Why is dirt brown?
Dear Straight Dope:
Why is dirt brown?
I can give you two answers.
(1) It's a misleading question, you dog! Well, at least in part. Turns out that soil can have a pretty amazing variety of colors--the "Munsell Soil Color Chart," a standard reference, distinguishes over 200 different soil colors that can be found somewhere in the world. Red soils tend to have high iron content, blue/gray soils tend to be high in a certain type of clay, while other clay soils are more yellow, others nearly white. Talking to a professional potter, it turns out that the different colors of clay are the essence of pottery-making culture. But, that's a whole other convoluted story. So let's cheat and rephrase the question:
(2) "Why is brown dirt brown?" Generally, it's because of a high content of plant fibers and the breakdown products of plant chemicals (especially tannins, as far as color goes) that constitute much of the organic fraction of the dirt. Less plant material, and you have soil whose color reflects more of its mineral composition, as above. (Compare lunar soil, which has no organic content.) Soils with lots of organic materials tend to converge in hue because whether the plant material is the result of chemical decomposition, microbial breakdown, burning, or defecation by herbivores and scavengers, it's still basically a limited set of chemicals (especially cellulose) with a pretty predictable process of decomposition. In a practical sense, the only real organic material in soil is plant material, since only plants turn CO2 into solid organic compounds--basically all that the rest of the food web functionally serves to accomplish is redistribution and some transformation of those same compounds--and that, too, follows predictable chemical pathways; waste is waste, in the end.
So I guess what I'm saying is, brown dirt is brown for pretty much the same reason poop is brown. Let's leave it at that.