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Soil Capillary Conductivity: Comparison of Measured and Calculated Values¹

ABSTRACT

Capillary conductivities were measured for two loess soils and two glacial till soils at four depths and at five values of water suction, up to and including 100 cm. of suction. For the same soils, curves of moisture content vs. suction were obtained and, from the latter data, capillary conductivities were calculated by the Childs and Collis-George method and by the Marshall method. The values obtained by the Marshall method were all considerably higher than the measured values. Those obtained by the former method—a method which utilizes a matching constant to fit the experimental data at one point—agreed satisfactorily for the loess soils, but not for the glacial soils. Suctions as small as 25 cm. of water reduced capillary conductivities by as much as 10^-4 for the glacial soils. The reduction was not as marked for the loess soils. The theories tested had been developed for nonshrinking materials such as sieved sand and slate dust. The present results show that the use of one of the theories may be extended to agricultural soils, and that the other theory could also be so extended, if a matching constant were used.

¹ Journal Paper No. J-3713 of the Iowa Agr. and Home Economics Exp. Sta., Ames, Project 998. Contribution from the Department of Agronomy (Soils).

² Research Associate, and Professor of Agronomy (Soils), Iowa State University, Ames; and Laboratory Technician IV, University of California, Davis, respectively. The senior author is now Assistant Professor of Irrigation, University of California, Davis.

Received for publication August 26, 1959. Accepted for publication November 13, 1959.