Soil—It's Not Just Dirt!

Soil is often mistakenly referred to as “dirt,” but the difference between the two could not be starker. Dirt is biologically dead—something to be swept off of a floor or washed out of clothes. Soil, by contrast, is incredibly biologically complex. One teaspoon of healthy topsoil contains more than one billion microorganismsbacteria, fungi, and other tiny life forms whose role, though essential to life on Earth, is only beginning to be understood.1

None of the plant communities found in the park could exist without these organisms, whose actions convert inorganic mineral nutrients into biologically active forms that can be readily taken up by plants. Microorganisms also transform decomposing organic matter into humus, the essential component that contributes to soil fertility, moisture-holding capacity, and many other desirable qualities for plants.

Most of this action takes place in the rhizosphere, an exceedingly thin, highly biologically active zone of topsoil that surrounds every plant root. This process is so specialized that many plant species are known to have their own unique microorganisms in the soil around their roots, suggesting that biological diversity in the soil is fundamental to a diverse flora.