Soil Profiles An Important Part of the Picture At SEGA Sites

There’s one very important piece of the climate change research puzzle which is easily overlooked – because it’s right beneath our feet. Soil. So to make sure that soils are very much part of the data available to SEGA researchers, completion of SEGA’s site infrastructure this year has included a collaboration with soil scientists from the USDA.

Soil scientists and surveyors Jim Harrigan and Harry Hosler from the Flagstaff MLRA Soil Survey Office (part of the USDA’s Natural Resource Conservation Service) have a wealth of experience describing the soils of the region. Over a combine 40 years of working at NRCS they’ve surveyed and mapped soils at sites ranging from the Aubrey cliffs to the Four Corners, Fredonia to Showlow and El Malpais National Monument to San Luis, Colorado. 

Their work at the ten core SEGA sites has involved a detailed characterization of the soil profile. This has involved digging one cubic meter soil pits, measuring the discrete soil horizons, as well as describing their most important physical and chemical properties (e.g. texture, color, and pH).

The collaboration between the NCRS Soil Survey and SEGA is enabling the collection of detailed meteorological data from the exact locations where the soil pits have been described. That’s because the SEGA meteorology towers have been installed in the soil pits dug at each site. The Soil Survey are using the detailed precipitation and temperature data collected by SEGA to help improve its classification of the soils.  Having good data on soil properties is useful for a wide variety of applications. For example, soil ‘hue’ (specific color), ‘chroma’ (color intensity) and ‘value’ (how light or dark a soil is) have been recorded routinely as part of soil surveys, for decades. But it was only in the last few years that it was found that these data could be used to derive albedo (the ratio of incoming solar radiation that is reflected from the ground surface) – an important component in parameterizing global climate change models.

Details about soil properties are also essential for evaluating wind and water erodibility, predicting the effect of landscaping and irrigation systems, positioning of septic tanks and construction projects. Soils are also tied closely to the above-ground ecological and habitat characteristics of sites – the more detailed and accurate the soil map – the better the flora and fauna likely to be found there can be predicted. 

By design, the soils at SEGA sites fall into two broad categories – those derived from dominantly limestones versus those derived from more acidic basalt parent materials. Both broad soil groups are similar in that they contain a lot of clay. But at sites where the clay is basalt-derived it’s very sticky, expanding and contracting a lot depending on moisture; soils at sites with limestone-derived carbonate-rich clay are not as sticky and don’t change moisture content nearly as much. Soils at the SEGA North Rim sites are limestone derived, while at Walnut Creek the very deep soils there are formed from a mixture of limestone and basalt parent materials. Bradshaw Ranch has very sandy loams, derived from the surrounding Supai sandstones – rocks formed from ancient aeolian (wind blown) sands, while the sandy soils at Black Point are derived from a similar sandstone but the drier climate at that site has led to a lower clay content compared to Bradshaw Ranch.

You can find a soil profile photo along with a table showing the soil properties of each soil horizon by going to the SEGA ‘Gardens’ webpage, clicking on the site of interest and scrolling down:

http://www.sega.nau.edu/gardens

You can find more information about the work of the NRCS and Soil Survey at:

https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/