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A Comparison of Five Methods for Expressing Aggregation Data¹

ABSTRACT

Five ways of expressing aggregation data have been in common use in recent years. These are: percent aggregates greater than 2 mm, 1 mm and 0.25 mm, and geometric mean and mean weight-diameter. To determine the advantages or disadvantages of the several indices, aggregation results obtained by the Yoder method on several hundred soil samples from three locations in Iowa have been compared. Correlation coefficients were obtained between the mean weight-diameter and the percent of aggregates > 2 mm, > 1 mm, and > 0.25 mm, and also the geometric mean. The geometric mean was also correlated with the three size separates. The results indicate that a single size fraction such as the > 2 mm or > 1 mm can be satisfactorily used to express soil aggregation. This method of expression would be as reliable (to within the 1% level of significance) as that indicated by the two indices, mean weight-diameter and geometric mean, which utilize all size fractions. However, it was shown that more replications are necessary for equal accuracy when only a single size fraction is used. If time is to be saved in making the laboratory determinations, it is suggested that a size fraction such as the > 2 mm or > 1 mm might be determined more quickly and with greater accuracy by developing a method using one or two sieves and a larger soil sample. The final selection of an index for expressing soil aggregation must be based on the ability of the index to correlate with crop response.

¹ Journal Paper No. 2242 of the Iowa Agr. Exp. Sta., Ames, Iowa, Project 739. Joint contribution of the Agronomy Dept., Iowa State College, and the Division of Research, S.C.S., U. S. Dept. of Agr. Presented before Division I Soil Science Society of America, Cincinnati, Ohio, Nov. 20, 1952.

² Soil conservationist and soil scientist, respectively.

Received for publication February 11, 1953.