Soil developed from weathered bedrock, Betasso gulch

Examining soil physical properties in the field
with Peter Birkeland, University of Colorado

Evey Gannaway (University of the South) taps
tube into soil to sample density near Nederland

Soil exposure at 11400 ft in the Boulder Watershed.  Martinelli Snowfield in the backgroun

Ken Nelson (Macalester College) describes buried soil (outlined by white dashes), Betasso gulch

 

Soil grows downward over time as parent material (bedrock or sediment) weathers and mixes with organic material and dust from the ground surface.   In time, soil layers (“horizons”) develop parallel to the surface.  Digging downward you find first the “O”, which consists of dark-colored fresh and decaying organic matter derived mainly from local vegetation. The “A” horizon is a mixture of organic materials from the O-horizon and minerals derived from soil parent material. Beneath the organic-rich soil is a yellow to orange “B” horizon, formed mainly from inorganic material and colored by iron hydroxides and clays transported from higher in the soil.  The “BC” (or Crt) horizon is a transition between the B-horizon and relatively unaltered parent material, the C-horizon.  Root mass, organic matter and biologic activity are highest above the C-horizon, but some tree roots reach this zone.  Where soil forms from altered bedrock the deepest layer is called Cr or saprolite.  The horizons, taken together, constitute a soil profile characteristic of the soil-forming factors at a site.

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Mapping, sampling and analysis of soils provides clues about soil fertility, slope stability, hydrologic properties and rates of soil formation.  Some soil properties, such as layer thickness, color and structure, are described by carefully examining and measuring exposures in the field.

Other important soil properties such as grain size, pH, bulk  and extractive chemistry, are measured in the laboratory using a variety of techniques.

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The Boulder CZO catchments—Betasso, Gordon Gulch and the upper Green Lakes basin—allow us to study rates of weathering and the evolution of soil properties from similar parent materials in three landscapes, across a steep gradient in temperature and moisture.  Study areas range from the icy reaches of the thinly vegetated, recently glaciated alpine zone of the upper Boulder Creek watershed to the warm, relatively dry area near the Betasso Water Treatment Plant, where soils have developed over a long time.

Soils in each area reflect local biogeochemical processes and help to record landscape history. At most sites, soil moves slowly downslope, mixing as it goes, at rates that depend mainly on climate and slope. Soil material also can move rapidly, eroding from one area and burying another in response to climate change or to local, intense 
storms. Burial preserves the properties of the old soil and begins the development of a soil profile beneath the new surface.

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Soil properties are influenced by local climate and evolve over time. Soil pH or clay mineralogy, for instance, may record changes in mean temperature, moisture and vegetation associated with climate change. Over time, clays, oxidized iron minerals and low-solubility elements such as Al, Ti and Zr accumulate, easily weathered cations such as Na are removed, and biologically essential elements such as K and P become part of tight biochemical cycles. Soil organic matter at Betasso was buried about 9000 years ago, when the overlying stony sand, silt and clay accumulated as slope debris. The buried soil is rich in clay and iron oxides compared to the “young” soil. Comparison of physical and chemical properties to those of dated Front Range soils suggests that the buried Betasso soil may have developed over a period of nearly 100,000 years before burial.

Soils act as biogeochemical reactors, fed by organic matter and atmospheric deposition from above and transformation of fresh rock material from below, stirred by a variety of processes and populated by countless microorganisms. In most soils of the Front Range area, water flow, organic matter and biochemical activity decrease downward in the profile. Plant nutrients, derived from organic material and from mineral weathering, are most abundant in the water- and CO2-rich A-horizon, where roots and microbe populations are most dense. Water that passes through the soil and into the fractured bedrock beneath flows slowly into channels as groundwater. The chemistry of water in Boulder Creek and other Front Range streams reflects soil processes as well as local weathering in bedrock beneath the soil. Water you are using today fell as snow or rain in the watershed and gained its chemical character as it reacted with the soil on a pathway to the channel.