Dirt Shirts: Soil as a dye

---

The Science, Part 1. Soil color is the result of mineral coatings on primary soil particles which often have little color. For example, red soils have coatings of oxidized and anhydrous iron (Fe₂O₃), a hematite mineral, while yellow soils have coatings of oxidized and hydrated iron (2Fe₂O₃·3H₂O), a limonite mineral. Dull greens and sometimes blues are the result of coatings of reduced forms of iron, manganese and sometimes other elements. Very light colors may be due to loss of the dye coatings. White colors also may be related to the presence of calcium and magnesium carbonates and pink occurs when some oxidized iron has been coated with carbonates. Dark soils typically have coatings of organic matter, or humus, though there are minerals that sometimes result in dark soil colors.

The Science, Part 2. Cotton is a long-chain of repeating cellulose molecules (see below) which have the chemical formula, (C₆H₁₀O₅)_n, where n represents how many cellulose molecules are present. Typically cotton fibers are more than 90% cellulose, with a few other radicals and molecules that may be attached. Notice all the hydrogen (H) atoms at the end of a line (branch). In the structure, the hydrogens are sharing an electron with the carbon (C). Under certain conditions, the hydrogen can be nudged off the carbon. The resulting hydrogen will be a cation – have a positive charge.

The Science, Part 3. The photos below show two reasons water may have color: suspension and solution.

Suspension (left image): The Red River in Texas is red due to the red soil particles “floating” (suspended) in the water. Sands (0.05 to 2.00 mm) fall out of suspension within 40 seconds, but I have witnessed clays still remaining in suspension after more than a year. Mud is essentially a very thick suspension of soil particles in water. A slurry has enough water that it will flow.

Solution (right image): Solutions are composed of free ions – anions (negatively charged) and cations (positively charged) – in the water. They are not particles, so they do not fall. Water in the soil is never pure, it is always a solution. The most common solution is salt water; put salt crystals (sodium chloride, NaCl) in water and stir, and they dissolve into water, sodium cations (Na⁺) and chloride anions (Cl^–). In the beakers below, a purple solution (grape Koolaid) was poured through different soils, resulting in the colored solutions. (See Soil is a Filter for more details.)

The Science, Part 4. The conditions in a soil slurry are messy – not just because it is mud. Almost all soils have a negative charge and attract cations (positively charged elements and molecules), holding onto them a bit like a magnet because opposites attract. This process is called adsorption. Because of their mineral composition, fine-textured soils like clays have hundreds, even thousands, more negative charges than coarse-textured soils like sands, and can adsorb that many more cations. In the soil slurry, some of the adsorbed cations on the soil particles swap places with cations in the soil solution.

And now, the magic of chemistry! A red, fine-textured soil will release a lot of those iron oxides into the soil solution (desorption) as cations in a slightly different form, typically H₂FeO₃⁺ or HFeO₃⁺², depending upon the pH of the soil solution. When a cotton swatch is placed in the slurry (mud), all the iron cations “bully” the hydrogen cations, forcing some of them off the cellulose molecule and taking their place. The iron oxides then share electrons with the cellulose, forming covalent bonds. These bonds are very strong, and the cotton will never be white again, not even with bleach.

---

---

Updated: 14 October 2024