05.14.06
Why is the ground brown?
Heres a short article at scientific american about why most ground is brown. A fraction of the compounds created from dead organic material (plants, bacteria, fungi) in the soil are chemically resistent to microbial enzymes in the soil.
Many soils are brown in color because they contain large amounts of carbon. In particular, carbon-containing polymers called humic compounds absorb most visible wavelengths of light and give soils a dark brown appearance. Often the majority of soil carbon is present as humic compounds, which means they have a large impact on soil chemistry and fertility.