Soils

Soil development (pedogenesis) is a function of bedrock regolith, climate, vegetation, and time (Jenny 1941). Three major soil orders are present at the Preserve. Entisols occur in the east valley and southern  third of the west valley, Mollisols occur along the valley borders and within the depressions between ridges, and Alfisols occur on steep slopes and high points of the eastern and western ridges. The mollisols at the Preserve are further  classified as belonging to the suborder Udolls, which are mollisols that form under Udic moisture regimes in humid continental climates. In the Udic moisture  regime, moisture inputs from cumulative precipitation and stored soil moisture exceed losses through evapotranspiration, and there are less than 90 cumulative dry days per year. Similarly, the Alfisols belong to the suborder Udalfs, which are Alfisols that also form under Udic moisture regimes.  Entisols are not divided into suborders based on moisture regime. Soil temperature regimes for both soil orders are thermic, possessing a mean annual soil temperature of 15 to 22°C to (97.2°F) at a standardized depth of 50 cm with a difference between mean summer and mean winter air temperatures greater than 5°C (41°F) (Birkeland 1984).

 

Soils Map

Entisols

Entisols are recent soils, meaning that they are not separated into readily discernable horizons, and entisols are characterized by their lack of significant profile development (Brady 1974). The entisols of the Preserve consist of highly productive soils on glacial and stream bank alluvium. Two series of entisols occur at the Preserve (Whitson et al. 2008):

  1. The Orion Series (Or) occurs in the bottomlands of the eastern valley and is characterized by a layer of silt loam at least five feet

The Orion Series entisol in the eastern valley sedge meadow remnant is a hydric mineral soil possessing redoximorphic features of soils subject to periodic inundation or saturation. This soil qualifies as a wetland soil under the definitions outlined in the Clean Water Act of 1973: The Orion Series posses a Thick Dark Surface Indicator of Hydric Soils (Hurt and Vasilas, USDA‐NRCS 2006), evidenced by a depleted matrix with greater than 60% chroma less than or equal to 2 at 30 cm below the surface, with a top layer (from 0 – 30 cm) having a value less than  or equal to 2.5 and chroma of 1. Consultants from Integrated Restorations, LLC measured a color of 7.5YR 2.5/1 in the upper 30 cm of these soils, and a color of 7.5YR 2.5/2 below 30 cm. Thus, the Orion Series soils of the east valley meet the legal criteria for wetland soils, and a permit is required to modify hydrology in the east valley. The entisols of the east valley are somewhat unique in that they developed partially from the weathering of deep layers of glacial outwash till, giving them a more sandy texture than the mucky entisols typical of sedge meadows in glaciated portions of the state. Textural analysis of the east valley entisols estimated sand‐silt‐clay fractionation at 11‐69‐20 (sand‐silt‐clay), categorizing these soils as silt loams (Brady 1974).

In 2007, when selecting donor sites for autumn transplants of Eastern Prairie Fringed Orchid (Platanthera leucophaea), Orion Series soils of  the eastern valley were sampled and three composite samples of (n = 5) subsamples were analyzed by the University of Wisconsin Soil and Plant Analysis Lab (Verona, WI). Soil bulk density was estimated at 1.02 g/cm3, indicating that these soils contain roughly equal amounts of soil and pore spaces, giving them medium to high water‐holding capacity. Calcium and magnesium levels were  high, at 2,474 ppm and 566 ppm, respectively.  This is not surprising since these cations are accumulating in the valleys following erosion and transport of weathered Prairie du Chien dolomite from the uplands. The soils of the Preserve’s eastern valley contained 3.9% organic matter and had a CEC of 18 milliequivalents per 100g of soil, meaning that 18% of the adsorption sites in the clay‐humus soil matrix were available to adsorb soil cations, principally divalent calcium (Ca2+) and magnesium (Mg2+) and univalent potassium (K+), while 100 – 18= 82% of the exchange sites were already occupied by these cations. We can therefore conclude that base saturation in the west valley soils is indeed high, evidenced both by their low CEC and also near‐neutral pH (soil pH in the eastern valley was 7.5).  Soil potassium and phosphorus were high, at 91 ppm and 57 ppm. Soil available nitrate‐nitrogen and ammonium‐nitrogen were low, at 9.2 ppm and 10.7 ppm.

2. The Elvers Series (Ev) occurs near the southern third of the west valley, and is characterized by a layer of silt loam to a depth of 35 inches subtended by a layer of muck from 35 – 60 inches.

Mollisols

Mollisols are characterized by their deep, dark surface horizons (mollic epipedons) and are relatively fertile (Brady 1974). A mollic epipedon can be identified in the field as having a minimum thickness of at least 10 cm over bedrock or a thickness of 18 to 25 cm over subsurface horizons, with organic matter content greater than 0.6% and base saturation exceeding 50% (Birkeland 1984). Base saturation is defined as the ability of exchangeable bases to  neutralize soil pH, and is expressed as a percentage of cation exchange capacity (CEC) by subtracting measured CEC from 100% (complete base saturation). Prior to Euro‐American settlement, mollisols were formed under mesic, wet‐mesic, and wet tallgrass prairie and sedge/wet meadow vegetation, and are rich in organic matter.

Three series of mollisols occur at the Preserve (Whitson et al. 2008):

  1. The Elburn Series (EgA), which occurs in the bottoms of the gulleys and ravines of the northern part of the west ridge, are characterized by slopes of 0 – 3%, with profiles consisting of silt loam (to a depth of 15 inches) overlying subsurface horizons of silty clay loam (from 15 to 44 inches) and sand and gravel (from 45 to 60 inches). Depth to bedrock is greater than 10
  1. The Otter Series (Ot), which can be found within the northern two‐thirds of the west valley and in the southwest bottomlands of the preserve, are characterized by five feet of silt loam with a depth to bedrock greater than 10 feet.
  2. The Port Byron Series (PrB and PrC), which can be found under the mesic prairie remnant (unit 7) and buffer strip grasslands of the east and west valleys, are characterized by slopes of 2 – 12% with up to five feet of silt loam and a depth to bedrock ranging from four to ten feet.

Although the Soil Survey of Dane County (Whitson et al. 2008) classifies the Sogn Series (SoD) as mollisols, this series does not meet the criteria for a mollisols. Its profile is only seven inches thick underlain with dolomite regolith to a depth of five feet, and is thus actually an alfisol.

Alfisols

Alfisols occur on the ridges, and are often associated with dry prairies and deciduous forests. Alfisols are characterized by a thin A1 horizon with a light‐ colored albic subsurface horizon formed by clay accumulation (Birkeland 1984). Alfisols are further diagnosed by medium to high (35 – 50%) base saturation (Brady 1974). Five series of mollisols occur at the Preserve (Whitson et al. 2008):

  1. The aforementioned Sogn Series (SoD), which occurs on 2 – 20% slopes of the unit 2 oak savanna remnants on the west ridge, is characterized by seven inches of loess overlying dolomite regolit
  2. The Dunbarton Series (DuC2 and DuD2), which occur on 6 – 20% slopes of the west ridge, have well‐defined soil profiles consisting of loess (to a depth of 11 inches) covering an albic horizon (from 11 – 18 inches) overlying dolomitic bedrock.
  1. The Elva Series (EhE2), which occurs on 20 – 30% slopes of the east ridge, is characterized by a top layer of sandy loam (to a depth of 22 inches) with a subsurface horizon of loamy sand (from 23 to 38 inches) overlying sandstone
  2. The Hixton Series (HbD2), which occupies 12 – 20% slopes of the east ridge, has well‐developed profiles consisting of loam (to a depth of 24 inches), covering subsurface horizons of sandy loam (from depths of 25 – 31 inches) and sand (from 32 – 39 inches), overlying sandstone
  3. The Seaton Series (SmD2), which occurs on 12 – 20% slopes of the west ridge, is characterized by silt loam to a depth of at least five feet.

The soils/substrate of the Preserve’s dry/dry‐mesic prairie remnant create suitable requirements for a Bullsnake (Pituophis catenifer sayi) population.

Stony and Rocky Soils

The Soil Survey of Dane County (Whitson et al. 2008) further segregates the substrates of the mid‐slope portions of the southwest ridge of the Preserve as stony and rocky. These substrates support a 30‐acre dry/dry‐mesic prairie remnant which was probably spared from the effects of grazing due to its slope and thin soil characteristics.