Fluorescein and pyranine were used as solute markers in field experiments under irrigation and under winter rainfall. Dye distribution was recorded by ultraviolet photography and was also measured from analyses of soil extracts.
Ultraviolet photographs showed uneven horizontal distribution of fluorescein. The main causes were (1) earthworm channels, which appeared to be more important for solute transport under irrigation than under winter rainfall; (2) fissures about 0.05 to 0.10 millimeters wide; and (3) loosely packed soil with varied pore sizes. Although the dye was adsorbed by soil, the vertical distribution followed a similar pattern to that of chloride, suggesting that it was conducted down the same channels as more mobile solutes.