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Porosity of Surface Soil Aggregates at Various Moisture Contents¹

ABSTRACT

Porosity of individual surface soil aggregates at moisture contents ranging from saturation to air-dry was determined by glass bead displacement. Measurements were made on aggregates having a diameter range from 0.5 to 10 mm at air-dry moisture content and 2.0 to 10 mm at suctions 15 bars. The coefficient of variation for a determination of aggregate density (g cm^-3) ranged from 1 to 11% depending on diameter and moisture content. The major contributor to precision loss was heterogeneity of soil aggregates and not precision errors of the method.

More than a 50% decrease in specific total pore volume (cm³ g^-1 of oven-dry aggregate) upon desorption from saturation to air-dry was found for three aggregate sizes each from a Chernozem and a Chestnut soil. In both soils, the percentage of pores having diameters > 29 µ increased as aggregate diameter increased. The volume fraction of air at a given moisture content increased as the diameter of aggregate increased. For all aggregate sizes, the volume fraction of air was 0.10 at 0.33 bar suction. The two soils exhibited different types of shrinkage curves.

¹ Contribution from the Corn Belt Branch, Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Minnesota Agr. Exp. Sta. Presented at a joint meeting of Div. S-1 and S-6, Soil Science Society of America, Kansas City, Mo., Nov. 15–19, 1964.

² Soil Scientists, Soil and Water Conservation Research Division, ARS, USDA, Morris, Minn. and Ames, Iowa.

³ The authors express their appreciation to Russell Rosenau Physical Science Technician, and to Elton A. Hallauer, Digital Computer Programmer, for their assistance in the collection and calculation of laboratory data.

Received for publication July 9, 1965. Accepted for publication October 20, 1965.