Organic Polymers' Effect on Soil Shear Strength and Detachment by Single Raindrops

Organic matter is known to influence soil credibility, but little work has been done to determine the effects of specific organics on erosional processes. Our study was designed to assess the influence of organic polymers known to increase aggregate stability on nearsurface soil shear strength and soil detachment by single raindrops during time. An index of shear strength was obtained with a Swedish fall-cone device and detachment was based on soil loss in the splash of a single raindrop. Two soils with similar texture but differing organic-matter contents were used. Amendments studied were polyvinyl alcohol (PVA), an experimental starch graft polymer (SCP), and a mixture of a microbial polysaccharide rich in levans and other cellular material (LEV). The amendments initially induced significant increases in soil shear strength ranging from 1.5 to S.5 times greater than untreated soil. The pattern of change of shear strength during time was nearly identical for all treatments, and was attributed to an aging process. Significant changes induced by the amendments in the amount of soil detached varied with time. The patterns of change during time seen in the PVA-amended soils were similar to the controls, while the LEV and SCP response curves differed. In conjunction with decomposition-rate data, it was concluded that the transformation products formed during the decomposition of the LEV and SCP were causing temporal changes in the soil that the Swedish fall-cone device was insensitive to, but influenced the response of the soil to raindrop detachment. T EXTENT to which a soil erodes depends on the ability of soil structural units to withstand the forces of raindrop impact and surface flow (Meyer, 1981). Soil properties are known to affect a soil's susceptibility to erosion (Wischmeier and Mannering, 1969). Still, no single property has correlated closely to soil loss due to erosion, which caused Wischmeier and Mannering (1969) to use a multiple regression analysis technique to relate several soil properties to erosional losses. Properties contributing significantly to soil-loss variance are soil texture, organic-matter content, measurements of soil structure including aggregate stability, parent material, and residual effects of crops P.V. Barry and R.F. Turco, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907; and D.E. Stott and J.M. Bradford, USDA-ARS National Soil Erosion Research Lab. , Purdue Univ., West Lafayette, IN 47907. Joint contribution of the National Soil Erosion Research Lab. , USDA-ARS and the Indiana Agric. Exp. Stn. Purdue Journal no. 117070. Received 12 Jan. 1990. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 55:799-804 (1991). and management practices. Current erosion models are separating the erosion process into two components, rill and interrill erosion (Foster, 1987). For intertill credibility, the process of detachment due to raindrop impact predominates (Young and Wiersma, 1973). Soil properties shown to specifically affect soil detachment include organic-matter content (Young and Onstad, 1978), aggregate stability (Young and Onstad, 1978; Luk, 1979; Meyer and Harmon, 1984), and infiltration rates (Poesen and Savat, 1981). Soil shear strength has been used to predict soil erodibility or other soil-related processes (Foster, 1987). A field study by Watson and Laflen (1986) shows a close relation between interrill erosion and soil shear strength after rainfall. Current efforts to develop erosion-prediction models that are more process based than the older predictive technologies include measurements of soil-strength indices (Foster, 1987; Elliot et al., 1989). Cruse and Larson (1977) showed that soil shear strength measured by triaxial compression is related to soil detachment by single-raindrop impact. Al-Durrah and Bradford (1981) related shear strength as measured by the Swedish fall-cone device to detachment by raindrop from a soil in an uncrusted condition. Bradford et al. (1986) showed that, if a raindrop's kinetic energy is known, splash from a single raindrop on finer textured soils can be accurately predicted from shear strength derived from fall-cone measurements without having to correct for a soil's friction angle. None of these studies included time or aerobic incubation as factors. Many studies have been done on the relation between organic matter and soil structure. Organic matter stabilizes soil structure by binding particles into durable aggregates, forming an organo-mineral complex (Martin, 1971; Hayes, 1980; Tisdall and Oades, 1982). The resistance of an organo-mineral complex to microbial degradation determines the complex's duration and effectiveness in stabilizing soil particles (Martin, 1971; Olness and Clapp, 1972). Portions of the soil organic matter are in chemical forms that are resistant to microbial decomposition and remain in soil for many years; however, other components, such Abbreviations: LEV, bacterial polysaccharide rich in levans; PVA, polyvinyl alcohol; SGP, starch graft polymer; T, near-surface shear strength; 8, soil detachment due to single raindrops; LSI, least significant interval. 800 SOIL SCI. SOC. AM. J., VOL. 55, MAY-JUNE 1991 as microbial and root exopolysaccharides, may last only days to months in soil (Martin, 1971; Tisdall and Oades, 1982; Stott and Martin, 1990). Polysaccharides that are long, flexible, and have charged side groups are most effective in binding soil particles together (Martin, 1971; Hayes, 1980). While most of the work on relating soil erodibility to soil properties has dealt with total organic matter, little work has been done on the relation between specific types of organics present in the soil and changes in erodibility. Cruse and Larson (1977) added PVA to soil, then measured changes in soil strength and detachment due to raindrop impact. The PVA increased soil shear strength and decreased soil detachment with increasing concentrations, but the amendments were not allowed to undergo microbial transformation within the soil before testing. The first objective of this study was to determine if different types of polysaccharides added to soil would influence soil detachment by single raindrops and near-surface soil shear strength in a similar manner. We used three organic polymers known to increase aggregate stability, PVA, SGP, and LEV. Polyvinyl alcohol, a soil polysaccharide analog, has been, and still is, used as an amendment to improve soil structure. Starch graft polymer, also a soil polysaccharide analog, is being developed as a soil stabilizer (Weaver and Fanta, 1987; Weaver, 1988). The SGP is synthesized by grafting charged side chains of polymethyl acrylate to a cationic corn starch backbone (Weaver and Fanta, 1987). The LEV, a polysaccharide produced by a common soil microorganism, is probably produced within the soil environment and could be expected to undergo profound transformations before becoming part of the natural soil humus (Stott and Martin, 1990). A second objective was to determine if the initial changes seen would be sustained during time within a given soil type when allowed to incubate under aerobic conditions. Another objective was to determine if the linear relationship between soil detachment by single raindrop and soil shear strength as measured by the fall-cone device shown by Al-Durrah and Bradford (1981) would be maintained during time in the presence of decomposing organic amendments. MATERIALS AND METHODS