These slides shows that a solute can move preferentially along the walls of a soil column.
Note the dark red dye stain at the edge of the soil disc and the lack of dye in the middle of the disc, below. This effect can be minimized by wetting the inside of the soil column and adding an swelling clay (i.e., montmorillonite), allowing the excess clay to fall out of the column, and carefully packing the column. Use a funnel when adding the soil to the column so that it doesn't touch the clay lining.
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