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Soil Aggregation in Relation to Various Soil Constituents¹

ABSTRACT

The relative importance of diverse factors affecting aggregation in four Wisconsin soil types was studied by multiple regression analyses. Single and combined effects of pH and contents of organic matter, microbial gum, clay, and free iron oxide were considered.

In general the most important single factor was the microbial gum. However, in the Kewaunee soil which has a relatively high content of clay and iron oxide, the iron oxide was of prime importance in aggregate formation. In all the soils, iron oxide showed a marked effect on aggregation with a tendency to be more important in the smaller aggregate size range. The effect of clay, however, was very small with the exception of the high clay soil (often > 30% clay), in which its effect was only exceeded by that of the free iron oxide. In the multiple correlation analysis the effect of organic matter was considered separately from that of microbial gums. By this statistical analysis only a slight positive relationship existed between organic matter and soil aggregation, i.e., the effect of organic matter was conditioned largely by its content of microbial gum; pH 6.5 was optimum. No relationship existed between soil aggregation and the rate of decomposition of organic matter as measured by CO₂ evolution from the soil.

¹ Contribution from the Department of Soils and the Department of Bacteriology, University of Wisconsin, Madison 6, Wis. Published with the approval of the director, Wisconsin Agr. Exp. Sta. under Project Heading No. 722. This work was supported by a federal grant under the Flannagan-Hope program. Presented before Div. III, Soil Science Society of America, Nov. 15, 1956, at Cincinnati, Ohio.

² Research assistant and Professors of Soils and Bacteriology, respectively.

Received for publication November 23, 1956. Accepted for publication December 6, 1956.