Should you ever desire crossing a soil scientist, use the word “dirt”. Seriously. The word offends people who spend careers describing and classifying the first few meters of material beneath our feet. In fact, it highly offends. It’s like telling the owner of a 1967 Corvette Stingray convertible with a 427 that they own just “a car”.
And yes, I said classify. Scientists classify soils into different orders and suborders based upon physical characteristics, much like plants or insects. The National Resource Conservation Service (NRCS) devised the classification system currently in use in the USA, and continually surveys and maps soil in the United States. These soil maps and reports are accessible to the public.
When the discovery of something called a “Mollic Epipedon” in a location other than the corn belt excites a soil scientist, one may consider the scientist obsessed. But, this information is extremely useful to farmers, civil engineers, and environmental scientists. The tiniest details within soil classifications help properly manage land, maximize crop yield, minimize erosion, design adequate foundations, and mitigate contaminants.
A soil map and data browser can be found on the NRCS website here:
This Web Soil Survey (WSS) is an online Geographic Information System that lets users browse and obtain soil information about their area of interest. I learned that the soil under my house is named “Estrella Loam”, part of the Estrella series of soils. Further information about the Estrella series and a typical pedon description is found here. The map also identified nearby loams from the Gilman and Mohall series. The description of those soils can be found here and here respectively.
Here is a map of my neighborhood from the WSS that shows the relation of the various soil units. (Es is Estrella Loam, Gm is Gillman loam, and Mv is Mohall Loam. I apologize that the soil units are hard to see; I have no control over that.)
So what does this tell me about the soil in my neighborhood? The NRCS classifies the Estrella Loam as an Entisol, and more specifically a Typic Torifluvent (If it ends in -ent, it is an Entisol). Basically, a Fluvent is a young soil formed in alluvium and has miniscule profile development except possibly an Ap horizon (a plowed layer). The area around my house is former agriculture land, so the Ap horizon makes sense. The pedon description also indicates the land surface is an alluvial fan, and that the profile includes buried B horizons from a previous soil. Geomorphically, this means there was a period of relative quiescence long enough for soil to develop, followed by a period of erosion and deposition to form the current land surface. The current land surface is not old enough to form distinct soil horizons. The description also conveys the soil as well drained and alkaline, and lists typical native vegetation and crops typically grown on this soil.
Comparison between the Estrella Series and the nearby Mohall and Gilman Series reveals clues to the pre-development landscape. Gilman Loam is also an Entisol, but typically lacks the buried B horizons of the Estrella Loam. This indicates the surface into which Gillman Loam developed is also young. The lack of buried horizons may indicate the older soil was completely removed by erosion prior to deposition of the modern sediment, or deeper sedimentation buried the older soil deeper than the 60-inch pedon.
The Mohall series is an Aridisol rather than an Entisol. This implies an older, more stable land surface allowing the development of soil horizons. The Mohall series is also formed in alluvium, and has clay and carbonate horizons similar to those buried in the Estrella profile. The buried horizons of the Estrella profile would probably constitute an Aridisol before they were buried.
The relation and comparison of the Estrella, Gilman, and Mohall soils in my neighborhood illustrates the dynamic nature of Alluvial fans. In a natural setting, sediment-laden water coming from the Camelback and Phoenix Mountains to the north would deposit sediment as the water spread into the valley. More recently eroded ares areas and those areas that have received new sediment have young Entisols, and older, more stable areas developed Aridisols. This pattern typifies the pattern seen when arroyos on alluvial fans migrate laterally, eroding old material and depositing new sediment.
All of this information came from a rudimentary review of a soil map and pedon descriptions. Considering the value of soil to food production and human civilization, and the information that can be gleaned from studying soil, is the word “dirt” apt?