EFFECT OF ROOT SYSTEMS ON PREFERENTIAL FLOW IN SWELLING SOIL

Permeability problems on irrigated soils may be alleviated by root sys tems that increase water flow by creating macropores. Infiltration rat es have been shown to increase where plant roots decay and serve as pr eferential flow paths. For low-organic-matter swelling soil, there is a question whether macropores are able to resist the lateral swelling forces of the soil. The objective of this study was to observe prefere ntial water flow paths in a swelling soil under two cropping systems. A Holtville silty clay (clayey-over-loamy, montmorillonitic Typic Torr ifluvent) was observed in situ. Two crops, alfalfa (Medicago sativa, L .) and wheat (Triticum turgidum, L.) provided sharply contrasting root systems, with wheat possessing fine, fibrous roots; alfalfa on the ot her hand, has a taproot system. Macropores were observed after applyin g soil-adsorbing methylene blue dye to irrigation water. Shrinkage cra cks failed to conduct dye after 10 minutes into a flood irrigation. Ea rthworm (Lubricus terrestris) channels were also not stable. However, decaying roots of alfalfa produced stable macropores, while wheat prod uced no such macropores. The influence of alfalfa-root-induced macropo res was demonstrated by the increase in final infiltration rate during alfalfa cropping which agreed with Meek et al.'s (1989, 1990) finding s on sandy loam soils.