ORGANIC POLYMERS EFFECT ON SOIL SHEAR-STRENGTH AND DETACHMENT BY SINGLE RAINDROPS

Organic matter is known to influence soil erodibility, 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 near-surface 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 (SGP), 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 5.5 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 SGP response curves differed. In conjunction with decomposition-rate data, it was concluded that the transformation products formed during the decomposition of the LEV and SGP 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.