Permeability problems on irrigated soils may be alleviated by root systems that increase water flow by creating macropores. Infiltration rates have been shown to increase where plant roots decay and serve as preferential 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 preferential water flow paths in a swelling soil under two cropping systems. A Holtville silty clay (clayey-over-loamy, montmorillonitic Typic Torrifluvent) was observed in situ. Two crops, alfalfa (Medicago saliva, L.) and wheat (Triticum turgidum, L.) provided sharply contrasting root systems, with wheat possessing fine, fibrous roots; alfalfa on the other hand, has a taproot system. Macropores were observed after applying soil-adsorbing methylene blue dye to irrigation water.Shrinkage cracks failed to conduct dye after 10 minutes into a flood irrigation.Earthworm (Lubricus terrestris) channels were also not stable. However, decaying roots of alfalfa produced stable macropores, while wheat produced no such macropores. The influence of alfalfa-root-induced macropores was demonstrated by 's (1989, 1990) findings on sandy loam soils.
The surface subsidence of shrink‐swell soils may be used to estimate the soil profile water content and soil behaving. Roots are known to affect cracking patterns by anchoring the soil mass, thus influencing soil shrinkage. Our hypothesis was that plant roots will influence soil shrinkage measured by vertical surface subsidence. We tested this hypothesis by measuring shrinkage in a large weighing lysimeter for bare soil with no root system and for soils under wheat (Triticum turgidum L.) and alfalfa (Medicago sativa L.) crops possessing fibrous and tap root systems, respectively. The volume‐loss shrinkage curves for both bare and cultivated soil were found to conform to the straight‐lines model, with distinct changes in shrinkage zones, in spite of uneven water‐content distributions in the profile. The shrinkage characteristic (i.e., the differential change in bulk volume divided by the change in volume water) was greater for fallow (0.677) than for wheat (0.380) and alfalfa (0.377), with the cultivated conditions being similar. These data suggest that plant roots may have large effects on in situ soil shrinkage rates and the water‐content zones throughout which they occur. This result means that a soil's shrinkage characteristic may change depending on the cropping condition. It also means that shrinkage measurements from soils without rooting systems (e.g., small cores or clods) are not necessarily representative of actual shrinkage properties observed in the field.
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