|Zobeck, Teddy - Ted|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2002
Publication Date: 3/1/2003
Citation: Zobeck, T.M., Popham, T.W., Skidmore, E.L., Merrill, S.D., Lindstrom, M.J., Mokma, D.L., Yoder, R.E., Lamb, J.A. 2003. Aggregate mean diameter and wind erodible soil predictions using dry aggregate-size distributions. Soil Sci. Soc. Am J.67:425-436. Interpretive Summary: Soils form naturally-occurring groups of particles called aggregates during soil formation. Aggregates are broken up during tillage and other soil manipulations into pieces often called clods. Water infiltration, soil-seed contact in the seedbed, wind and water erosion, and many other soil conditions are significantly affected by the size and arrangement of soil aggregates. Accurate information on the sizes of soil aggregates is needed to relate soil management to the resulting soil behavior. The sizes of aggregates are predicted using mathematical equations. In the past, several different methods have been used to develop these equations to describe aggregates. This study evaluates four methods of describing the sizes of soil aggregates and clods in dry, tilled surface soils. Over 5,000 surface samples of soil dry aggregates were collected for two to four years at 24 locations in six states. The soils represented a wide range of management systems and kinds of soils, including mineral and organic soils. Surface soil samples were sieved through a series of sieves to determine the amount of aggregates of various sizes. The four methods were used to describe the relationship of amount of aggregates versus size of aggregates. The mean aggregate diameter and the amount of wind erodible soil predicted by each method were compared. One method, called the Weibull distribution, was shown to be the most accurate and least variable of the four methods tested. In addition, this study demonstrated the importance of using a standard number of sieves with similar size openings.
Technical Abstract: Temporal estimates of surface soil dry aggregate size distributions (DASD) are needed to evaluate soil management systems and estimate wind erosion. Over 5,000 surface samples of soil dry aggregates were collected for 2 to 4 years at 24 locations in 6 states. The soils represented a wide range of management systems and intrinsic soil properties, including mineral and organic soils. We evaluated three DASDs: the log-normal (LN), fractal (F), and Weibull (W) distributions, and compared estimates of the aggregate mean diameter (MD) and amount of wind erodible soil (WES) derived from the distributions. We evaluated LN expressed as amount oversize (LNO) and undersize (LNU), and tested the effect of using different smallest and largest sieve openings sizes. For comparisons of MD, the W and LNU distributions generally produced about the same estimates. The LNO distribution tended to overestimate MD. Estimates of MD made using F were similar to W and LNU, but sometimes lower. Estimates of WES were lowest using LNU. In about half the sites tested, estimates of WES made using W were the same as those made using LNO and estimates made using F were the same as those using LNU. The Weibull distribution was the most accurate and precise DASD distribution of those tested. The F distribution had the lowest accuracy and the LNO distribution had the lowest precision of those tested. Using substantially different sizes for the smallest sieve size openings had a great effect on estimates of MD and WES, but only when using LNO. Using substantially different sizes for the largest sieve size openings had a great effect only on estimates of MD for all DASD tested.