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Simplified Accounting for Membrane Impedance in Capillary Conductivity Determinations¹

ABSTRACT

Miller and Elrick's method of computing capillary conductivity for nonnegligible membrane impedance has been modified to reduce the amount of calculation required for converting pressure plate outflow data to capillary conductivities. Plate impedance need not be measured and variable contact impedance is accounted for. The method is most practical if accurate time and outflow measurements can be made shortly after making a small step change in pressure. An apparatus is described to make such measurements. To minimize diffusivity changes associated with moisture outflow, only the initial portion of the outflow data resulting from the step change in pressure is used. The relationship between capillary conductivity and moisture content is shown for two soils.

¹ Journal Paper No. J-4214 of the Iowa Agr. and Home Econ. Exp. Sta., Ames, Project No. 998. Contribution from the Department of Agronomy. The authors gratefully acknowledge the financial assistance of the National Science Foundation in supporting this work.

² Assistant Professor of Soils and Professor of Soils and Physics, respectively.

Received for publication November 2, 1961. Accepted for publication February 13, 1962.