HOW SOILS DEVELOP UNDER GRASS

James Thorp

RELATIONSHIPS between grassland soils and their dominantly grassy cover are close and complex. The soils owe many of their properties to the kinds of vegetation they support; the kinds and qualities of grasses and associated plants depend considerably upon the characteristics of the soil.

Native dense stands of tall grasses in the United States usually have produced thick, dark-colored soils, high in fertility and suitable for growing many economic crop plants, especially members of the grass and legume families.

Less luxuriant grasses in drier areas have produced lighter colored soils with less organic matter and nitrogen, but frequently with more mineral plant nutrients than those of tall-grass regions. The variety of adapted crops on these soils is narrower, and yields are limited by the lower rainfall, except where crops are irrigated.

In the Atlas of American Agriculture, 1936, H. L. Shantz said this about the location and conditions in the United States under which grasslands develop : "Lying between the western and eastern forest belts, and extending from Canada on the north to Mexico on the south, is the great grassland area, broken only by river courses and occasional buttes or low mountains.

"Grasslands characterize areas in which trees have failed to develop, either because of unfavorable soil conditions, poor drainage and aeration, intense cold and wind, deficient moisture supply, or repeated fires. Grasses of one kind or another are admirably suited to withstand conditions of excess moisture, excess drought, and fires which would destroy tree growth."

Important smaller areas of grassland in this country occur also on the Coastal Plain of Alabama and Mississippi, in north-central Oregon and southeastern Washington, in the Central Valley of California, and in hundreds of narrow strips along the mountains and basins from the eastern limit of the Rocky Mountains to the Pacific coast.

The type of grass and the density of cover both important to soil formation are determined by the interaction of many factors. For example, rank water-loving grasses and sedges and other grasslike plants grow in poorly drained areas and contribute enormous quantities of organic matter to soils of marshy and semimarshy lands. Wiesenboden (wet meadow soils) and Half Bog soils develop in these situations.

The well-drained black Prairie and Chernozem soils have thick covers of sod-forming tall-grass associations. Heavy, dark, grayish-brown clay soils like the Pierre clay developed from Pierre shale of South Dakota and northwestern Nebraska seem to provide conditions where western wheat-grass is best able to survive as the dominant grass. Shantz's map of the grasslands of the United States, in the Atlas of American Agriculture, shows an area of western wheatgrass coextensive with a large area of Pierre clay soil. On very sandy soils, as in the sand hills of Nebraska, are associations of sand sage and sand reedgrass with bluestem grass. Farther south, as in sandy soils of Texas, a low scrubby growth of shinnery oak is associated with bunchgrasses.

Blue grama appears to be the dominant grass of medium-textured soils in the zone of Brown soils from Colorado to the Canadian line, while mixed grama and buffalograss are widely distributed in the dry parts of the Chestnut and Reddish Chestnut and the Reddish-Brown soils zones. Red Desert soils and some areas of Reddish-Brown soils in New Mexico and western Texas are dominated by black grama grass, associated with other grasses and desert shrub.

The formation of grassland soils involves the accumulation of mineral soil materials, the invasion of these materials by grass, and the accumulation of organic matter and development of soil structure.

Mineral soil materials accumulate through the direct chemical and physical weathering of rocks and through the deposition of broken and chemically weathered rock fragments (sand, silt, clay, and gravel) by streams, lakes, glaciers, wind, and down-slope gravitational movement. A very rough estimate is that one-third to one-fourth of the soil materials of the natural grasslands in the United States are the products of direct weathering of rocks of greatly varying composition and hardness. The remaining materials have been deposited by streams and the other agencies I have listed.

The larger areas of grassland soil materials, developed directly through the weathering of rock, came from soft rocks that either were weathered easily to form soil or were already soft enough in their original state to be penetrated easily by the fibrous roots of grasses. The great area of Pierre shale in South Dakota, Colorado, and Nebraska, and the still larger areas of tertiary shales, silt-stones, and soft sandstones of Montana, North Dakota, Wyoming, Colorado, and Utah are good examples.

The extensive loess (wind-blown dust) deposits of the Great Plains east of the Rockies and of the Palouse region of Oregon and Washington are ideal for the growth of grass. Loess is a uniform unstratified mixture of silt, very fine sand, and clay. It varies in thickness from a few inches to more than 100 feet in the United States. Probably loess is the most extensive single kind of parent material of grassland soils in the world as a whole.

Glacial deposits composed of clay, sand, silt, and stone fragments of many kinds are parent materials of thousands of square miles of grassland soils of the northern Great Plains of the United States and of the Great Plains of Canada. Possibly wind-laid and water-laid sands are third in importance. For example, the grassy sand hills of Nebraska are only slightly smaller in area than all of the cultivated land of Japan, and very large areas of aeolian sands occur in Kansas, Colorado, Wyoming, Oklahoma, Texas, Oregon, and Washington.

Stream alluvium and lake sediments of medium to clayey textures also are important, especially along the large river valleys and mountain fronts.

Where climatic conditions are favorable, grass invades areas of freshly deposited soil materials very soon after they are first exposed. If accumulation of sediments or weathered rock materials is slow, grasses become well established and begin immediately the work of soil building. Roots spread through the soil and sooner or later die, providing organic, humus-forming waste.

Indeed, where the accumulation is slow, soil formation can almost keep pace with weathering and deposition, and upper soil layers rapidly take on a dark color. Where accumulation of soil material is rapid, as is true of some sedimentary deposits, vegetation has relatively little effect on soil development until accumulation slows down or stops. Plants are covered before they can contribute much to the soil.

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 gultinosa; Bg, blue grama, Bouteloua gracilis; Mc, globemallow, Malvastrum coccineum; Pt, a legume, Psoralea tenuiflora; Ss, Sideranthus spinulosis; Bd, buffalograss, Buchloe dactyloides; Ap, western ragweed, Ambrosia psilostachya; and Lj, skeleton weed, Lygodesmia juncea.