Demonstrations in Solute Transport Using Dyes: I. Procedures and Results

The mathematical formalism surrounding concepts of solute transport in soil often deters the uninitiated. Classroom demonstrations with visually stimulating results are an excellent tool to inspire and motivate students. Toward this end, a mixture of two organic dyes (FD&C yellow no. 5 and blue no. 1) was drip‐applied to clear‐walled soil columns and observed as it moved through the soil. In analogy with chromatography, the dyes separate and disperse to an extent depending on the chemistry of the dyes and properties of the soil including structure, texture, organic matter content, and pH. Under near neutral to basic conditions, the anionic yellow dye moves with water ahead of the partially cationic blue dye. At neutral pH the retardation of the blue dye is oberved to be slightly greater in a fine sand than in a clay loam with organic matter. This surprising result suggests movement of the blue dye with soluble organic matter in the clay loam. The hypothesis of transport by adsorption to mobile organic matter is supported by demonstrations using humic acid in sand. The results of all the demonstrations are interpreted based on the chemistry of the dyes and chemical and physical properties of the soil.