Factors Which Affect the Development of Prismatic Structure in Soils of the Southern Great Plains

Structural development in soil is more easily observed in subhumid regions than in areas which receive more abundant rainfall because the expansion and contraction of clay and organic matter develop different structural patterns which are poorly defined in soils containing a good supply of capillary moisture. Some of these structural characters are associated with surface soil and others appear only in the subsurface horizons. In the Southern Great Plains, soil profiles are frequently dry to a depth of several feet. Under natural conditions grass roots remove subsoil moisture during periods of drought and lateral and vertical shrinkage occurs in the soil mass unless the clay content is very low. As a result of this condition structural characteristics may be observed where soil profiles are exposed in cuts along highways and in ditches which have been formed by gully erosion; and in areas where wind erosion is severe subsurface structures can be readily observed where the surface soil has been removed from cultivated land. The surface layer of a normal soil contains more pore space and is usually more friable than deeper horizons, consequently, forces which cause expansion and contraction combined with chemical effects produced by soluble organic matter develop small aggregates. Single grain, granular, and loamy structure are terms applied to the surface soil by Lyon and Buckman (3), and conditions which are favorable for the development of these structural characters are described. In the Southern Great Plains the organic matter content of the average soil is relatively low, and granulation which is commonly observed in Chernozem-like soil is absent. Subsurface soils are also low in organic matter and the deeper layers increase in density with a corresponding decrease in total pore space. Longwell (2) observed several types of desert mud cracks and suggested that rapid drying produces wide-spaced cracks, whereas slow desiccation produces closely spaced, cracks and that texture of the sediment is an important factor in determining the shapes which are produced. Fine .sand with some clay and silt will crack into polygons. The presence of salt tends to interfere with the formation and preservation of definite units. Twenhofel (6) suggests that the crossing of the cracks defines the shape of the polygons and that cracks which develop as a result of freezing at minus ten degrees Centigrade are approximately sixteen feet apart. Prismatic structure, although it'is commonly observed as a subsurface development in the soil profile, frequently extends from the subsurface layers to the surface of the ground. In most cases the prisms are stabilized by the accumulation of surface soil which is less plastic in the vertical cracks occurring in the soil profile; consequently, the soil masses always separate in the same place as a result of desiccation, although good prisms cannot be easily removed from the soil when it is wet. When prisms extend from the subsoil to the surface of the ground it has been observed that natural or accelerated erosion has removed the surface soil more rapidly than soil-forming processes could modify the structure in the top of the prismatic horizon. In order to determine what soil characters were associated with prismatic structure, composite samples of so.il were collected from twenty-three different soil profiles in Oklahoma, Kansas, and Texas. Each profile was divided into five parts. Where a distinct surface layer was present, samples were taken from this horizon. In all cases the prisms were divided into three parts, and composite samples were collected from the upper one-third, the center, and the lower onethird of several prisms occurring in each profile. A fifth sample was obtained from the subsoil occurring below the prisms. In most cases the soil profiles were dry when the samples were obtained. In a few instances subsoil samples were collected from moist material. The length of the prisms which were studied varied from eight to twenty-four inches. The average diameter varied from four to seven inches. In several instances the vertical cracks which separate the prisms extended from the surface of the ground