A Study of the Shrinking and Swelling Properties of Rendzina Soils

E most striking physical characteristics of the 1 Rendzina soils developed in the black-land area of Texas are extreme stickiness and unusual swelling and shrinking which accompany wetting and drying. These physical properties are responsible for much ' of the difficulty encountered in managing these soils, particularly in regard to' erosion control, moisture conservation, and tillage operations. The "dry weather" cracks that develop in these soils during prolonged dry periods, illustrated in Fig. i, constitute a major factor in water infiltration. Slow, gentle rainfall, which wets the surface soil slowly, causes these cracks to become sealed at the top while remaining open at lower depths; rainfall of high intensity fills the cracks by gravitational movement of water, with the result that swelling of the soils shuts the entire crack. There are two general methods of measuring soil loss by erosion. The first is to measure the soil washed off by individual storms and the second is to determine changes in surface elevation. Woodruff (7) found that plates set at different depths in a Shelby loam profile rose and settled as the soil moisture content rose and fell. Hill, et al. (3) found'that bench marks set at depths of 3, 5, 8, 10, 15, and 20 feet in Houston black clay, settled during dry weather and rose during wet weather. During one year the 3-foot bench mark moved approximately J4 mcn P d M inch down — a total movement of I ̂ 2 inches. The rising and falling of the surface soil make it necessary that soil-movement lines be run at times when comparable soil moisture content and state of tilth exist; otherwise, the data obtained will be unreliable. Because this requirement is seldom met under field conditions this method cannot be successfully used for measuring sheet erosion, but it is a satisfactory method for measuring gully erosion when cross-slope profiles are run. Tempany (6) has shown that cubical contraction •of soil is numerically equal to volume loss of water during the earlier stages of shrinkage. During the later stages of drying a marked decline of shrinkage occurred. Hardy (2) pointed out that differences in shrinkage coefficient between soils of similar colloid content were probably due to the kind of soil colloids. His data show that soils in plastic bricks cease to shrink when the moisture content has been reduced to a point which approaches the hygroscopic coefficient. Haines ( i ) collected data for puddled prisms of soil that agree with Tempany's hypothesis that cubical contraction is numerically equal to volume loss of water during the early stages of shrinkage. His data also showed that as dehydration progressed to near dryness a distinct break in the curve developed. This break signifies the point at which air enters the soil. Haines termed the shrinkage from this break to dryness "residual shrinkage", and the shrinkage from maximum plasticity to the break in the curve "normal shrinkage". Lauritzen and Stewart (5) did some work on the shrinkage and swelling properties of natural soil clods from a Houston black clay profile. They found that shrinkage per unit loss of weight was greatest when the moisture content was near the wilting point. -The purpose of the work reported herein was to obtain some laboratory measurements of the shrinking and swelling properties of natural soil clods from Houston black clay and Austin clay profiles, and to relate these properties to the development of dryweather cracks and to desirable tilth of these soils.