Aggregate-mean diameter and wind-erodible soil predictions using dry aggregate-size distributions

Temporal estimates of surface soil dry aggregate-size distributions (DASD) are needed to evaluate soil management systems and estimate wind erosion. This study was conducted to determine the most accurate and precise DASD to estimate aggregate-mean diameter and amount of wind-erodible soil. Over 5400 surface samples of soil dry aggregates were collected at various times throughout the year for 2 to 4 yr at 24 locations in six states. The soils represented a wide range of management systems and intrinsic soil properties, including mineral and organic soils. We evaluated four DASDs: the log-normal determined by two methods, fractal and Weibull distributions, and compared estimates of the aggregate-mean diameter and amount of wind-erodible soil derived from the distributions. We evaluated lognormal distributions expressed as amount oversize (LNO) and undersize (LNU) and tested the effect of using different smallest and largest sieve openings sizes. The Weibull distribution is the most accurate because the ranges of error of the Weibull were generally smaller than all other distributions over the full range of sieve sizes tested, rarely exceeding 0.15. The Weibull distribution is the most precise because only the Weibull had an error mode of +/-0.05 in all sieve-- size classes tested. Using substantially different sizes for the smallest sieve-size openings had a great effect on estimates of aggregate-mean diameter and wind-erodible soil, but only when using LNO. Using substantially different sizes for the largest sieve size openings had a great effect on estimates of aggregate-mean diameter but little effect on estimates of wind-erodible soil.