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Influence of Soil Bulk Density on Matric Potential¹

ABSTRACT

Unsaturated soil samples were compressed at three different moisture contents and temperatures to evaluate these interrelations with soil moisture matric potential. It was found that in all cases increasing bulk density, at constant moisture content and temperature, resulted in an increase in soil moisture matric potential. The moisture contents were such that all suctions were less than 1 bar. Temperatures of 10°, 25°, and 40°C. were studied. Compression was performed such that only mechanical pressure was applied to the soil; thus, air pressure within the soil sample remained constant and equal to that of the free atmosphere at all times. Mechanical pressures up to 27 bars were studied. Matric potential was measured with a null-point tensiometer which rendered water flux between the soil and tensiometer systems negligible.

An expression is proposed for the chemical potential of soil water which includes the influence of pressure, temperature, moisture content, bulk density, and composition, which includes implicitly all other variables. The term matric potential has been used to describe the residual chemical potential when pressure, temperature, and solute content were constant.

It is felt that the proposed expression is more adequate for describing soil moisture matric potential than previous expressions. This expression provides a way for describing, at least in part, hysteresis of soil water that is associated with shrinking and swelling.

¹ Research work reported here was done in cooperation with the 12 Western States and the ARS, USDA under regional research Project W-68. The work was supported in part by the US Signal Corps under Contract No. DA-36-039-SC-80263. Published with the approval of the Director of the Utah Agr. Exp. Sta., Logan, as Journal Paper 170.

² Formerly graduate assistant, Utah State University, now Soil Scientist, USDA, U. S. Big Spring Field Station, Big Spring, Tex., and Professor of Agronomy, Utah State University, respectively.

Received for publication February 6, 1961. Accepted for publication December 28, 1961.