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Bulk Density, Saturation Water Content and Rate of Wetting of Soil Aggregates¹

ABSTRACT

Bulk density, degree of saturation, volume increase on wetting, and rate of wetting were measured for aggregates and clods of Ormstown silty clay loam (Humaquept) of sizes from 5 cm to 0.036 cm diameter.

Bulk density increased with a decrease in diameter. For 2.2-mm to 0.36-mm diameter aggregates this increase was due to a surface area effect, i.e., loss in porosity as a result of subdividing larger aggregates. Both surface area effects and increased porosity from root channels and fissures were required to explain the increase for clods between 5 and 1 cm diameter.

An evaporation and a sorption technique are described and used to estimate the water content of aggregates at saturation, i.e., the intra-aggregate water content. The evaporation technique gave reliable results at specific rates of evaporation, but the sorption technique was not as useful. Clods and aggregates of the soil and of pumice samples did not fully saturate; entrapped air accounted for 25 to 27% of the total pore volume.

Aggregates of 1 and 2.2 mm diameter and clods of 1.0 and 5.0 cm diameter took about 1, 1.7, 45, and 1,000 seconds, respectively, to saturate. This rapid wetting would minimize differences in potential between inter-and intra-aggregate water during infiltration.

¹ Contribution from the Dep. of Soil Science, Macdonald College of McGill University, Ste. Anne de Bellevue, Quebec. Part of the work submitted as a Ph.D. thesis by the senior author. This study was supported by a Grant-in-Aid of Research from the National Research Council, Canada.

² Graduate Research Student and Professor, Dep. of Soil Science. The senior author is now lecturer in Soil Science, University of the West Indies, Trinidad.

Received for publication December 6, 1974. Accepted for publication September 9, 1975.