A cursory overview of pigmenting agents in the beautiful, diverse, living below ground soil ecosystem.
RED + ORANGE + YELLOW – these gorgeous colors are thanks to iron oxides. Some minerals are ferrihydrite, hematite, goethite, lepidocrocite – and many more. They form a wide range of colors that when mixed together due to varying amounts in soils – they can span the whole spectrum (even purple, black, brown, blue, and green).
GREEN – thank you ancient marine sediments (and other geologic materials – especially those of volcanic origin). Minerals including glauconite, celadonite, serpentinite, chlorite, olivine – greens ranging from very pale gray green to the greens that dreams are made of. Hard to find and often hard to work with due to their mineralogical properties – some are associated with salts and other shrink and swell upon wetting and drying.
BLUE + PURPLE - wow – these colors are among the rarest to see in soils but when you are able to glimpse them – smitten. Blues are often the reduced form on an iron phosphate mineral, vivianite, but can also be seen in azurite, Ti-rich magnetite (an iron oxide), and in smaller amounts in many semi-precious gems – not super common in soils but a treat when you spy them for sure. Purple colors come from iron oxides like hematite but also magnetite and maghemite.
WHITE – volcanic ash, carbonates, other salts like gypsum, and even the whites and grays of uncoated mineral grains.
BLACK + GRAY – many of the dark, black colors you see in soils are the results of decaying or charred organic matter (not mineral as the rest mentioned in this thread are). Magnetite and maghemite are also black and regularly form in small clusters smaller than your pinky fingernail and can of course be weathered into smaller particles.
BROWN – saved the best for last! Probably the reason we all think of brown when we think of soil – because soils are often colored by a wide range of materials. If you put all the above-mentioned materials in a bucket and mixed them together – you’d see brown! Organic matter is a driver in brown colors found in the upper part of soils – think, the root zone – but many iron oxides, when added with other minerals, impart a beautiful brown color.
There you have it. Enough information to spur curiosity and fuelconversation. If you want to learn more, check out the following sites.
Soil Color via Science Direct. "Soil color is a clear way to see and distinguish specific soil properties, especially for the trained soil scientist."
The Color of Soil via USDA-NRCS
Soil Colors of the United States - maps that show soil color at various depths for individual States and the Continental United States.
Soil Colors of the National Parks
If you have anything to share or questions for us - please comment below.