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Moisture, Pressure, and Formation of Water-Stable Soil Aggregates¹

ABSTRACT

Moisture and pressure treatments were applied to soil samples consisting of particles of 0 to 0.5 mm. diameter obtained by grinding 2 to 8 mm. aggregates of a Webster silty clay loam collected at the field moisture capacity and then air dried. The application of the moisture and pressure treatments formed the soil material into cylindrical briquettes and these, while moist, were broken into aggregates; these aggregates were then air-dried and tested for water stability and finally the water-stabilities resulting from the moisture-pressure treatments were compared with the water-stability of 2 to 8 mm. diameter aggregates of untreated soil material. It was found that, although pressure and moisture applied to ground soil material did increase water stability, the water-stability which resulted from the pressure (maximum 1000 psi.) and moisture (maximum 30.5% referred to oven-dry weight) was much less than was the water-stability found in soil material not ground and not subjected to the pressure and moisture treatments. An important conclusion is that, since pressures as high as 1000 psi., which are in excess of pressures exerted by farm machinery, plant roots and soil fauna, failed to produce water-stability comparable to that of untreated soil material, other forces are more responsible for water-stable aggregate formation. These other forces may be due to chemical, biological or other natural factors. Concurrently with the work on 0 to 0.5-mm. soil, identical moisture and pressure treatments were also applied to 2- to 8-mm. diameter aggregates of this same Webster silty clay loam soil material. It was found that the moisture and pressure treatments resulted in decreased water-stability of these treated 2- to 8-mm. aggregates in comparison with the untreated 2- to 8-mm. aggregates.

¹ Journal Paper No. J-4152 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Iowa, Project 1235. Contribution from the Department of Agronomy (Soils). This investigation was supported in part by Consolidated Hatch Regional Research Funds, as a contributing project to Regional Project NC-17. Presented before Div. I. Soil Science Society of America at Chicago, Ill., Dec. 6, 1960.

² Research Associate and Professor, respectively.

Received for publication July 10, 1961. Accepted for publication January 10, 1962.