The color of soils has long been considered to indicate to some extent, at least, their fertility. For centuries farmers in the humid regions have been accustomed to select their land very largely on the basis of color. Dark colored soils have been preferred because experience has shown that such soils are usually more productive than light colored types. But just what the relation is between the color of soils and their fertility or crop-producing power has never been determined. It has been assumed that the organic matter or humus content, which is mainly responsible for the color of soils, has a fertility value. It has been believed also that the temperature, aeration, and moisture content of soils bear a direct relation to their color and content of organic matter and hence to their fertility. The actual relation of color to chemical composition, however, has never been studied in detail, altho it has been generally understood that dark-colored soils, with more organic matter, naturally contain more nitrogen and organic carbon.
Brown, P. E. and O'Neal, A. M.
"The color of soils in relation to organic matter content,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 5
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol5/iss75/1