been in a state of press the structure particles into a still temporary dynamic equilibrium for a firmer consistence. Where clayey soils fe\v thousand years wh(^n the region swell and shrink greatly with changes was invaded by white colonists. Most of in moisture, roots are somí^times un- the Chernozem and Reddish Chestnut able to expand in the normal way and soils and a smaller proportion of the so take on flattened shapes. Some roots others have been plowed and planted do not confine themselves to the crev- for two or three generations, and it ices but pierce the structure particles cannot be denied that much of the soil and so tend to make them more porous has deteriorated under this regime. or to break them into smaller units. Some of the organic matter has been Although soils are considered to lost by wind erosion, some has been reach equilibrium with their environ- oxidized, and some has been w^ashed ment it is probable that soils of the away. And crops also have made seri- grasslands gradually change with time. ous inroads on the supply of easily Mineral plant nutrients are removed available plant nutrients. Soil man- slowly in solution from subhumid agement practices are now in a state grasslands (Prairie and Chernozem of slow evolution that we hope will soils). The clay content of the soils is eventually correct a bad situation. gradually increased by chemical proc- esses, and is concentrated in subsoil Development of Soils horizons where it impedes water move- ment in the soil and makes soil water The dark-colored Prairie and Cher- less quickly available to plants. In nozem soils, with the associated Wie- other words, a slow deterioration sets senboden (dark-colored wet meadow in as soils become older. soils) and Planosols (soils with clay- In a fe\v places natural erosion and pan), show the maximum eftects of soil formation seem to have reached a true grasses and other grasslikc plants balance that keeps the soils fertile and on soil formation. These soils occur in productive for a long time. Generally, subhumid and dry-subhumid areas however, it appears that soils are either where climatic conditions are generally improving gradually through the slow better suited to the growth of grasses accumulation of organic matter and than to forest, but wehere moisture is fresh sediments, or are deteriorating sufficient to support a dense growth of HOW SOILS DEVELOP UNDER GRASS 6l grass. In the Prairie soils zone, forests moisture nearly every year to pass en- will spread over the grasslands fairly tirely through the profile. Chernozem readily if young trees are protected soils generally are neutral or slightly from fire until they are well estab- alkaline in reaction and typically have lished. Trees can also be made to grow- a horizon, or layer, of subsoil enriched On medium- to sandy-textured soils in with carbonate of lime. This horizon the Chernozem soils zone, and even in is developed because rain w^ater fre- drier areas, with still greater care; but quently is insufficient to pass entirely it is doubtful whether any extensive through the soil. Percolating water forest could establish itself on these usually reaches only a few feet into the soils with or wdthout the aid of man. subsoil, where it gradually evaporates Only on steep north- or east-facing into the soil air or is picked up by grass slopes and along stream valleys do for- roots and carried back to the surface ests obtain a permanent foothold in and transpired through the leaves. zones of Chernozem and Chestnut soils. Soluble material like carbonate of lime Parent materials of Chernozem and is thus carried to the subsoil and left Prairie soils are developed from a very behind wh(^n the water is dissipated. wide variety of rocks, loess, aeolian It is generally true on the Great sand, and alluvium, but loess, glacial Plains of the United States and-Can- drift, alluvium, and soft sedimentary ada that grassland soils in the cool re- rocks comprise the greater part. Most gions have more organic matter than of the parent materials are calcareous those in the warmer regions. Soils in (contain carbonate of lime) or have subhumid grassland areas of Texas been in the recent geologic past. generally have less organic matter than Typical soils of these groups are all those in areas of southern Manitoba well drained, either because of moder- and North Dakota where eíTective soil ate slopes or because substrata arc moisture is comparable. porous and permeable to water and After an exhaustive study of the have suitable natural outlets to remove nitrogen and organic matter of hun- excess water. The land surface ranges dreds of soils in the United States, Hans from level to gently sloping. Soils of Jenny, in Research Bulletin No. 152 of other groups occur on the steep slopes the Missouri Agricultural Experiment and in wet depressions. Station, drew a number of conclusions Prairie and Chernozem soils sup- regarding the accumulation of soil or- port a natural cover of tall prairie ganic matter in relation to climate. grasses dominated by a few species, Among them are : with a minor constituent of legumes 1. Within regions of equivalent and other shrubby perennial and an- eíTective moisture, the nitrogen and or- nual plants that are not true grasses. ganic matter contents of soils of me- Under virgin conditions the original dium textures increase 2 to 3 times tall-grass cover formed a dense sod at with every 18° F. fall in average annual the surface of Prairie soils, but the temperature, from south to north in cover was less complete in the drier the United States. The statement holds parts of the Chernozem soils zone for both forested and grassland soils. where bunchgrass associations arc the 2. Within regions of equal average rule. annual temperatures, the nitrogen and While Prairie and Chernozem soils organic matter content of grassland have in common dark colors, much or- soils increases with increasing humid- ganic matter, and similar structure, ity. The rate of increase is greater in they differ in one important respect: the cool northern regions than in the Prairie soils generally are acid in re- warmer southern regions. This conclu- action and have lost most of their sion docs not hold for forested soils. original carbonate of lime through 3. From data available to Jenny at leaching. Rainfall provides enough the time his bulletin was wTitten in 62 YEARBOOK OF AGRICULTURE 19 48 1930^ he felt that climate and vege- ground plant parts than little bluestem tation were more significant to the ac- in the same region. In the same soil, cumulation of nitrogen in loamy soils the dark humified organic matter than topography, parent material, or ranges from about 25 to about 40 tons age of the soil. in the top 6 inches of soil on each acre. Those who have studied the grass Analyses of Prairie and Chernozem roots in the Great Plains tell us that soils in North Dakota, Manitoba, and different kinds of grasses and associ- Saskatchewan show that the weights of ated herbaceous plants distribute their humified organic matter are consider- roots in different layers of the soil. ably greater in the northern parts of Some species have concentrated their the Plains and range from about 40 to roots in the uppermost layer where possibly 1.00 tons an acre in the top 6 they depend on moisture from rains inches of Manitoba and Saskatchewan which penetrate the soil only to a rel- soils. In warmer southern areas, total atively shallow depth. Other plants weights of humified organic matter to draw their water from intermediate the acre of the dark-colored Reddish- layers. Still others reach down to ob- Prairie and Rendzina soils average con- tain water from the deep layers of the siderably less than in Nebraska— parent material and even from the roughly 10 to 20 tons in the top 6 ground water, many feet below the inches of soil, on the basis of very lim- surface. Most of the deepest-rooted ited information. plants of the prairies are perennial C. C. Nikiforoff estimates totals of herbaceous or shrubby plants (known from 120 to 240 tons of humus an acre technically as forbs) that grow in close in Chernozem soils of the United association with the grasses. In times States. His maximum figure is for the of protracted drought, many of these entire profile of Chernozems with ex- plants survive while some of the grasses ceptionally thick dark surface soils. with shallowxu' roots die. Dr. Weaver and Ellen Zink have Prairie and Chernozem soils have estimated the rate at which roots of díícp, nearly black surface soils with various perennial grasses die in the well-developed granularorcrumb struc- soil under various conditions. The esti- ture and much organic matter. Fer- mates wcu'c based on periodic exam- tility is high. The dark surface layers, ination of a large number of roots under natural conditions, are matted banded with soft tin strips. They found with grass roots to depths of 1 or 2 feet, that big bluestem lost 19 percent and many grass roots penetrate to of its original coarse roots in 3 years, depths of 5 to 8 feet. little bluestem and porcupinegrass each J. E. Weaver and others estimated 90 percent, blue grama 55 percent, and that 60 percent of the underground side-oats grama 86 percent. parts of little blucstem grass on Lan- All the losses were gradual, and re- caster loam in Nebraska were in the placements doubtless were made by the top 6 inches of soil and that 68 percent plants, but it is interesting that even the of underground parts of big bluestem relatively long-lived big bluestem roots grass were in the top 6 inches of Wa- are able annually to contribute about bash clay loam. 312 to 540 pounds of raw organic mat- S. B. Shively and Dr. Weaver report ter an acre to the top 6 inches of soils that the top 6 inches of Prairie soils in in western Iowa and eastern Nebraska western Iowa and eastern Nebraska and Kansas, assuming pure stands of contain from about 4.5 to about 2.6 this grass. This would mean that from tons of underground plant parts (dry about 520 to 900 pounds per acre of weight) per acre, and Chernozems raw organic matter is added annually from 2.34 to 2.6 tons per acre in Ne- to the entire soil profile. braska and Kansas. Big bluestem L. E. Andrew and H. F. Rhoades produced a larger amount of under- analyzed a sample of soil developed in HOW SOILS DEVELOP UNDER GRASS 63 calcareous glacial till thrown out from and Chernozem zones. The Chestnut- a railroad cut in Lancaster County, soils zone grades imperceptibly into the Ncbr., 75 years ago. Organic matter is zone of Brown soils, and the Reddish most concentrated in the topmost 2 Chestnut into the Reddish-Brown soils, inches of soil, which contains 4.3 pcr- where short grasses are dominant and ccnt. Possibly part of this organic mat- shrubby plants characteristic of still ter came from wind-blown dust. The drier regions fairly common. average for the top 6 inches is about The Sierozem (gray-earth) soils are 2.9 percent. Assuming that 0.4 percent in a still drier climate with a dominant of the material deposited with the cover of sagebrush and only a sparse recent dust or present in the original growth of short grasses. This zone is material w^as organic material, it seems not very distinct and is considered to safe to estimate that organic material be just a little more moist than true accumulated in the last 75 years com- Desert and a little drier than the zone prises 2.5 percent of the top 6 inches of Brown soils. The break between of soil. Assuming further a volume Sierozem soils and Desert soils is ap- weight of about 1.1 for this layer, the proximately the break between the total estimated weight of dry organic complex of sparse short grass and sage- matter per acre to 6 inches is about brush and the zone where desert shrub 18.7 tons. At a uniform rate of ac- plants, like creosotebush, shadscalc, or cumulation, this w^ould be 500 pounds white bur-sage are dominant, and grass a year. The figure seems high ahd sug- is almost nonexistent. gests that organic accumulation might Soils in semiarid regions generally reach its maximum for the region arc not covered completely by grass. within as little as 200 or 300 years. Bare spots are subject to erosion by With a diminishing rate of accumula- wind and water and to beating by the tion, the period might be somewhat occasional torrential rains. The wind longer. The development of a clayey removes fine soil from the bare spots, subsoil would take considerably longer, deposits some of it in adjacent grassy perhaps a few thousand years. patches, and carries the rest greater Traveling from east to west across distances to form dust deposits, fre- the Great Plains, from subhumid to quently in Chernozem and Prairie soils semiarid or arid climates, one can ob- zones. The ultimate result is that the serve a gradual decrease in the dark- surface of the land in semiarid regions ness and thickness of the surface soil. has a peculiar micro-relief with minute The horizon of carbonate of lime lies basins a few inches deep interspersed at shallower depths than in Chern- between grassy mounds a few inches ozem soils and is proportionally thicker high. Where soils contain pebbles or on soils of equal age and like parent rock fragments, these are left behind material. These changes correspond to form a cover or pavement that pro- directly with the decrease in the height tects the soil from further wind erosion. and concentration of grasses on the One or more groups of soils within soil, as well as with the decrease in each soil zone have peculiar properties effective moisture. that set them apart from soils consid- Virgin areas of dark grayish-brown ered "normal" for that zone. Usually Chestnut and dark-brown Reddish one or more horizons of each of these Chestnut soils occur under a mixed soils is overdeveloped and in marked cover of tall, medium-height, and contrast to the normal soil. These are short grasses, with a considerably less called intrazonal (within-zone) soils. dense cover on the ground. Moisture Planosols of the Prairie and Cherno- is not sufficient in the zone of Chest- zem soils zones, and claypan soils nut and Reddish Chestnut soil to (some of which are known technically support the luxuriant growth of tall as Solonetz and solodized-Solonetz) of grasses characteristic of the Prairie drier regions arc intrazonal soils char- 64 YEARBOOK OF AGRICULTURE 19 48 actcrized by grassy vegetation. The gray layer have a more dense grassy most extremely developed of the Plan- vegetation than those in which the gray osols have very thin surfaee horizons layer has become well developed. The of dark-eolored material rieh in or- development of a thick gray layer above ganic matter. Beneath the thin dark the claypan usually is interpreted as horizon is light-colored, friable, platy evidence of soil deterioration. material^ several inches thick, with Morphology of solodized-Solonetz little organic matter^ and usually mod- soils of the drier regions resembles that erately to strongly acid in reaction. of Planosols. Most solodized-Solonetz Beneath is a strongly developed clayey soils have a less dense grass cover than horizon, or claypan. most Planosols. Many of them have in- While the claypan may appear to be numerable small bare spots where wdnd practically impervious^ it is easy to erosion has removed the surface hori- demonstrate that grass roots will pierce zon and has exposed the intractable it if nutrients and moisture are there clay subsoil. These "scab spots" or to attract them. E. G. Fitzpatrick, for- "slick spots" are abundant in areas of merly soil scientist of the Division of imperfectly drained soils of the semi- Soil Survey in the Department of Ag- arid plains from southern Texas to riculture, once made root counts in a northern Saskatchewan and Alberta, claypan soil of Oklahoma. He found and in the drier grasslands of Cali- more roots in the claypan layer than in fornia and the Pacific Northwest. the friable silty leached soil immedi- A peculiar group of intrazonal soils ately above it. known as Rendzinas occurs in regions Furthermore, dry weather so shrinks ranging from subhumid climates wehere and cracks many of the claypans that Chernozems are normal to humid cli- water penetrates them readily follow- mates wehere leached, strongly acid, ing dry weather. Grass roots work their light-colored forest soils are dominant. way between the blocky- and prismatic- These soils are covered largely with structured particles into deeper layers grass and other herbaceous plants and of the subsoil in search of moisture, have dark-colored, thick surface layers and some of the roots actually pierce much like those of the Chernozems. the dense blocks and prisms of clay. They are developed in soft calcareous Such penetration tends to lessen the clays, marls, and chalk deposits (soft undesirable physical conditions of the pow^dery limestone), all of which con- soil. Shrinking and expanding of the tain much soft carbonate of lime. It clay blocks with changes in moisture seems that the excessive amount of have the effect of flattening the roots, easily available lime carbonate and the but this process does not kill them ex- clayey texture of the parent material cept where extreme shrinkage in dr)^ encouraged the growth of grasses at weather breaks them asunder. Only in the expense of forests. Only wehere soils soils with subsoil horizons very low in of this kind have been exposed for a plant nutrients and moisture do the long time to leaching without erosion roots fail to penetrate the claypans. do the soils become acid and the for- But roots generally encounter consider- ests invade the grasslands. The Hous- ably more difficulty in penetrating clay- ton soils of the "blackland prairies" of pans than friable soil layers. Texas, Mississippi, and Alabama are All gradations of claypan soils exist good examples. These soils are gen- from those in which there is a well- erally more fertile than the associated developed gray layer like the one de- strongly acid forested sandy soils. scribed to one in w^hich there is only The Rendzinas in scattered belts of a thin sprinkling of light-gray silt on the Southern States from central Texas the aggregates at the top of the claypan to Alabama are nearly black in color, subsoils. Under like climate, the day- but they contain less organic matter pan soils with only the beginnings of a on the whole than Chernozems and HOW SOILS DEVELOP UNDER GRASS 65 Prairie soils of similar colors in the light-colored except for a very thin North Central States. It is generally layer immediately under the decaying thought that the higher temperatures leaves of the forest. The important ex- of the South accompanied by greater ceptions to this are in the Brov/n forest activity of bacteria keep the organic soil and a few^ areas of forested Rend- material of the soils at a somewhat zinas, which have dark-(X)lored surface lower percentage than in grassland soils not greatly diíTerent in superficial soils of the cooler Northern States. appc^arance from the Chernozem. Azonal soils are those whic^h are so The usual explanation for this dif- young that they have not yet developed ference^ between the black soils of the distinct soil horizons. The group in- Prairie and Chernozem zone and the cludes soils with only the beginnings light-colored soils of the forested areas of organic accumulation in the upper is that the organic matter of the grass- layers where grass roots are abundant. land soils is developed largely through Many of these soils are younger than the decay in plac^e of myriads of grass 100 years. The development of older roots rather evenly distributed through ones has been retarded by rapid geo- the soil, and secondarily by the de- logic erosion on steep slopes or accumu- composition of grass tops which die lation of sediments on flood plains. back annually. Every year a certain J. E. Weaver and F. W. Albcrtson proportion of grass roots, varying with have demonstrated the harmful effects sDixnes, and all of the tops, decay and on the soil brought about by the in- become mixtxl with the soil. Part of vasion of the blucstem prairie by west- the material disappears as gas, part of ern wheatgrass during and immedi- it seeps aw^ay in ground water solu- ately after the great drought of the tions, and part of it remains behind in mid-1930's. Western wheatgrass lacks the form of dark-colored, fairly stable, the dense mass of fine leaves character- organic matt(T. The amount of grass istic of the bluestems and so permits roots thus contributed annually to the the beating rains to destroy the soft soil decreases with decreasing rainfall crumblike soil aggregates at the sur- westvv'ard across the Plains. Certain face. The soil runs together and pre- roots of grasslike plants like those of vents rapid penetration of water. As a the sedge commonly called black root result, the soil absorbs less of the rain decay so slowly that their contribution water, runoff is promoted, and erosion to humified soil organic matter is very is accelerated. In this w^ay drought con- little, indeed. It is possible to identify ditions are maintained longer than the roots of this sedge in Brown soils would be expected otherwise. of the Great Plains many years after the plants have been killed by plowing. Forested Soils and Deserts The difTercnces in the amount of or- ganic matter between the dark-colored While it is altogether possible that grassland and light-colored forested the total percentage of living and dead soils is ascribed partly to the fact that organic matter of soils of many forested forest soils are generally more acid in areas is equal to or greater than that of reaction than grassland soils. The the Chernozem and Prairie soils, the "mild" or nonacid humus of grasslands organic matter of most forest soils is is more stable than the relatively solu- not quite so dark-colored as that of the ble acid humus of forests. Part of this grasslands and is distributed through difference may be due to the fact that greater thicknesses of soil materials. the molds (fungi) are more active in Probably, also, the ratio of humified the organic matter of forest soils than soil organic matter to roots in most bacteria, whereas the reverse is true in forest soils is less than in the grasslands. most grasslands. Generally speaking, the majority of Grasses will survive better than for- forest soils of the United States are est trees in subhumid and semiarid re- 757150"—-4S- - -() 66 YEARBOOK OF AGRICULTURE 19 48 gions in soils of clayey textures, so that the occasional rains. Sand is heaped up we find many areas of grasslands in by the wind around clumps of brush heavy clayey soils surrounded by for- or cactus and silty materials arc carried ests on soils of medium to sandy tex- farther to be deposited in denser vege- ture. Examples of this may be seen in tation around the desert border. Texas, where the forested Cross Tim- Where soil materials contain coarse bers areas arc interspersed wdth grass- rock fragments, these are left behind covered Rendzina soils. Furthermore, to form a "desert pavement" which grassland parks have persisted within helps to check further erosion. As a net areas of forests where subsoils are very result, desert soils contain less organic droughty, as on some of the gravelly matter than soils of grasslands. and sandy outwash deposits of north- Medium-textured Desert soils with ern Indiana and southern Michigan. well-balanced mineral nutrients can Deserts of the United States gen- become just as productive under irri- erally are not grassy, although certain gation as grassland soils of similar tex- species of grasses are able to survive in ture, but this can be achieved only some regions commonly called desert. after the organic material and nitro- Even in the Yum a Desert of Arizona gen content have been built up where rainfall averages less than 5 through good agricultural practices. inches a year, bunchgrasses can be found on certain sandy soils. THE AUTHOR^<- ]ames Thorp, a None of the desert plants can sup- native of Pennsylvania and a graduate port a dense growth of fine roots in of Earlham College, has been engaged upper soil layers for any long period in research on soils since 1921 in the of time and so provide enough organic United States, the West Indies, China, matter to darken the soil appreciably. and Japan. Mr. Thorp, principal soil Furthermore, vegetation in the desert scientist in the Bureau of Plant Indus- nowhere covers the soil sufficiently to try, Soils, and Agricultural Engineer- prevent erosion by wind and water. ing, is principal soil correlator for the Fine-textured soil materials are re- Great Plains, with headquarters at the moved by the wind or washed away by University of Nebraska.

THE TOOLS OF FLOOD CONTROL

HUGH H. BENNETT

FLOOD CONTROL begins where the the orderly disposal of runoff from rains fall and runoíT originates. Flood fields, pastures, and woodlands through control ends only when that runoff has stabilized waterways and the tempo- safely reached the ocean. rary detention of runoff in small up- The work, then, of preventing floods land storage basins and temporary (which annually cost citizens of the pools where practicable. It requires, United States 250 million dollars in too, the control of the erosion al de- crops, equipment, and other property bris—silt—that shoals waterways, fills damage) does not consist only of in- ditches, and reduces the capacity of stalling major engineering works along flood-control reservoirs. the main channclways of our trunk Sometimes it involves the improve- streams. It consists also of treating the ment and maintenance of minor land surface in such a way as to obtain stream channels. In many watersheds maximum infiltration consistent with large detention reservoirs are required proper use of land for crops, pasture, at critical locations to hold back excess forest, and other purposes. It covers floodwaters that would otherwise over-