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The Influence of Aggregate Size on Soil Moisture Content-Capillary Conductivity Relations¹

ABSTRACT

The capillary conductivity of beds of 0.5- to 1-, 1- to 2-, 2- to 3-, 3- to 5-, and 5- to 9-mm aggregates separated from Nicollet, Kranzburg, Aastad, and Miami soils was determined over a suction range of from 0.02 to 12 bars by using the pressure plate outflow technique. When moisture content-moisture suction relations were essentially unaffected by aggregate size, capillary conductivity was a function of volumetric moisture content. However, if moisture content-moisture suction relations were affected by aggregate size, then at any given moisture content, conductivity values for a given aggregate size were inversely related to the suction corresponding to said moisture content. For beds consisting of mixtures of aggregate sizes, changes in moisture content-moisture suction brought about by mixing, affected capillary conductivity in much the same way as in the case of beds of single-sized aggregates. Data from sand and uniformly porous ceramic "aggregates" indicate that capillary conductivity is a function of size only if size affects moisture content-moisture suction relations. Other factors influencing moisture retention may be expected to influence capillary conductivity accordingly.

¹ Contribution from the Cornbelt Branch, Soil and Water Conservation Research Division, ARS, USDA, and the Iowa Agr. and Home Econ. Exp. Sta. cooperating. Journal Paper no J-5002 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Project no. 1486. Presented before joint session of Div. S-1 and S-6, Soil Science Society of America, Kansas City, Mo., Nov. 16, 1964.

² Research Soil Scientist, ARS, USDA, and Associate Professor of Agronomy, Iowa State University, Ames, Iowa.

Received for publication February 10, 1965. Accepted for publication August 30, 1965.