Genesis, Morphology, and Classification of Mounded Soils in Eastern Oklahoma1

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Genesis, Morphology, and Classification of Mounded Soils in Eastern Oklahoma1F. P. ALLGOOD AND FENTON GRAY2ABSTRACTNumerous small mounds are unique features of landscapesthat extend from the southern part of Missouri to the coast ofTexas. Many hectares of eastern Oklahoma are included in thisvast area. The mounds occur in densities of 8 to 16/ha.These mounds were studied with special emphasis on thegenesis, morphology, and classification of associated soil pedons.Laboratory measurements included particle size distributions,bulk density, extractable cations, and organic matter ex-tractions.Soil morphological features and laboratory analyses indicatedthat the development of the mounded soils differs from that ofthe associated intermound pedons by being highly altered bythe many organisms of the landscape that assemble in theselected elevated soil to escape seasonal wet soil conditions.Mounded soils of Site I are classified as Aquic Paleudolls orAquic Argiudolls in the present system of soil classification(12). But most of the mounded soils which were studied wouldbe better classified with a vermic modifier.Additional Index Words: Verma, Mima, Faleudoll, Haplu-dalf, Argiudoll.NUMEROUS SMALL MOUNDS are unique features of land-scapes that extend over many hectares in easternOklahoma. This is part of a vast area containing moundsthat extend from the southern part of Missouri to the coastof Texas. They occur in densities of 8 to 16/ha averagingabout 15 m in diameter and nearly 1 m in height and oc-cupy a volume of 75 to 100 m3. Representative moundedand intermound soils were investigated in east central Ok-lahoma (F. P. Allgood, 1972. Genesis and morphology ofmounded soils. M.S. Thesis. Oklahoma State University,Stillwater. It contains an extensive literature review). Theobjectives of this study were to relate the soil forming fac-tors and processes to the soil properties of mounded andALLGOOD & GRAY: MOUNDED SOILS IN EASTERN OKLAHOMA97 102747LEGENDWEST BOUNDARY OF - - MOUNDED SOILS INOKLAHOMA. . . . . . . . .NORMAL ANNUAL TOTALPRECIPITATION. .PP!....S I T E - I STUDY AREA Fig. 1Map showing mounded soil area relative to annual rainfall and location of study area.intermound soils and to classify these soils accurately bya soil taxonomy system. The present mapping units ofmounded phases do not adequately describe these soils.A companion paper will report the ecological relation-ships to the construction of these unique, small "verma"mounds.Special attributes of the mounded soils in eastern Okla-homa may accommodate more specialized cropping thanthe associated intermound soils and should, therefore, becorrectly classified and mapped, thus, enhancing the inter-pretations for suitability potentials for both agricultural andengineering uses of these soils.GEOGRAPHY OF THE STUDY AREASThe field study of these unique mounds was conducted inMuskogee and Mclntosh Counties in Oklahoma (Fig. 1). Theelevation is approximately 180 ms above sea level. A mound ina 5.2 ha meadow in the southern part of Muskogee County wasused as the nucleus of the study and is referred to as Site I.ReliefCommon slopes associated with landscapes contain-ing mounds range from nearly level to gently sloping. Only oc-casionally are landscapes with slopes greater than 5% found tocontain mounds.The general slope associated with Site I is nearly level. Run-off is medium to slow with no discernable drainage pattern.GeologyRocks that lie below the regolith in the landscapeof Site I are of the Boggy unit of the Des Moines series of thePennsylvanian geological system. The Boggy formation aver-ages about 150 m (500 feet) thick in the general area. It con-sists of medium grained, tan to brownish sandstone, siltstone,and gray to dark gray shale (1) (R. A. Meeks, 1957. The geol-ogy of the Onapa-Council Hill area, Muskogee and MclntoshCounties, Oklahoma. M.S. Thesis. Oklahoma University, Nor-man) Everett R. Neff, 1961. Surface geology of MclntoshCounty, Oklahoma. M.S. Thesis. University of Oklahoma, Nor-man.). Shale is encountered beneath the soil solum of Site I.ClimateA. temperate, continental, moist, subhumid type ofclimate prevails in the area where the mounded soils were stud-ied (This is transitional to humid which prevails to the east.Described by Stanley G. Holbrook, State Climatologist of USDep. of Commerce Weather Bureau in Okmulgee County, SoilSurvey, 1968). The mean annual precipitation is 105.51 cmwith a mean annual temperature of 16.6C.The mounds in Oklahoma are closely associated with the 102cm plus rainfall belt (Fig. 1). The soils of the intermound areasof Site I are saturated with water during the spring of each year.As indicated by the degree of soil mottlings, the mounded soilsare not commonly saturated in the upper fringes of the mollicepipedon. The Thornthwaite annual P-E index is about 68 forthe location.PlantsSoils occurring in the study area developed under acover of herbaceous plants consisting mostly of tall grasses. Thesmall upland streams that form the drainage system of the up-land prairies are generally surrounded by trees. Rough brokenareas that lie within 5 to 10 km are also forested with decidu-ous trees. Vegetation measurements were made of Site I whichshowed a greater production of forage on the mounds than onthe intermound soils.TimeThe elapse of time pertaining to soil weathering inthe mound and intermound pedons is considered a constant fac-tor. Knechtel (4) placed the age of the surface containingmounds in the associated region to be no older than late Pleis-tocene. However, enough time has evolved to permit the devel-opment of soils with strong horizonations. These well-developedB2t horizons indicate that soil materials have been subjected toleaching and weathering for a considerable length of time.METHODS AND PROCEDURESMounds sampled were in an area extending between 10 and40 km north of Eufaula in Muskogee and Mclntosh Counties.The area selected for the nucleus of the study was a mound in anearly level meadow containing 46 mounds (Site I) Fig.2).This figure shows the elevation and location of the study moundrelative to other mounds in the field. A backhoe was employedin making excavations in the mound and intermound. Zeromarked the intermound excavation with numbers one throughnine locating pedons in the mound (Fig. 5 shows referenceno. 0 through 5). Soil horizons were marked with string, andcrotovinas and rodent passageways were flagged for ease intaking measurements. Bulk density samples were taken in trip-licate of all soil horizons of Pedons 0 and 3 in Site I and in ninedifferent mounds and associated intermounds of five differentlandscapes.Soil samples or chemical, mineralogical, and physical analy-sis were air-dried and processed to pass a 2-mm screen. Me-chanical analyses were determined by procedures outlined byDay (2). The pH of the soil was determined on a 1:1 soil/water paste and on a 1:1 soil/KCL mixture using a corning pHmeter. Exchangeable sodium, potassium, calcium, and magne-sium were determined by leaching a soil sample with neutralIN ammonium acetate. Sodium and potassium were deter-mined with a Perkin-Elmer #303 atomic absorption spectro-photometer. Calcium and magnesium were measured by theEDTA Method (11).148 SOIL SCI. SOC. AMER. PROC., VOL. 37, 1973Hori-zonDepth,(cm.)oo: Mound30.6 M = Elev. (Highest Point)Scale = 9. l!!-2.*>-4" \Location: NESec3ITI3NRI7E \Muskogee Co. Oklahoma27.3727.44o27.07e26.9880.7 M-^ELEV.Fig. 2Drawing showing location and elevation of moundsin a field of native meadow at Site 1.Soil organic matter was determined by the potassium dichro-mate wet combustion method (9). Exchangeable hydrogen wasdetermined by the barium chloridetriethanolamine method (6).Total phosphorus measurements were made by digestion of thesoil with perchloric (72%) acid (10). The molybdemeous bluecolor was developed by using the ascorbic acid reduction pro-cedure. Bulk density samples were oven dried and bulk densitydeterminations were made in the laboratory (11).MORPHOLOGY OF MOUNDED ANDINTERMOUNDED SOILSSite IIntermoundPedon0Hori-zonAllA12Depth,(cm.)0-1923-46Description(All colors moist unless otherwise stated)Very dark grayish brown (10YR 3/2) siltloam; grayish brown (10YR 5/2) when dry;few distinct dark brown mottles; weak finegranular structure; friable; many fibrousroots; porous; common worm casts; mediumacid; pH 5.5; gradual boundary.Dark grayish brown (10YR 4.5/2) silt loam;light brownish gray (10YR 6/2) when dry;Description(All colors moist unless otherwise stated)common fine distinct reddish brown (SYR4/3) mottles; weak fine granular structure;friable; many fibrous roots; common wormcasts; medium acid; pH 5.5; smooth clearboundary.A21 46-66 Brown (10YR 5/3) loam; very pale brown(10YR 7/3) when dry; many fine distinctyellowish brown mottles; weak medium gran-ular structure; friable; common fine roots;few fine brown concretions; few worm casts;strongly acid; pH 5.0; gradual boundray.A22cn 66-79 Light brownish gray (10YR 6/2) silt loam;light gray (10YR 7/2) when dry; many finedistinct strong brown (7.5YR 5/6) mottles;moderate medium granular structure; fria-ble; few fine roots; many fine and mediumconcretions; porous; few worm casts; stronglyacid; pH 5.0; smooth abrupt boundary.B21tcn 79-109 Grayish brown (10YR 5/2) silty clay loam;light brownish gray (10YR 6/2) when dry;common medium distinct strong brown(7.5YR 5/6) and a few fine distinct red(2.SYR 4/8) mottles; moderate mediumblocky structure; very firm; few fibrousroots; thin continuous clay films on faces ofpeds; common hard brown and black con-cretions; slightly acid; pH 6.0; gradualboundary.B22tcn 109-140 Grayish brown (10YR 5/2) silty clay loam;light brownish gray (10YR 6/2) when dry;common medium distinct strong brown(7.5YR 5/6) and few fine red (2.5YR 4/8)mottles; moderate medium blocky structure;very firm; very few fine roots; thin continu-ous clay films on faces of peds; few fineand medium concretions in soil mass; manyin pockets of pale brown clay loam texturedsoils; slightly acid pH 6.0; gradual boundary.B3cn 140-180 Brown (10YR 5/3) crushed gray (10YR6/1) with fine dark gray and fine distinctyellowish red (SYR 5/6) mottles; silty clay:many, fine and medium, brown and blackconcretions in broad areas and pockets:pockets contain lighter textured lenses oflight brown soil; weak very fine blocky struc-ture; firm; roots rare; mildly alkaline; pH7.5; gradual boundary.C 180-203+ Brown (10YR 5/3) gray (10YR 6/1) siltyclay loam; moderately alkaline; pH 8.0.Site IMounded SoilPedon390Hori- Depth,zon (cm.) Description(All colors moist unless otherwise stated)All 0-48 Very dark grayish brown (10YR 3/2) siltloam; grayish brown (10YR 5/2) when dry;weak fine granular structure; friable; manyfibrous roots; porous; many fine channelsand worm casts; numerous crotovinas 6 to18 cm in diameter; medium acid; pH 5.5;gradual boundary.A12 48-77 Dark grayish brown (10YR 3.5/2) silt loam;grayish brown (10YR 5/2) when dry; few,fine, distinct dark brown (7.5YR 4/4) mot-tles; medium fine granular structure; friablewhen moist; many fibrous roots; porous;many fine channels and worm casts (color ofsome casts lighter than surrounding soil);numerous crotovinas 6 to 18 cm diameter;medium acid; pH 5.5; gradual boundary.A21 77-91 Dark brown (10YR 3.5/3) silt loam; brown(10YR 5/3) when dry; few fine distinct,dark brown (7.SYR 4/4) mottles; mediumfine granular structure; friable; many fibrousroots; porous; many fine channels and wormcasts (color of some casts darker than sur-rounding soil); crotovinas 5 to 10 cm;strongly acid; pH 5.2; gradual boundary.ALLGOOD & GRAY: MOUNDED SOILS IN EASTERN OKLAHOMA 749Table 1Chemical and physical analyses of a typical mounded soil, Aquic Argiudoll (Vermic Paleudoll)Profile Description: Mounded Soli (VeSample no.70-OK-51-3-170-OK-51-3-270-OK-51-3-370-OK-51-3-470-OK-51-3-570-OK-51-3-670-OK-S1-3-770-OK-51-3-870-OK-51-3-9Sample no.70-OK- 51-3-170-OK-51-3-2 .70-OK-51-3-370-OK-51-3-470-OK-51-3-570-OK-51-3-670-OK-51-3-770-OK-51-3-870-OK-51-3-9HorizonpHH205. 22A 23B21TB22TB3C111KC14.34.74.85. Thickness Colorrmlc Paleudolls)(moist)cm0-48 48 10. 0 YR3 5/248-73 25 10. 0 YR 4/273-91 18 10.0YR3*5/391-101 10 10.0YR4/3101-119 18 10.0YR5/4119-137 18 10.0 YR 5/4137-173 36 10.0YR5/4173-227 54 IO.OYR5/4227-230 3 10.0YR6/1Chemical Data: Analyst: D. BakhtarTextureSILSILSILSILSILCCCExtractable cations, meq/100 KCEC8. Base saturationNAAC50.856.168.676.974.574.279.885.684.1Sum of cat.46.759.469.380. 176.278.884.289.289.8OM%1.861.450.550.470.460.580.540.500.47Totalpppm291.1257.3179.2213.7206.1205.4280.2338.0316.2Physical Data: Analyst; D. BakhtarSand suberactlonsSample no.70-OK-51-3-170-OK-51-3-270-OK-51-3-370-OK-51-3-470-OK-51-3-570-OK-51-3-670-OK-51-3-770-OK-51-3-870-OK-51-3-9Sand16.116.715.719.38.813.29.711.214.4Silt74.176.774.974.782.267.452. 4 ..51.948.7Clay9.>2mm4. 16.47.311.0Interpretive CalculationsClay free particle size distributionHori-zonSample no.70-OK-51-3-170-OK-51-3-270-OK-51-3-370-OK-51-3-470-OK-51-3-570-OK-51-3-670-OK-51-3-770-OK-51-3-870-OK-51-3-9Depth,(cm.)Ca/mg0.540.500.530.460. 100.85Depth,(cm.)MS0.690.660.740.590.530.660.600.990.83FS2.843.002.522.872.622.352.233.012.93VFS10.4311.7810.8814.434.1110.0110.2211.5717.50Description(All colors moist unless otherwise stated)A22 91-102 Dark brown (10YR 4/3) silt loam; brown(10YR 5.5/3) when dry; few fine distinctdark brown (7.SYR 4/4) mottles; weak finegranular structure; friable; common fibrousroots; porous; few fine concretions; manychannels and worm casts (colors of somecasts darker than surrounding soil); croto-vinas 10 X 15 cm; strongly acid; pH 5.0;gradual boundary.A23cn 102-119 Yellowish brown (10YR 5/4) silt; very palebrown (10YR 7/4) when dry; common fineand medium, distinct dark brown (7.SYR4/4) mottles; weak, fine, granular structure;friable; many fine and medium, yellowishbrown and very dark gray concretions; por-ous; few fibrous roots; many worm casts;slightly acid; pH 6.0; clear smooth boundary.B21tcn 119-137 Yellowish brown (10YR 5/4) heavy siltloam; very pale brown (10YR 7/4) whendry; common fine distinct yellowish brown(10YR 5/6) mottles; moderate fine blockystructure; very firm; few roots; patchy clayfilms on faces of peds; many fine and me-dium brown and black concretions; slightlyacid; pH 6.0; gradual boundary.B22t 137-173 Composed of colors, yellowish brown (10YR5/4), grayish brown (10YR 5/2) and specksof reddish brown (SYR 4/4); silty clayloam; weak, medium subangular blockystructure; very firm; very few roots; thincontinuous clay films on faces of peds; com-mon, medium, and fine, brownish and blackconcretions occur in pale brown silty clayloam pockets; neutral; pH 7.0; gradualboundary.B3 173-226 Composed of colors, yellowish brown (10YR5/4), gray (10YR 6/1) with very dark graystains; silty clay loam; weak, medium blockystructure; very firm; roots are rare; scatteredbrown and black concretions; alkaline; pH7.5; gradual boundary.C 226+ Gray (10YR 6/1) with coarse brown (10YR5/4) mottles; silty clay loam and shale; mas-sive; moderately alkaline; pH 8.0.RESULTS AND DISCUSSIONChemical AnalysesChemical analyses of the mounded soil, Pedon-3 andintermound, Pedon-0, are shown in Tables 1 and 2.Hydrogen Ion ConcentrationThe pH values of the soilhorizons of the mounded soil show a gradual increase inpH or decrease in acidity from the surface down, whilethat of the intermound deviates with a pH of 6.0 in theA22 horizon.Organic MatterThe maximum organic matter content750 SOIL SCI. SOC. AMER. PROC., VOL. 37, 1973Table 2Chemical and physical analyses of a typical intermound soil, Albaquic HapludalfProfile Description:Sample no.70-OK-51-0-170-OK-51-0-270-OK-51-0-370-OK-51-0-470-OK-51-0-570-OK-51-0-670-OK-51-0-770-OK-51-0-8Sample no.70-OK-51-0-170-OK-51-0-270-OK-51-0-370-OK-51-0-470-OK-51-0-570-OK-51-0-670-OK-51-0-770-OK-51-0-8HorizonAHA12A21A22B21TB22TB3CNCpH lilH20 KC15. 9 4. 96. 2 4. 86. 1 4. 56. 0 4. 56. 1 5. 16. 8 5. 67. 0 6. 07. 1 6. 1Intermound Soil (Albaquic Hapludalfs)Depth Thicknesscm0-23 2323-46 2346-66 2066-79 1379-109 30109-139 30139-179 40178-216 38Chemical Data: AnalytColor (moist)10.0 YR 3/210.0 YH 4/210. 0 YR 5/310.0 YR 6/210. 0 YR 5/210. 0 YR5/210. 0 YR 5/310. 0 YR 5/3rt: D. BakhtarTextureSILSILSILSILCCCSCIExtractable cations, meq/100 g % BiCEC9. Ca3. 18 2. 261.52 2.381. 26 1. 580. 73 1. 244.23 8.451.91 7.361. 55 9. 950.99 10.79PhysicalMg. 2.801.811.361.848.319.0311.139.57K0. Analyst: D.Na0.150.240.300.572.833.004.153.80BakhtarAl0.140.110.530.470. saturationSum of cat.62.774.872.583.782.691.294.496.2OM%2.261.611.100.570.740.960.510.39Totalpppm214.8193.7161.8152.3145.0183.2184.6201.4Sand suberactlonsSample no.70-OK-51-0-170-OK-51-0-270-OK-51-0-370-OK-51-0-470-OK-51-0-570-OK-51-0-670-OK-51-0-770-OK-51-0-8Sand15.615. 76.879.248.878.953.955.149.052.7Clay Texture7.>2mm0.51.35.616. % CalculationsClay free particle size distributionSample no.70-OK-51-0-170-OK-51-0-270-OK-51-0-370-OK-51-0-470-OK-51-0-570-OK-51-0-670-OK-51-0-770-OK-51-0-8Ca/mg0.811.311.170.671.020.820.891.13CEC/Clay123. 20142.4184.2575.1269.1561.4766.3959. 19Silt83.1184.0753.0587.4084.8786.4583.5885.98VCS0.570.871.582.461.160.511.020.79CS1. 0.760.960.61MS0.650.620.560.590.790.661.110.65FS2.422.352.502.262.462.563.032.51VFS12.3611.0541.286.3210.019.3710.649.79is in the surface horizons (Fig. 3a and 3b). A secondprominent accumulation is in the upper part of the argillichorizon of the intermound pedon. This illuviated accumu-lation is an indication of transformation of organic matterand transfer of humus along with illuviated clay to theargillic horizon.Accumulations of organic matter are increased in croto-vinas because of the higher concentration of residue de-posited as rodent bedding or from "downwash" materialfrom the surface. Crotovinas are numerous in the epipe-dons of the mounded soils but are absent from the argillichorizon. Crotovinas are not present in the intermoundpedon of Site I.The production per hectare of herbaceous material onthe mounds was 4,997 kg compared to 3,227 kg from theassociated intermound areas. This could imply that agreater amount of organic residues may be returned to themounded soils. This source of organic matter coupled withhighly porous surface horizons and a humid warm climatepresent conditions for a rapid mineralization of the or-ganic matter. This along with high base saturation may ac-count for the higher organic matter contents in the imme-diate surface of the intermound pedon than the moundedsoil pedon. Also, the intermound soil may receive organicmatter in runoff from the mounds.Extractable Cations, Cation Exchange Capacity, andBase SaturationExtractable cations are dominated by cal-cium and magnesium in both soils (Tables 1 and 2). Po-tassium is very low throughout the pedons while sodiumincreases with depth. The cation exchange capacity (CEC)correlates highly with the clay distribution of the soils.However, where cation exchange capacity/clay ratios arecompared, some irregularities can be seen (Fig. 4a and4b). The cation exchange capacity percent clay ratiosshow more clay mineral stratification in the mounded soilthan in the intermound soil. This may pertain to differ-ences in illuviated fine clays which are dominated by mont-morillonite or interstratified illite and montmorillonite.Sodium increases in the B2t horizons of both profiles.The exchangeable sodium of the mounded soil increasesto a maximum of 2.45 meq/100 g in the C horizon,whereas that of the intermound soil increases to a maxi-mum of 4.15 meq/100 g in the B3cn. The excessive so-dium may be from the weathering of sodium feldspars andthe leaching of it to lower depths. Sodium clays are impor-tant to the development of "puffs" in gilgai in Australia(3). The profile descriptions show glossic tongues to ex-tend upward in the pedons (Fig. 5). These gray tonguesthat interfinger the argillic horizon contain bleached soilthat is similar in texture and color to the A2 horizon.These tongues contain many concretions. This suggests thatthere has been much shrinking and swelling of the clays inthe previous years. The tongues have been in place for anextended period as indicated by many medium size blackALLGOOD & GRAY: MOUNDED SOILS IN EASTERN OKLAHOMA 7513 PERCENT ORGANIC MATTER (I) D .PERCENT ORGANIC MATTER (M)0 1 2 30 1 210CATION EXCHANGE CAPACITY/CLAY (M)20 30 40 50 60 70 80 90 100A12 46B21T109B22T139B3CN179A21 91A22 101B22T17383 227C 230Fig. 3a, Computer-graph showing organic matter distribu-tion in intermound soil, Pedon 0; b, Computer-graph show-ing organic matter distribution in mounded soilPedon 3.concretions lining the interior of the silt textured tongues.This suggests that the fine clays moved down first, fol-lowed by the coarser, less expanding clays which formed aseparation between the tongues and the albic horizon. Sincethe tongues do not extend into the upper part of the argillichorizon, they are not emphasized in classification.Percent base saturation exceeds 50% throughout the soilprofile of the mound, while that of the intermound de-creases to a 49.2% in the A21 horizon. These high basesaturations may be significantly related to the increasednumber of worm casts in the A2 horizons of the moundedsoils and to the effects of rodent activity in their surfacesoils. Movement of soil from one horizon to another, cou-pled with earthworm digestive effects on soil, are processesthat could affect soil development (7). The many wormcasts about the mound agree with the higher concentrationof earthworms in the mound (Pedon 3). Results of analy-ses show a greater increase in soluble or available phos-phorus, potassium, nitrogen, calcium, and magnesium inworm casts (5). Calcium in worm casts is mobile. Whenthis is considered, the analyses may be interpreted to fur-ther substantiate the concentration of earthworms in themound. Percentages of calcium are less in the A horizonsof the mounded soil than in the intermound soil but in-crease to a higher percentage with depth. Magnesium may beless mobile than calcium. Magnesium averages much higherin the mounded soil pedon than in the intermound pedon.Cm iAHA 1 2 7 3 IA21 91 iA22 101 iA23119]B21T137'iB22T173'B3227,..C230 1"CATION EXCHANGE CAPACITY/CLAY RATIO 11]0 10 20 30 40 50 60 70 80 90 100A11 23 >;A1246J| jA21 66 !"i"A22 79 jjjB21T109 jj;B22T139JJJi"B3CN179J';'i