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Properties of Soil Aggregates: I. Relation to Size, Water Stability and Mechanical Composition¹

ABSTRACT

Material from three different soils in Arkansas was sieved through a series of standard screens and material collected from each of the sized aggregates separated. This was collected in two forms: (a) by dry screening, and (b) by wet screening in water. Mechanical analysis, exchange capacity and organic matter were determined for the water-stable aggregates for each of the sizes separated and related to the same properties of the whole soil.

Results indicate that the properties of the individual waterstable aggregates are somewhat different from those of the whole soil. The smaller aggregates of 0.15 –0.42 mm in diameter tend to contain a larger content of clay and organic matter than do the large aggregates or the whole soil. Also the aggregates larger than 0.42 mm possess lower exchange capacities than does the whole soil. A shielding of the effect of clay inside the aggregates is indicated by the lack of uniform ratio of clay to cation exchange capacity. This ratio is generally larger in the aggregates than in the whole soil.

A theory indicating the formation of aggregates from small secondary particles by cementation with clay and organic matter to account for the differences observed is proposed.

¹ Research paper No. 1097, Journal Series, University of Arkansas, Fayetteville, Ark. Published with the permission of the Director of the Ark. Agr. Exp. Sta. Presented before Division 1, Soil Science Society of America, Nov. 20, 1952, Cincinnati, Ohio. Received for publication Dec. 8, 1952. Financial support through a grant by the Mathieson Chemical Corp. is hereby acknowledged.

² Assistant Professor of Agronomy.