Dry‐Soil Aggregate Stability as Influenced by Selected Soil Properties

Dry aggregate stability is an important soil physical property for evaluating tillage and wind-erosion research. Research in this study was aimed at developing a model for predicting soil aggregate stability as influenced by intrinsic soil properties, with particular application to wind erosion. Aggregates from 10 Kansas soils with a wide textural range were tested periodically for stability during a 3-yr period. The logarithm of aggregate stability was regressed against the intrinsic soil properties (geometric mean diameter of primary particles, specific surface area, water content at -1500 J/kg matric potential, and clay content). Clay content and water content at —1500 J/kg were both good predictors of mean aggregate stability. A resulting empirical model estimates mean aggregate stability from either clay or water content with coefficients of determination of 0.97 and 0.96, respectively. Further testing is planned by running the model with several independent data sets to estimate the probability of aggregate stability within specified limits for particular soils. W RESEARCH SCIENTISTS have been charged with developing improved wind-erosion prediction technology as a possible replacement for the wind-erosion equation (Hagen, 1991a). Improved wind-erosion prediction technology requires that we improve our ability to measure and predict, in time and space, the susceptibility of soil to wind erosion. The main properties of dry soil aggregates affecting E.L. Skidmore, USDA-ARS and Kansas State Univ., and J.B. Layton, Agronomy Dep., Kansas State Univ., Manhattan, KS 66506-4006. Joint contribution of the USDA-ARS and Kansas Agric. Exp. Stn. Contribution no. 91-201-J. Received 8 Feb. 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:557-561 (1992). their susceptibility to wind erosion are stability and size distribution. Soil aggregate stability and size distribution vary widely in time and space. Aggregate density affects soil credibility to a lesser extent but is much less variable than stability and size distribution. Chepil (1950), using a wind tunnel, determined relative credibility of soil as a function of the proportions of dry soil aggregates in various sizes. Chepil (1960) later converted relative credibility to actual field soil loss for specified conditions, which was the basis for soil -credibility factor of the wind-erosion equation (Woodruff and Siddoway, 1965). Their data show nonaggregated sandy soils with only 1% of the sand having diameters >0.85 mm are 100 times more erodible than aggregated soils with 77% of their aggregates >0.85 mm (Woodruff and Siddoway, 1965). Similarly, dry AS may differ a hundredfold between soils. Aggregates with low stability fracture easily and break into small sizes. Hagen (1991b) found that AS was the dominant predictor of soil erosion from surface abrasion. He concluded that major improvements in predicting the abrasion coefficient of the abrasive flux equation can come only from improved predictions of the stability of aggregates and crusts. The aggregate status of a soil at any instant in time is the result of many aggregate-forming and -degrading processes. Those processes comprise a complex interrelationship of physical, chemical, and biological Abbreviations: AS, aggregate stability; SD, standard deviation; CDN, calendar-day number; CDMAX, calendar-day number when aggregate stability is at maximum; GMD, geometric mean diameter; GSD, geometric standard deviation; CV, coefficient of variation; PWP, water content at 1500 J/kg. 558 SOIL SCI. SOC. AM. J., VOL. 56, MARCH-APRIL 1992 reactions. Aggregates may form by the breakdown of consolidated soil mass into smaller sizes or by the coalescing of finer materials. Chepil (1955) observed a definite relationship between amounts of erosion in a wind tunnel and the percentage of clay contained in the soil. Soils containing 20 to 35% clay were least credible. Coarsetextured soils apparently lack sufficient amounts of silt and clay to bind individual sand particles together, or they may form weakly cemented clods that are readily broken down and eroded. On medium-textured soils, the proportions of silt and clay appear to be sufficient to bind the sand grains together, yet not so high as to cause excessive cleavage by weathering and consequent granulation. This research was aimed at developing a model for predicting soil AS as influenced by intrinsic soil properties, with particular application to wind erosion. 1965). In our application of the error function, Z, = (InAS, mAS)/(V2 SD) [4] where InAS, is the natural logarithm of AS corresponding to a probability, P,, for a soil with a mean log stability of InAS and SD. Because Eq. [3] is for only one-half of the area integral, it follows that P, = 0.5 + erf(Z,)/2 [5] where P, is the cumulative probability that AS will not exceed the value specified in Eq. [4] for the soil with the given mean and SD. Combining Eq. [4] and [5] gives P( = 0.5 + erf[(lnAS, mAS)/(V2 SD)]/2. [6]