Characteristics of Red River clays pertaining to vertisolic criteria and macropore flow

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Cavers, Curtis G.
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Clay soils predominately composed of montmorillonite possess a high shrink-swell potential that has taxonomic and practical implications. The presence of slickensides and evidence of high shrink-swell in these soils may meet revised criteria which would place these soils under a Vertisolic Order, or as vertic intergrades of Chernozemic, Gleysolic, Luvisolic or Solonetzic Orders in the Canadian System of Soil Classification. The presence of large cracks and biological pathways in these soils may result in higher rates of water flow and potential for pollutant transport than previously expected. Several analyses were conducted on three clay soils with contrasting hydrologic regimes. The purpose was to identify features and processes which may serve as diagnostic criteria for classification, and which may affect the permeability of the soil. Visual analysis of contiguous cores from two sites identifred physical features such as texture, structure, carbonates, salts, mottles, slickensides, roots, cracks, and biopores and mapped their location according to depth. Micromorphological analysis of the soil fabric, tritium analysis of groundwater, and 14 C analysis of root material were conducted to aid in assessing the potential for downward movement of water and dissolved materials in these soils. Saturated flux versus hydraulic gradient relationships were examined in the laboratory using a liquid bubble flow meter and pressure transducer on four contiguous core samples. At hydraulic gradients less than 1.2 m/m, there is a deviation from the straight-line relationship of Darcy's Law, with saturated fluxes at low hydraulic gradients about one order of magnitude lower than predicted by Darcy's Law. This non-Darcian flow occurs due to a portion of the water molecules in the pores immobilized by the attractive forces of the clay. Samples with pathways conducive to macropore flow follow Darcian behavior but saturated flux may be an order of magnitude higher than expected for samples of heavy clay texture. Saturated hydraulic conductivity was measured at three depths on four different cropping/tillage practices over two years on a Black Lake series (Cumulic Regosol) using a Guelph Permeameter. Of the four cropping/tillage practices, soil under continuous alfalfa had the highest saturated hydraulic conductivity at all three depths. Analysis of slickensides and shrink-swell properties confirmed that the other two soils investigated (Red River series, Dencross series) would meet criteria to be classified as a Humic Vertisol and a Vertic Rego Black respectively. Analysis of water movement through these soils revealed that under low-gradient flow these heavy clay soils is conducive to non-Darcian flow. However, the presence of physical features that produce preferential pathways may result in macropore flow that could have serious environmental implications.