Munterville: A New Soil Series in Iowa

Agronomy Publications Agronomy 2011 Munterville: A New Soil Series in Iowa Mostafa A. Ibrahim Ohio State University - Main Campus C. Lee Burras Iowa State University, [email protected] Jason Steele United States Department of Agriculture Mark La Van United States Department of Agriculture Michael L. Thompson Iowa State University, [email protected] See next page for additional authors Follow this and additional works at: http://lib.dr.iastate.edu/agron_pubs Part of the Agriculture Commons, and the Soil Science Commons The complete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ agron_pubs/57. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Agronomy at Digital Repository @ Iowa State University. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Digital Repository @ Iowa State University. For more information, please contact [email protected]. Authors Mostafa A. Ibrahim, C. Lee Burras, Jason Steele, Mark La Van, Michael L. Thompson, and Michael Sucik This article is available at Digital Repository @ Iowa State University: http://lib.dr.iastate.edu/agron_pubs/57 Munterville: A New Soil Series in Iowa Mostafa A. Ibrahim,* C. Lee Burras, Jason Steele, Mark La Van, M.L. Thompson, and Michael Sucik Before 2009, the Natural Resources Conservation Service (NRCS) staff noticed plant roots and argillans deep within the B horizon of Gosport polypedons (a fine, illitic, mesic Oxyaquic Dystrudept) while updating Major Land Resource Area (MLRA) 108 (Illinois and Iowa deep loess and drift) and 109 (Iowa and Missouri heavy till plain) in the central United States. In 2009, Gosport soils in MLRAs 108 and 109 were remapped and reclassified. Both map unit transects and the relevé method were used to identify pedons for detailed laboratory analyses. Eight pedons representing Gosport soils were collected from eight counties in southern Iowa, USA (Davis, Jefferson, Keokuk, Lucas, Mahaska, Marion, Monroe, and Van Buren). All of the pedons were described, analyzed, and classified. The results revealed two groups of soils, which are different than that of Gosport: Alfisols, newly named as the Munterville series (fine, mixed, smectitic or kaolinitic, active or superactive, mesic Oxyaquic Hapludalfs), and Ultisols (fine, mixed or kaolinitic, active, mesic Oxyaquic Hapludults) that have not yet been assigned a series name. The aims of this work were to reclassify the Gosport soils and detect the lithologic discontinuities within their sola. A lthough most of it is buried under the various Pre-Illinoian drifts, Illinoian drifts and Peoria Loess that comprise the modern Southern Iowa Drift Plain (SIDP), the Pennsylvanian shale is still extensive across the SIDP (Prior, 1991; Horick, 1974; Hallberg and Boellstorff, 1978). The SIDP’s rolling topography has developed continuously since the Yarmouth interglacial period of 300,000 yr ago (Prior, 1991). Consequently, the SIDP has experienced extensive erosion, and Pennsylvanian shale is now exposed on many hillsides. Some of the shale remains in place; some has broken loose and occurs interbedded with loess- and till-derived colluvium. Shale was defined by Pettijohn (1949) as a laminated or fissile claystone or siltstone that had been buried by geologic processes at some point. When that shale is found near the Earth’s modern surface, soils that form in it often inherit many of the shale’s properties, such as texture and pH. Soils formed in shale may have a range of particle-size distributions determined by the original composition of shale, which may have a wide range of texture. For example, its clay content has been reported to range from Mostafa Ibrahim, School of Environment and Natural Resources, The Ohio State Univ., 422 A Kottman Hall, 2021 Coffey Road, Columbus, OH 43210, USA *corresponding author ([email protected]); C. Lee. Burras and M.L. Thompson, Dep. of Agronomy, Iowa State Univ., Agronomy Hall, Ames, IA 50011, USA ([email protected]); Jason Steele ([email protected]) and Mark La Van (retired), USDANRCS, 1805 West Jefferson Ave. Ste. 2, Fairfield, IA 52556-4235; Michael Sucik, USDA-NRCS, 2402 S. Duff Ave., Ames, IA 50010-8037 ([email protected]). doi:10.2136/ssh2011-52-4-1 Published in Soil Surv. Horiz. 52(4):103–110 (2011). 17% to more than 50% by weight (Boggs, 1992; Prothero and Schwab, 2004) or from 23 to 79% (Slusher, 1960). Iowa’s shale is acidic. This acidity can be attributed to the oxidation of sulfur in pyrite, forming sulfuric acid when it is exposed to air and water (Peterson 1946; Slusher, 1960). Kaolinite, illite, muscovite, and quartz have been identified in the lower horizons of soils formed in shale in Iowa (Slusher, 1960). The pedogenic relevance of this geologic history is that a complex range of parent materials may be found on an individual hillslope of the SIDP, yet the predicting where a given parent material will be found can be challenging. Given that matrix mineralogy, architecture, and color are generally inherited in even well-developed sola, the identification of each parent material is essential in deducing pedon formation and classification (Schaetzl and Anderson, 2005; Oganesyan and Susekova, 1995). In fact, the importance of identifying parent materials was recognized by Hilgard (1860). He suggested that this is one of the single most important activities for pedologists. Ideally, different parent materials within a solum can be distinguished by lithologic discontinuities that are identifiable in the field and confirmed in the laboratory (Schaetzl, 1998). Wang and Arnold (1973) suggested that lithological discontinuities are observed first in the field by detecting abrupt changes in texture, structure, consistency, or horizon boundaries. The absence of lithologic discontinuities often—but Abbreviations: BS, base saturation; CEC, cation-exchange capacity; EC, electrical conductivity; MLRA, Major Land Resource Area; SIDP, Southern Iowa Drift Plain; XRD, X-ray diffraction. SOIL SURVEY HORIZONS 103 not always—means the soil was formed in a uniform parent material (Chadwick et al., 1990; Beshay and Sallam, 1995). An immediate confounding factor is that discontinuities can be pedogenic or geologic. Where they are not coincident, pedogenic discontinuities usually exhibit gradual changes while geologic discontinuities are abrupt (Schaetzl and Anderson, 2005; Smeck et al., 1968). There are many different approaches for detecting lithologic discontinuities. Each entails examining depth trends of some soil feature such as total sand content, content of a sand fraction, total silt content, ratio of sand/silt, or mineralogical differences (Schaetzl, 1998). A break in any of these features suggests the presence of a lithologic discontinuity (Schaetzl and Anderson, 2005). However, pedologists recognize that it is not easy to detect lithologic discontinuities when younger parent materials gradually accrete onto older parent materials and/or if different parent materials share primary mineral provenance (Ruhe and Olson, 1980). Soil welding may also mask the discontinuity between layered parent materials (Ruhe and Olson, 1980). McDonald and Busacca (1990) suggested that when the rate of addition of loess is less than the rate of pedogenesis, the profile gets thicker, especially in the B horizon, yet no lithologic discontinuity seems to occur. Another example of a “disappearing” discontinuity is when sand-size fragments, especially in shale, weather and/or disintegrate to smaller particles (Smeck and Wilding, 1980). Those authors also noted sand weathering can lead to differences in the ratios of resistant minerals such as Ti and Zr, which in turn can result in identification of extra or too few parent materials. Soil plasma includes all components of soil materials that are capable of moving within the soil profile (e.g., clay particles and soil solution). Eroded materials are not considered soil plasma (Schaetzl and Anderson, 2005). This mobility of soil plasma during pedogenesis is yet another challenge in detecting lithologic discontinuities. That is why Smeck and Wilding (1980) recommended that discontinuities should be detected using immobile components that are resistant to weathering. Thus, the use of pH, cation-exchange capacity (CEC), electrical conductivity (EC), organic matter content, base saturation (BS), clay content, clay mineralogy, and/or soil color is generally avoided because they can move or change within the soil profile and result in erroneous parent material identification (Schaetzl and Anderson, 2005). In 2009, as part of the Iowa Cooperative Soil Survey, the NRCS reexamined Gosport map units and collected characterization pedons from MLR As 108 and 109 to investigate whether or not a new series needed to be created. The Gosport series was established in 1938 as a moderately deep series because the surveyors at that time did not closely examine the shale. It is thought they assumed the presence of any shale implied a Cr horizon. The MLR A surveyors decided to remap Gosport map units after they sampled and investigated many Gosport soils for ponds and animal waste lagoons as well as examining Gosport locations being used in the Conservation Reserve Program. During these investigations, they discovered that plant roots actually went past the original Cr horizon that was described in older soil surveys. The goals of this study were to classify pedons sampled as Munterville, previously Gosport, by the NRCS MLR As 108 and 109 field surveyors, and to evaluate various lines of evidence for lithological discontinuities in these pedons. 104 Materials and Methods Study Area and Field Work In 2009 a number of pedons were collected for laboratory analysis as part of the remapping project of the soil map units in MLR As 108 (Illinois and Iowa deep loess and drift) and 109 (Iowa and Missouri heavy till plain) by the MLR A soil scientists. The ones of interest to this study are those representing Gosport, currently Munterville, soil map units. To maximize similarity in Pleistocene landscape evolution between locations, only pedons from the Des Moines River Valley were used. This resulted in eight pedons, with one from each of the following counties: Davis, Jefferson, Keokuk, Lucas, Mahaska, Marion Monroe, and Van Buren (Fig. 1). Each pedon was identified using the relevé method of statistics which is known in pedology as “representative pedons.” Readers interested in relevé statistics are encouraged to examine the handbook assembled by the Minnesota Department of Natural Resources Staff (2007). Each pedon was collected as one 7-cm-diameter soil core extending 1.5 m deep. All cores were taken using a truck mounted soil sampler (Giddings Machine Company, Windsor, CO, USA). All pedon locations were recorded using a handheld Garmin-GPS Map 76 unit (Garmin International Inc., KS, USA). Other pertinent field conditions (e.g., slope) were determined via standard NRCS field methodologies. The Iowa Soil Properties and Interpretations Database (ISPAID, 2004) was used to determine the occurrence and distribution of Gosport, currently Munterville, soil map units. The digital maps were obtained from the Natural Resources Geographic Information Systems Library (2011). ArcGIS 9.3 (ESRI, The Redlands, CA, USA) software was used to show the positions of the study pedons, where Munterville series occurs, and to determine the total area occupied by Munterville in the whole state and in each individual county. Fig. 1. Distribution of Munterville series across Iowa, USA and locations of study pedons. SOIL SURVEY HORIZONS Pedon Description Pedons were described according to the methods in Schoeneberger et al. (2002). Horizon type, horizon depth, boundaries between horizons, texture by feel, structure, consistency, presence of redoximorphic features, clay films, roots, and pores were determined for each pedon. Soil color was determined by comparing soil samples to a Munsell Soil Color Chart. Physical and Chemical Laboratory Analyses Soil samples representing all of the horizons of each pedon were air-dried, ground, and then sieved through a 2-mm sieve to separate the fine earth materials from the coarser materials, although due to the soft rock and other parent materials involved no coarse material was identified. Soil texture was determined by the pipette method (Pansu and Gautheyrou, 2006). Soil organic carbon was determined according to the dry combustion method described by Soil Survey Staff (2004) with a LECO LC2000 (LECO, St. Joseph, MI, USA). Total C was assumed to be equal to total organic C in horizons where the pH was £6.8 because acid conditions decrease the probability of inorganic carbon sources. Soil pH of each soil horizon was measured in water and CaCl2 with an Orion pH meter (Thermo Scientific, Beverly, MA, USA) according to Sparks (1996). Both a 1:1 volumetric soil to water solution and a 1:2 volumetric soil to 0.01 M CaCl2 solution were used. Cation-exchange capacity was measured by Na-saturation displacement using the centrifuge method (Soil Survey Staff, 2004). Base saturation was determined by extracting Ca 2+, Mg2+, Na+ and K+ by displacement with 1 M NH4OAc at pH 7.0 (Soil Survey Staff, 2004) The extracted cations were determined by atomic absorption spectrometer (PerkinElmer Instruments, Waltham, MA, USA). Clay mineralogy for selected horizons was determined according to Poppe et al. (2001). In brief, approximately 45 g of the selected soil samples were weighed in a 1-L beaker. CaCO3 was removed by using 1% CH3CO2H. Organic matter was removed by adding 3% H2O2 gradually until foaming ceased. Sodium hexametaphosphate was used as a dispersion reagent. The clay fraction was isolated after dispersion and settling according to Stokes’ Law. Each clay suspension was coagulated by adding MgCl2 and excess MgCl2 was removed by washing the sample with distilled water and then 95% ethanol. The isolated clay was frozen at −80°C and then dried using a freeze-dryer. Oriented mounts on glass slides were prepared. Each sample was suspended in distilled water and filtered through a 47-mm-diameter Millipore cellulose filter paper using a Millipore filtration apparatus. The filter paper with the remaining clay on it was wrapped on a 200-mL glass beaker with the clay film up and then transferred to a labeled glass slide. Each Mg-saturated clay sample was treated with 1:1 glycerol: water solution before X-ray analysis. A separate sample was treated with 1M KCl and similarly prepared on a glass slide. CuKa radiation from a Siemens D 5000 X diffractometer (Siemens, Germany) operated in q-2q mode and equipped with a solid state Li(Si) detector and q-compensating slits on both sides of the sample was used for mineralogical analysis. The scanning speed was 0.05° 2q min−1. Clay mineral peaks in the X-ray diffraction (XRD) patterns were identified and then quantified. Intensities of clay minerals were quantified using the following equation: area = height of the peak ´ width at half peak height (Moore and Reynolds, 1997, p. 298–329.). After determining the area of all peaks, their areas were summed together to calculate the Winter 2011 percentage of each clay mineral using the following equation: clay mineral% = (its peak area/total area) × 100. Classification All pedon descriptions and the results of laboratory analyses were used to classify the eight pedons. Pedons classification was accomplished via Keys to Soil Taxonomy (Soil Survey Staff, 2010). Detecting Lithologic Discontinuities Two laboratory methods were used to detect lithologic discontinuities: total sand as a depth function (Oertel and Giles, 1966) and sand/ silt ratio as a function of depth (Tsai and Chen, 2000). Evaluation of each method was via comparison with field descriptions as well as crossmethod comparisons. Results and Discussion Munterville soils are distributed in 16 counties in Iowa (Fig. 1). Its map units occupy 42,620 ha, or 0.29% of Iowa’s total land area. Figure 2 shows the distribution of Munterville soil maps units across the eight counties of interest in the Des Moines River valley. Marion and Monroe Counties have 10.6 and 7.4% of their area, respectively, mapped as Gosport (or now, Munterville). The extent of Munterville in other counties is much less. For example, Jefferson and Keokuk Counties have 0.2 and 0.03% of their total land areas mapped as Munterville, respectively. Munterville soils form in multiple parent materials. The lower parent material is Pennsylvanian shale or interbedded shale and limestone. The upper parent material could be Pre-Illinoian till, Peoria loess, or local colluvium. Munterville soils are brown to yellowish brown in the upper 70 cm of the soil profile (Table 1). Below that, the color is often gray or even black. The gray color is inherited from the Pennsylvanian shale and is not an indicator of drainage class, which is moderately well drained. The black color is due to coal and/or carboniferous shale fragments in the lower horizons. In general, the soil textural class is clayey, specifically often silty clay (Table 1). The pH of some horizons of certain pedons is unusually acidic (Table 2), at least by the normal standards of the upper Mississippi River valley. Fig. 2. Distribution of Munterville series in study counties in Iowa, USA. 105 Table 1. Physical and morphological properties of pedons from Iowa, USA. Pedon ID Mahaska Monroe Davis Jefferson Van Buren Keokuk Lucas Marion Depth cm 0–13 13–41 41–63 63–112 112–140 140–203 0–18 18–33 33–48 48–64 64–94 94–142 142–203 0–20 20–36 36–61 61–81 81–112 112–163 163–203 0–13 13–23 23–43 43–71 71–102 102–130 130–176 176–203 0–10 10–21 21–41 41–61 61–84 84–145 145–175 175–203 0–13 13–36 36–61 61–94 94–112 112–135 0–20 20–40 40–50 50–69 69–99 99–122 122–178 0–20 20–33 33–64 64–102 102–156 156–173 Horizon Ap Bt1 2Bt2 2Bt3 2C 2Cr Ap Bt1 Bt2 2Bt3 2Bt4 2C 2Cr Ap Bt1 2Bt2 2BC 2C 2Cr1 2Cr2 A E Bt1 2Bt2 2BC 2C1 2C2 2C3 A E Bt1 2Bt2 2BC 2C1 2C2 2Cr Ap 2Bt1 2Bt2 2BC 2C1 2C2 Ap 2Bt1 2Bt2 2BC 2C1 2C2 2C3 Ap 2Bt1 2Bt2 2BC 2C 2Cr Sand Silt Clay ———————————% —————————— 2.2 63.4 34.3 2.0 48.4 49.6 9.6 46.6 43.8 9.1 42.0 48.9 12.2 41.6 46.1 13.2 46.3 40.5 24.0 42.6 33.4 24.9 41.4 33.7 15.4 34.2 50.4 7.6 27.2 65.3 3.6 40.3 56.1 3.6 41.2 55.1 7.4 40.0 52.6 14.7 64.3 21.0 18.5 39.0 42.5 33.1 24.3 42.6 6.3 36.3 57.5 11.1 37.6 51.3 3.0 37.3 59.7 13.8 30.7 55.4 11.5 66.9 21.6 10.1 63.3 26.6 11.7 39.8 48.6 4.9 35.7 59.4 10.8 52.5 36.7 9.1 46.5 44.4 4.5 36.5 59.0 0.6 31.7 67.7 35.1 45.8 19.2 34.2 48.7 17.1 12.5 41.4 46.1 2.0 33.4 64.5 2.4 23.7 73.9 35.4 38.5 26.1 12.1 62.7 25.3 8.2 50.2 41.6 28.6 43.8 27.6 7.9 32.9 59.2 2.5 37.6 60.0 2.8 44.9 52.3 2.9 46.8 50.2 2.8 68.6 28.5 15.0 49.4 35.5 9.6 42.6 47.8 2.8 37.6 59.6 0.4 45.0 54.6 5.6 44.8 49.6 0.9 49.6 49.5 1.9 44.9 53.2 11.7 54.7 33.6 4.2 42.4 53.5 13.9 39.9 46.2 7.6 32.8 59.6 6.2 30.6 63.2 7.9 35.2 56.9 Sand/ silt Texture† Horizon boundary‡ 0.0 0.0 0.2 0.2 0.3 0.3 0.6 0.6 0.5 0.3 0.1 0.1 0.2 0.2 0.5 1.4 0.2 0.3 0.1 0.4 0.2 0.2 0.3 0.1 0.2 0.2 0.1 0.0 0.8 0.7 0.3 0.1 0.1 0.9 0.2 0.2 0.7 0.2 0.1 0.1 0.1 0.0 0.3 0.2 0.1 0.0 0.1 0.0 0.0 0.2 0.1 0.3 0.2 0.2 0.2 SiCL SiC SiC SiC SiC SiC CL CL C C SiC SiC SiC SiL C C C C C C SiL SiL C C SiCL SiC C C L L SiC C C L L SiC L C C SiC SiC SiCL SiCL SiC C SiC SiC SiC SiC SiCL SiC C C C C CS CS CS GS GS – CS CS CS CS GS GS – CS CS CS GS GS GS – AS AS CS CS CS CS GS – AS AS CS GS GS GS GS – AS CS CS GS GS – AS CS CS GS GS GS – AS CS GS GS AS – Color (moist) 10YR4/3 10YR5/3 10YR5/4 2.5Y5/2 10YR5/4 10YR5/4 10YR4/3 10YR4/4 10YR5/4 10YR5/4 5Y5/2 N5/0 2.5Y4/1 10YR4/3 10YR5/4 10YR5/4 10YR5/2 10YR5/2 10YR6/2 N2.5/0 10YR3/2 10YR4/2 10YR4/3 10YR5/4 2.5Y6/1 10B7/1 5Y5/1 2.5Y6/1 10YR4/2 10YR5/2 7.5YR5/4 2.5Y7/2 5Y6/2 2.5Y7/2 5Y7/2 N6/0 10YR4/3 10YR5/4 10YR5/4 2.5Y4/1 2.5Y3/1 10YR5/4 10YR4/3 2.5Y6/3 2.5Y5/2 N5/0 N5/0 N6/0 N6/0 10YR4/3 10YR6/3 2.5Y5/2 N6/0 N5/0 N2.5/0 Structure§ 1f sbk 2 m sbk 2 m sbk 2 m sbk M M 1 f sbk 2 m sbk 2 m sbk 2 m sbk 2 m sbk M M 1 f sbk 2 f sbk 2 m sbk 2 m sbk M M M 2 f gr 1 m pl 2 m sbk 2 m sbk 2 m pr M M M 1 f gr 1 tk pl 2 f sbk 2 m sbk 1 m sbk M M M 2 f gr 2 m sbk 2 m sbk 2 m sbk M M 1 f sbk 2 m sbk 2 m sbk M M M M 1 f sbk 2 f sbk 2 m sbk 1f sbk M M Clay films yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes † Texture: SiCL (silty clay loam), SiC (silty clay), CL (clay loam), C (clay), SiL (silt loam), L (loam). ‡ Horizon boundary: C (clear), S (smooth), G (gradual), A (abrupt). § Structure: 1 (weak), 2 (moderate), f (fine), m (medium), tk (thick), sbk (subangular blocky), M (massive), gr (granular), pl (platy). The strongly acidic lower sola are due to the oxidation of high contents of pyrite and siderite, which are common in this region’s Pennsylvanian shales and coals (Einspahr et al., 1955). The mechanism that created the unusually acidic upper sola is less clear, although we speculate it is due, at least in part, to colluvial mixing and oxidation of shales into epipedons. Table 2 also shows the total carbon distribution in the studied pedons. In most of the horizons, the total carbon is equal to organic carbon content since no carbonate minerals are present; however, the presence of carbon-rich shales 106 and coal causes elevated total carbon in the lower sola of pedons from Monroe and Keokuk Counties (Table 2). Each pedon has an ochric epipedon with the thickest “mollic epipedon colors” being only 13 cm (Table 1). All eight pedons have argillic horizons. The evidence for argillic horizons are field descriptions that note argillans, Bt horizons, and laboratory measured clay enrichment in the Bt horizons that meet the criteria given in Keys to Soil Taxonomy (Soil Survey Staff, 2010). SOIL SURVEY HORIZONS Table 2. Chemical properties of pedons from Iowa, USA. pH Pedon ID Mahaska Monroe Davis Jefferson Van Buren Keokuk Lucas Marion Depth cm 0–13 13–41 41–63 63–112 0–18 18–33 33–48 48–64 0–20 20–36 36–61 61–81 81–112 0–13 13–23 23–43 43–71 71–102 0–10 10–21 21–41 41–61 61–84 84–145 0–13 13–36 36–61 61–94 94–112 0–20 20–40 40–50 50–69 0–20 20–33 33–64 64–102 102–156 Horizon Ap Bt1 2Bt2 2Bt3 Ap Bt1 Bt2 2Bt3 Ap Bt1 2Bt2 2BC 2C A E Bt1 2Bt2 2BC A E Bt1 2Bt2 2BC 2C1 Ap 2Bt1 2Bt2 2BC 2C1 Ap 2Bt1 2Bt2 2BC Ap 2Bt1 2Bt2 2BC 2C OC % 2.7 0.6 0.4 0.3 2.6 1.2 0.9 0.8 1.3 0.5 0.4 0.5 0.5 2.1 0.7 0.6 0.3 0.2 1.4 1.0 0.4 0.4 0.4 0.2 2.6 0.6 0.8 1.2 1.1 2.6 0.4 0.3 0.3 1.7 0.5 0.5 0.3 0.3 (1:1) H2O 5.26 4.46 4.96 6.26 6.54 6.48 6.61 6.17 5.87 4.23 4.17 4.10 4.12 5.7 5.35 4.93 4.66 4.67 5.29 5.15 4.6 4.41 4.23 4.57 5.37 4.74 4.75 5.17 5.45 5.35 5.53 5.58 6.01 5.97 5.05 4.94 4.55 4.13 Exchangeable cations (1:2) CaCl2 4.83 4.13 4.35 5.55 6.45 6.19 6.16 5.79 6.14 3.88 3.70 3.60 3.55 5.21 4.46 4.04 3.98 4.23 4.99 4.65 3.96 3.67 3.79 3.85 5.20 4.13 4.07 4.67 5.41 5.14 5.06 5.27 5.88 5.46 4.36 4.18 3.80 3.56 By definition, pedons that have ochric epipedons and argillic horizons are either Alfisols or Ultisols (Soil Survey Staff, 2010). The distinction between Alfisols and Ultisols is whether BS is greater than 35% (by sum of cations) at a critical depth, which is determined using solum and argillic horizon thickness, as well as depth to a parent material change (Soil Survey Staff, 2010). Five pedons are clearly Alfisols, those being the pedons in Jefferson, Keokuk, Lucas, Mahaska, and Monroe counties (Tables 2 and 3). The pedon in Marion County is clearly an Ultisol. The two pedons in Davis and Van Buren counties are ambiguous, depending on where the shale parent material is identified and on how the 35% BS rule is interpreted (Table 3). Since the soil moisture regime of all moderately well-drained soils in MLRAs 108 and 109 is udic, these eight pedons are Udalfs and Udults. Given that they have no other diagnostic features, they are Hapludalfs and Hapludults, respectively (Soil Survey Staff, 2010). The presence of redoximorphic features at 40 cm and deeper results in their respective subgroup classification as Oxyaquic Hapludalfs and Oxyaquic Hapludults. To completely classify the soil pedons under study to the family level, the particle size, mineralogy, and CEC classes within the control section of each pedon (Table 4) were determined. Control sections within each pedon were identified per Keys to Soil Taxonomy (Soil Survey Staff, 2010). In brief, Winter 2011 Ca++ Mg++ K+ Na+ CEC ——————————————————cmolc kg−1—————————————————— 10.9 6.97 0.70 0.13 25.3 11.3 10.03 0.65 0.29 32.9 8.6 8.67 0.45 0.36 24.8 8.8 8.50 0.20 0.48 21.5 15.3 2.55 0.30 0.11 20.3 12.2 2.21 0.25 0.10 19.9 16.9 3.06 0.35 0.13 26.6 27.1 3.40 0.40 0.15 40.8 7.9 2.89 0.20 0.12 15.6 3.2 3.06 0.30 0.11 20.0 1.8 3.23 0.35 0.15 20.0 1.6 3.06 0.25 0.14 20.6 2.3 3.23 0.20 0.15 16.0 7.4 2.89 0.20 0.16 16.8 4.5 2.38 0.20 0.18 14.7 5.6 4.76 0.35 0.28 26.2 5.8 5.44 0.35 0.27 31.8 4.6 4.40 0.20 0.29 16.7 3.7 1.53 0.20 0.09 11.0 2.6 1.19 0.15 0.10 9.2 3.2 2.55 0.30 0.29 21.7 1.5 2.04 0.40 0.15 29.1 2.8 2.72 0.40 0.24 29.0 1.1 1.36 0.15 0.12 6.2 8.9 2.38 0.20 0.14 18.0 5.6 2.38 0.35 0.17 27.2 6.2 3.74 0.30 0.18 25.9 12.9 4.42 0.35 0.18 26.7 5.4 4.76 0.20 0.34 23.9 11.6 3.40 0.25 0.11 24.9 13.6 4.93 0.25 0.22 25.1 10.6 5.27 0.25 0.31 23.2 8.2 5.95 0.20 0.38 19.5 9.3 3.40 0.15 0.26 21.9 5.0 3.06 0.15 0.14 23.5 3.0 2.40 0.15 0.15 20.6 1.7 1.50 0.25 0.14 22.7 2.0 1.40 0.30 0.15 20.8 BS % 73.9 67.6 72.9 83.6 89.9 74.0 76.9 76.1 71.2 33.4 27.8 24.5 36.8 63.5 49.3 42.0 37.3 56.8 50.1 43.7 29.2 14.1 21.3 44.4 64.7 31.2 40.2 66.9 44.8 61.7 75.7 70.8 75.4 59.7 35.5 27.6 15.9 18.4 the control section of each pedon started from the top of the argillic horizon. If the argillic horizon thickness was more than 50 cm, only the upper 50 cm of its thickness was used. However, if the argillic horizon thickness was less than 50 cm, its entire thickness was used (Table 4). Family particle-size class was determined by calculating the weighted average of clay content within each pedon’s control section. Results show that the weighted average of clay content ranges from 40 to 60%, which qualifies the particle size class as fine (Tables 3 and 4). Within the control section, family mineralogy class was determined by identifying the clay mineral peaks of the XRD patterns and then quantifying them. To identify the peaks, degrees 2 theta and the concomitant d-spacing were considered and then compared with the d-spacing values given in the literature. There are a variety of layer silicate clay minerals within the control sections of the study pedons. These layer silicate clay minerals (smectite, vermiculite, illite, and kaolinite) were quantified and revealed that some of the soil pedons are mixed while others are kaolinitic. To determine the family activity class, the ratio between CEC and clay content was calculated. Table 4 shows the ratios range from 0.4 to 0.6 in most pedons, resulting in the “active” class. However, the Mahaska pedon has a 0.62 ratio, which qualifies the cation exchange activity class to be “superactive” (Table 4). Four pedons, the Davis, Jefferson, Keokuk, 107 Table 3. Taxonomic classification of the pedons studied in Iowa, USA. Pedon ID Mahaska Monroe Davis Jefferson Van Buren Keokuk Lucas Marion Classification (using the BS% value at the paralithic contact) Classification (using the depth weighted average of BS% from the top of the argillic to the paralithic contact) Fine, smectitic, superactive, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludults Fine, smectitic, superactive, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludults Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, kaolinitic, active, mesic, Oxyaquic Hapludults Fine, kaolinitic, active, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludalfs Fine, mixed, active, mesic, Oxyaquic Hapludults and Van Buren county pedons, are kaolinitic and “active” at the same time. This is attributed to the high content of organic carbon in these pedons within the control section, which ranges from 0.3 to 0.8% (Table 2). The mean annual soil temperature is about 10 to 12°C; thus, the soil temperature class is mesicIn summary, the complete taxonomic classification of the Alfisols group of pedons is fine, mixed, smectitic or kaolinitic, active or superactive, mesic Oxyaquic Hapludalfs (Table 3). The Ultisol pedons are classified as fine, mixed or kaolinitic, active, mesic Oxyaquic Hapludults. Both types of pedons occur in Munterville map units, with the Ultisol pedons representing a minor component that is not named as a separate series. Lithologic discontinuities were evaluated via total sand and sand/silt ratio as depth functions (Fig. 3 and 4) and compared with the trend breaks of lithological discontinuities described in the field. The major landscape position of the Munterville series is the backslope position. Its occurrence in this landscape position results in the upper portion of the sola being formed in till, loess, or local colluvium. Lithologic discontinuities identified using laboratory analyses were compared with those found in the field to find whether they match well and to identify the parent materials in each pedon. To identify lithologic discontinuities in the field, transitions in soil texture were carefully evaluated. Investigators often noted the “soapy” feel of the “shaly stuff.” Depth distributions of total sand indicated lithological discontinuities at the same depths as those detected in field descriptions (Fig. 3). Lithologic discontinuities detected by using depth trends of sand/silt ratio mostly match with those detected in the field (Fig. 4). Table 4. Parameters of the family level in classifying study pedons from Iowa, USA, within their control sections. Both clay content and CEC values were determined as a weighted average within their control sections. Briefly, if the control section contained one horizon, the value of clay content or CEC stayed the same. However, if the control section contained two horizons, each horizon value was multiplied by the thickness of this horizon; the two new values were summed and then divided by the total thickness of the control section. Pedon ID Mahaska Monroe Davis Jefferson Van Buren Keokuk Lucas Marion Control section Clay CEC cm 50 50 41 48 40 48 50 44 % 47 51 43 55 55 59 55 48 cmolc kg−1 29 29 20 29 25 27 24 21 0.62 0.57 0.47 0.53 0.45 0.46 0.44 0.44 50 – 30 – – 5 – – Vermiculite Illite Kaolinite ———————% ——————— 20 5 45 20 – 10 5 15 30 20 15 20 30 40 30 30 25 35 60 80 50 60 30 40 Fig. 3. Lithologic discontinuities in eight pedons from Iowa, USA using total sand as a depth function. Black lines show the position of the discontinuities. Fig. 4. Lithologic discontinuities in eight pedons from Iowa, USA using sand/silt ratio as a depth function. Red lines represent lithologic discontinuities detected in the laboratory that do not agree with those detected in the field, and black lines represent lithologic discontinuities detected in the field that agree with those detected in the laboratory. The XRD patterns of the Lucas and Marion county pedons (Fig. 5 and 6) show clay mineralogy variability between the upper horizons (Ap or Bt1 horizons) and the lower horizons (2Bt2 108 CEC/clay Smectite or 2Cr horizons). For example, in the Lucas County pedon (Fig. 5), the Ap horizon has vermiculite, illite, kaolinite, and lepidocrocite that is slightly different from the mineralogy of both the 2Bt2 and 2C3 hori- SOIL SURVEY HORIZONS Fig. 5. X-ray diffraction patterns of <2-mm clay from the study pedon in Lucas County, Iowa, USA. Fig. 7. Possible topographic positions of Munterville soils and Ultisols represented in the study pedons (Positions 1, 2, and 5 represent Munterville soils, and Positions 3 and 4 represent Ultisols) in Iowa, USA. Conclusions Remapping of Gosport soil map units resulted in the identification of pedons that are Alfisols and pedons that are Ultisols. Both are part of the range of field properties now considered part of Munterville, which is officially a fine, mixed, active, mesic, Oxyaquic Hapludalf. Identification of lithologic discontinuities in these pedons remains problematic because different approaches often result in differing interpretations. References Fig. 6. X-ray diffraction patterns of <2-mm clay from the study pedon in Marion County, Iowa, USA. zons, which have zeolites in addition to what was found in the upper horizons. Both of those horizons, the 2Bt2 and 2C3, do not have any lepidocrocite. However, they have two strong peaks (d-spacing 7.96 and 3.97 Å). These peaks were identified as representing heulandite (a zeolite mineral). Zeolites have been detected as components of sedimentary rocks, especially of volcanic origin (Boettinger and Ming, 2002). Volcanic ash has been recognized in Iowa within till from the Pre-Illinoian glaciations (Boellstorff, 1978). Similarly, Fig. 6 shows the same difference between the upper horizon and the lower horizons of the Marion County pedon. Overall, by comparing the depth at which the clay mineralogy changes with detected lithologic discontinuities in the field descriptions, both were found to match. Considering Smeck and Burras (2006) and Goebel et al. (1989), a suggested stratigraphic landscape was designed showing the possible positions of Alfisol or Ultisol occurrence in the study area (Fig. 7). Munterville may form in positions number one, two, and five because the majority of the sola exist in non-shaly materials or in an interbedded shale and limestone layer that causes the BS to be >35%. However, Ultisols may form in positions three and four because acidic shale participates significantly in determining the properties of the sola that cause the base saturation to be <35% (Fig. 7). 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