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Comparison of Preferential Flow Paths to Bulk Soil in a Weakly Aggregated Silt Loam Soil

USDA-ARS, Columbia Plateau Conservation Research Center, PO Box 370, Pendleton, OR 97801. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA

Correspondence: ^* Corresponding author (stewart.wuest{at}ars.usda.gov).

Received for publication 3 September 2008. Preferential flow paths are common in both tilled and untilled soil, but how or why soil in preferential paths differs from neighboring bulk soil is not well understood in either. We used a 10-min pulse of dye to mark preferential flow paths at the end of 2 h of ponded infiltration. Small cores were taken from dyed and undyed soil at the 10- to 25-cm depth. Compared with the bulk soil, preferential flow zones had more large and fewer small aggregates, greater water content, and lower bulk density (1.20 vs. 1.23 Mg m^–3). Root mass and total soil C were not significantly different. In comparison to untilled soil, tilled soil had more unaggregated soil, greater water content, and lower bulk density (1.16 vs. 1.24 Mg m^–3). Statistical analysis suggested no interactions between the preferential flow zone–bulk soil factor and the tilled–untilled factor. Infiltration rates in untilled soil were greater despite greater bulk density in both preferential flow zones and the non-preferential-flow matrix soil. These results suggest that greater saturated water conductivity in untilled soil is not due to better developed individual preferential flow pathways, but rather to a greater number of potential pathways being well connected to the surface water source. This could have more to do with better aggregation of surface soil than with specific properties of the preferential flow paths.