A novel method for measurement and characterization of soil macroporosity

Quantitative macropore characterizations were performed in large zero-tension soil lysimeters of a Maury silt loam (fine, mixed, mesic Typic Paleudalf) and a Loradale silt loam (fine, silty, mixed, mesic Typic Argiudoll) soil in an effort to assess potential colloid transport. Steel pipe sections (50 cm diameter × 100 cm length) were hydraulically driven into the soil for lysimeter establishment. A dye tracer was applied into each lysimeter under saturated conditions at eight-hour intervals using 500 mL pulse applications. After one pore volume elution, the center section of the stained lysimeters was removed, dissected, and photographed. Kubiena tins were inserted into the lysimeter's remaining soil at 15 cm intervals. Vertically oriented thin sections were prepared from each tin. Pore area, diameter, and orientation were evaluated from thin sections and photographs using commercial grade scanners and Photoshop® software. At an input scanning resolution of 60 pixels per cm, the software program was capable of identifying pores with diameters > 160 μm. Thin section analysis provided more detail of smaller sized pores (< 1.0 mm), while the cationic nature of the methylene blue dye restricted its presence primarily to the larger macropores. The Loradale soils tended to exhibit increased pore area and pore diameter over that of the Maury in both thin sections and photographs. The Loradale soil also showed a more elaborate and vertically oriented network of pores. Although standard soil physical measurements did not suggest the presence of a restrictive layer in these soils, morphological evidence generated from this methodology suggested otherwise. Colloid leaching experiments performed in separate lysimeters of the same soils in a fashion similar to that used for the dye tracer application showed colloid breakthrough and filtration patterns consistent with the macropore characterization observations obtained through the application of this method.