Discriminating dyes in soil with color image analysis

Dyes are useful tools for revealing now patterns, but because analysis of dyes is time-consuming and often only qualitative, soil scientists have not used them to their full advantage. We developed a method for discriminating dyed from undyed soil in the field and for determining the dye concentration from point to point. The method requires a 35-mm camera, a specially constructed camera stand for uniform illumination during field work, and a color scanner. Working with Brilliant Blue FCF dye on a gray (chroma of one) soil, we determined dye concentrations <2.0 g L-1 with an r(2) of 0.85 and a median absolute error of 0.17 g L-1. At concentrations >2.0 g L-1, analysis was more problematic due to dye precipitating out of solution onto the soil surface, but we obtained an r(2) of 0.70. Use of this method allows convenient and rapid quantitative analysis of dye concentration and other color contrast patterns.