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Soil is a natural filter! Demonstrate the ability of soil to bind and store essential nutrients, and exchange them with plants, by filtering dye solutions through soil and sand. Two organic indicator dyes are used-methylene blue, which has a positive charge, and eosin Y, which is red and has a negative charge. Why is one dye retained by the soil, while the other dye moves through the soil unaffected? Students observe that clay particles selectively absorb cations, and learn how potassium, calcium, magnesium, and other trace metal nutrients are taken up by plants from the soil. Demonstration also provides a nice visual aid to tie in larger environmental themes, such as the ability of soil to process “chemical waste” and protect against runoff and groundwater contamination. Kit includes enough chemicals and consumable supplies to perform the demonstration as written seven times, along with comprehensive Teacher Demonstration Notes and optional student worksheets.