Soil color

Soil color is often the most visually apparent property of soil. While color itself does not influence the behavior or practical use of soils, it does indicate important information about soil organic matter content, mineralogy, moisture, and leaching.

Soil can display a wide range of colors including brown, red, yellow, black, gray, white, and even blue or green, and vary dramatically across landscapes, between the various horizons of a soil profile, and even within a single clod of soil.

The development and distribution of color in soil results from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colorful compounds. Soil conditions produce uniform or gradual color changes, while reducing environments result in disrupted color flow with complex, mottled patterns and points of color concentration. Sometimes, a distinct change in color within a soil profile indicates a change in the soil parent material or mineral origin.

Dark brown or black colors typically indicate that the soil has a high organic matter content. Organic matter coats mineral soil particles, which masks or darkens the natural mineral colors.

Sodium content also influences the depth of organic matter and therefore the soil color. Sodium causes organic matter particles such as humus to disperse more readily and reach more minerals. Additionally, soils which accumulate charcoal exhibit a black color.

Red colors often indicate iron accumulation or oxidation in oxygen-rich, well-aerated soils. Iron concentrations caused by redox reactions because of diffusion of iron in crystalline and metamorphic rock, in periodically saturated soils, may also present red colors, particularly along root channels or pores.

Soil in anaerobic, saturated environments may appear gray or blue in color due to redox reduction and/or depletion of iron. In anaerobic soils, microbes reduce iron from the ferric (Fe³⁺) to the ferrous (Fe²⁺) form. Manganese may also be reduced from the manganic (Mn⁴⁺) to the manganous (Mn²⁺) form, though iron reduction is more common in soil. The reduced iron compounds cause poorly drained soil to appear gray or blue, and because reduced iron is soluble in water, it may be removed from the soil during prolonged saturation. This often exposes the light gray colors of bare silicate minerals, and soils with a low chroma from iron reduction or depletion are said to be gleyed.

Iron reduction may impart greenish gray colors, though certain minerals including glauconite, melanterite, and celadonite can also give soil a green color. Glauconite soils form from select marine sedimentary rocks, while melanterite soils are produced in acidic, pyrite-rich soils. Celadonite in hydrothermally-altered basalt within the Mojave Desert has been observed to weather into a green colored smectite-rich clay soil.