Using Two Sieves to Characterize Dry Soil Aggregate Size Distribution

SURFACE soil aggregate size distribution affects many facets of agriculture from wind erosion susceptibility to seedbed suitability. The log-normal distribution generally provides a good description of aggregated soil size distribution. Unfortunately, other measures of aggregation have often been adopted, because they were perceived as easier to apply. In this study, a method to calculate the geometric mean diameter, Dg, and geometric standard deviation from two sieve cuts was developed for log-normal distributions. Results from 10 soil samples using the two-sieve procedure were compared to results from the same samples using multiple seive cuts. The multiple sieve data were analyzed using both a traditional graphical and a linearized least-squares procedure to predict Dg and percentage aggregates greater than 0.84 mm. All the methods gave nearly equal size distribution parameters. The two-sieve procedure is least laborious but does not permit easy detection of samples that deviate from a log-normal distribution.