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Factors Influencing Swelling and Shrinking in Soils¹

ABSTRACT

The mechanism of the swelling process and its influence on soil structure and plant growth is critically evaluated. The tendency of a soil to swell in relation to mineralogical composition, adsorbed organic compounds, exchangeable cations, and iron content is discussed. A description is given of the apparatus developed to measure the swelling pressure of extracted soil clays. A comparison is made of the swelling of extracted soil clays and shrinkage curves obtained from clods taken from four Texas soils in natural structural condition.

It has been assumed generally that a soil which swells is predominantly montmorillonitic, but factors such as organic materials, exchangeable cations, and iron content in addition to its mineralogical composition are important in modifying the tendency of a soil to exhibit swelling characteristics. Removal of iron increased the swelling tendency of extracted clay from some of the soils. Removal of organic matter from the clay increased its swelling capacity, while adsorption of soil conditioners on one of the clays tended to increase its swelling capacity.

¹ Contribution of the Texas Agr. Exp. Sta., College Station, Tex. The work reported in this paper is in partial fulfillment of the requirements for the Ph.D. degree of the Agricultural and Mechanical College of Texas, College Station, Tex. Presented before Div. I. Soil Science Society of America, Davis, Calif., Aug. 16, 1955.

² Graduate Assistant, Department of Agronomy and Texas Agr. Exp. Sta., and Professor of Agronomy and Head of Department, A. and M. College of Texas, respectively.

Received for publication August 16, 1955.