Magnetic Susceptibility for Use in Delineating Hydric SoilsD. A. Grimley* and M. J. Vepraskas

ABSTRACTField indicators are used to identify hydric soil boundaries and to

delineate wetlands. The most common field indicators may not beseen in some soils with thick, dark, mollic epipedons, and do not formin Fe-poor soils. This study evaluated magnetic susceptibility (MS)meter as a field tool to determine hydric soil boundaries. Five Mollisol-dominated sites formed in glacial deposits in Illinois were evaluatedalong with one Ultisol-dominated site formed in Coastal Plain sedi-ments of North Carolina. Measurements of volumetric MS were madealong transects at each site that extended from wetland into uplandareas. One created wetland was evaluated. Field indicators were usedto identify the hydric soils. Results showed that volumetric MS valueswere significantly (P < 0.01) lower in hydric soils than in nonhydricsoils formed in glacial deposits. Volumetric MS values also decreasedslightly with depth. In a created marsh, significant differences in MSwere found between hydric and nonhydric soils at 15 to 30 cm but notat shallower depths where topsoil from a wetland had been artificiallyapplied. No significant (P > 0.15) differences in MS were found forCoastal Plain hydric and nonhydric soils where MS values were low(<10 X 10~s SI). Critical MS values that separated hydric and nonhy-dric soils varied between 20 X 10~s and 30 X 10"5 SI for the loessal soilsevaluated in Illinois. Such critical values will have to be determined onsite using field indicators until specific values can be defined for hydricsoils within a given parent material. With a critical MS value in hand,a wetland delineator can make MS measurements along transectsperpendicular to the envisioned hydric soil boundary to quickly andquantitatively identify it.

JURISDICTIONAL WETLANDS are areas that contain hy-drophytic vegetation, wetland hydrology, and hydric

soils (Environmental Laboratory, 1987). Hydric soilsare saturated, flooded, or ponded long enough duringthe growing season to develop anaerobic conditions inthe upper part (USDA-NRCS, 1998). They are identi-fied on site by specific field indicators that formed inthe upper 30 cm when the soils were anaerobic andchemically reduced. The field indicators are in mostcases layers of soil that have specific thicknesses, colors,and depths. At least 35 hydric soil field indicators havebeen identified to date which are used for identifyinghydric soils in every portion of the USA (USDA-NRCS, 1998).

The most common field indicator is the depleted ma-trix, which forms by the reduction, translocation, andoxidation of Fe oxides and hydroxides. This indicatoris a layer of soil that generally must be 15 cm or morethick with an upper boundary within 25 cm of the soilsurface, and have a Munsell value of 4 or more andchroma 2 or less (USDA-NRCS, 1998). The depleted

D.A. Grimley, Illinois Dep. of Natural Resources, Illinois State Geo-logical Survey, 615 E. Peabody Dr., Champaign, IL 61820; and M.J.Vepraskas, North Carolina State Univ., Dep. of Soil Science, Box7619, Raleigh, NC 27695-7619. Received 7 July 1999. Correspondingauthor (grimley@isgs.uiuc.edu).

Published in Soil Sci. Soc. Am. J. 64:2174-2180 (2000).

matrix is difficult to see in soils with mollic epipedons,which are dark in color and may be >60 cm thick. Thedark color virtually masks the gray color of the indicator.In Ultisols, regardless of drainage condition, the samegray colors found in a depleted matrix frequently occurin E horizons of nonhydric soils. Depleted matrices inE horizons of hydric Ultisols are required to have Feconcentrations in quantities that exceed 2% (USDA-NRCS, 1998). This requirement is difficult to meet incoarse-textured E horizons because of a general lack ofFe. For example, some sandy Aquults are known to beseasonally saturated and reduced, but simply do notmeet a hydric soil field indicator as currently defined(Hayes, 1998).

While hydric soil field indicators are powerful toolsfor hydric soil identification, other tools may be usefulfor identifying hydric soils in areas that lack visible,distinctive, color patterns that are diagnostic of fieldindicators. Measurements of magnetic susceptibilitymay be one such tool. Volumetric MS is the ratio ofinduced magnetization in a volume of material to theintensity of the magnetic field applied to that volume.Measurements of MS are reproducible, nondestructiveto samples, and can be made in <15 s in the field.Factors and processes that affect magnetic susceptibilityin soils were reviewed by Mullins (1977) and Maher(1986). In general, a soil's magnetic susceptibility is di-rectly related to the concentration of strongly magneticminerals present, their grain sizes, and to some extenttheir grain shape. Most MS changes in soils are dueto variations in the concentrations and grain sizes ofmagnetite and maghemite. Bulk density can also affectthe volumetric MS of soils, but usually it is only a minorfactor (Grimley, 1996).

In C horizons of glacial deposits in Illinois, the con-centration of depositional silt-size or sand-size magne-tite is largely responsible for the MS signal (Jones andBeavers, 1964; Grimley, 1996). In soil sola, secondarymagnetite and/or maghemite (commonly <0.03 Jim indiam.) also contribute to the MS signal, particularly inthe A horizons of well-drained Mollisols and Alfisols(Maher and Thompson, 1995; Dearing et al., 1996; Grim-ley, 1996; Maher, 1998).

Williams and Cooper (1990) found that MS valuesvaried among soils that formed from the same parentmaterial across different landscape positions. Magneticsusceptibility values are lowest in soils that have beenseasonally saturated and Fe-reduced compared withsoils that have been better drained and not Fe-reduced(Le Borgne, 1955; Vadyunina and Babanin, 1972; Yuet al., 1986; Maher, 1986; Mullins, 1977; Grimley, 1996;Maher, 1998). In Fe-reduced soils, ferrimagnetic miner-als dissolve under microbial influence following reduc-

Abbreviations: ANOVA, analysis of variance; MS, magnetic suscepti-bility; OC, organic C.

2174

GRIMLEY & VEPRASKAS: MAGNETIC SUSCEPTIBILITY FOR DELINEATING HYDRIC SOILS 2175

tion of Fe(III) to Fe(II) (Maher, 1986; Grimley, 1996).Magnetic depletion is especially prevalent in stronglyreduced, anaerobic soils in the presence of organic mat-ter (Maher, 1986, 1998). Silt-sized anthropogenic mag-netite has dissolved from strongly acidic and reducedbogs in Scotland within 150 yr (Williams, 1992). In oxi-dized soils, silt-sized magnetite probably remains stablefor thousands of years before it is oxidized to maghemiteor hematite.

The differential stability of magnetically susceptibleminerals in reduced and oxidized soils suggests that MSmeasurements made in situ could be used to separatehydric and nonhydric soils. The objective of our researchwas to compare changes in volumetric MS values acrosshydric soil boundaries to changes in the hydric soil fieldindicators. If MS values can be correlated with hydricsoil field indicators, then MS will have potential in delin-eating hydric soils in the field.

MATERIALS AND METHODSParent Materials and Soils

Six sites in two geographic regions, northeastern Illinois andeastern North Carolina, were studied to determine whethervolumetric MS measurements could separate hydric and non-hydric soils that formed in different parent materials. Location,parent materials, and soil classifications found at each site areshown in Table 1. The five sites in Illinois (Sites 1-4 and 6)were located in Lake County, =80 km north of Chicago, inLand Resource Region M (USDA-SCS, 1981). This portionof the midwestern USA was glaciated several times during theQuaternary Period, and the modern soils developed duringapproximately the last 11 000 yr in sediments deposited duringthe late Wisconsin Episode of glaciation (Hansel and Johnson,1996; Paschke and Alexander, 1970). Mean annual precipita-tion is 81 cm. Soil sola are generally 0.6 to 1 m deep, andargillic horizons in the Alfisols and Mollisols of the regioncontain carbonates in most places (Paschke and Alexander,1970). Parent material sediments include glacial till, lacustrinedeposits, and outwash all covered by a thin (<l-m) layer ofloess. Relatively unaltered (C horizon) loess in Illinois typi-cally contains 0.05 to 0.35% magnetite in silt fractions andhas a volumetric MS value between approximately 30 X 10~5

and 80 X 1XT5 SI units (Grimley, 1996; Grimley et al, 1998).Each of these sites have areas of deep marsh containing cattail(Typha latifolia L.) and bulrushes (Scirpus spp.).

Site 4 was a created deep marsh that was 5 yr old at thetime of sampling. To create the marsh, the A horizon wasremoved and stockpiled from an area of =0.5 ha. A basin was

then excavated that was up to 2 m below the original surfacewith sides that sloped to the upland. The basin and adjacentupland were then covered to a depth of 15 cm with the Ahorizon material that was originally covering the area. Themarsh was filled with water to a depth of =0.6 m. Wetlandplants consisting of broad-leaved cattail and bulrushes wereallowed to develop naturally. Details of the construction werereported previously (Vepraskas et al., 1999). The study areaincluded the modified upland (nonhydric) soils that sur-rounded the marsh, as well as hydric soils that were adjacentto the marsh but not inundated, and also soils in the marshthat were under water.

Site 5 was located on the Lower Atlantic Coastal Plain nearGreenville, NC, in Land Resource Region T (USDA-SCS,1981). The site is on the Wicomico morphostratigraphic unitwhich lies =7.6 m above mean sea level (McCortan et al.,1984). Soil parent materials are fluvial and marine sedimentsof mid-Pleistocene age that were deposited during a periodof high sea level. The sediments have been exposed to weather-ing for between 220 000 and 440 000 yr (Daniels et al., 1978).Black, opaque minerals (ilmenite and magnetite) occur pri-marily in the very fine sand fraction where they occupy =0.2%or less of the bulk volume (Daniels et al., 1975). Based onthe low MS values for soils in this region, we believe thatilmenite, a weakly magnetic mineral, is more abundant thanmagnetite, which is a strongly magnetic mineral. Mean annualprecipitation is 121 cm. Soil sola in the Ultisols of this regionare more than 1 m thick and do not contain carbonates.

TransectsAt Sites 1 through 5 hydric soil field indicators were identi-

fied and volumetric MS measurements made along transectsof plots that included the wetland center, edge, and the upland.Most transects included at least two plots with hydric soils;however, at Site 5 each transect contained only one plot witha hydric soil. One transect with four plots each was used forSites 1 to 3 (natural marshes), two transects were used at Site4 (created marsh), and four transects were used at Site 5 (wetmineral flat) with three plots per transect. Along each transectin Sites 1 to 3, a hole was excavated with a spade to a depthof =40 cm and an undisturbed vertical section of soil wasremoved. Profile descriptions were completed that includedhorizon depth, type, textural class, matrix color, and redoxi-morphic feature color and abundance (Vepraskas, 1996). Hy-dric soil field indicators were identified from the profile de-scriptions (USDA-NRCS, 1998). If an indicator was found, thesoil profile was considered to be from a hydric soil; otherwise itwas considered to have come from a nonhydric soil.

After the soil profile was described, volumetric MS mea-surements were made in the field for Sites 1 to 3. Triplicate

Table 1. Location of study sites and soil classifications for hydric and nonhydric soils.Soil classification

Site Location Parent materials Nonhydric Hydric

123

4

5

6

87° 56' W, 42° 25' N87° 56' W, 42° 26' N87° 54' W, 42° 19' N

87° 56' W, 42° 26' N

77° 26' W, 35° 34' N

87° 56' W, 42° 26' N

Natural Wet Meadows, IllinoisLoessLoessLake Plain and Till

Created Deep Marsh, IllinoisGlacial outwash and Till

Wet Mineral Flat, North Carolina

Tj pic HapludalfOxyaquic HapludollUdollic Epiaqualf

Typic Hapludalf

Coastal Plain Sediment Aquic PaleudultTest site for hydric soil delineation, Illinois

Glacial outwash and loess Mollic Endoaquept

Cumulic EndoaquollCumulic EndoaquollVertic Endoaquoll

Mollic Endoaquent

Typic Paleaquult

Typic Endoaquoll

2176 SOIL SCI. SOC. AM. J., VOL. 64, NOVEMBER-DECEMBER 2000

Table 2. Soil properties which were averaged across the 0- to 15-cm depth range of each site.Nonhydric Hydric

Site

123456

Horizon

AAAAAA

Color(moist)

10YR 2/12.5Y 2.5/12.5Y 2.5/12.5Y 3/22.5Y 2.5/12.5Y 2.5/1

Texturalclass

Silt loamSilt loamSlit loamSilt loamSandy loamSilt loam

Organic C

3.23.73.4

10.42.93.7

PH

7.07.16.17.85.07.1

Horizon

OAAA

Oa/AAA

Color(moist)

5Y 2.5/12.5Y 3/12.5Y 2.5/1N 2.5/12.5Y 2.5/1N 2.5/0

Texturalclass

Mucky silt loamSilty clay loamSilt loamMucky silt loamSandy loamSilt loam

Organic C

4°45.92.7

12.86.15.9

pH

7.16.87.17.35.06.8

Fieldindicatorf

A10, FlF5, TF7F4F3, FlF13F3, F4, F5

t Field indicators are defined in Table 3.

measurements were made for each depth interval for depthsof 0 to 15 and 15 to 30 cm using a Bartington magnetic suscepti-bility meter, model MS2, and an MSF probe (Bartington In-struments Ltd., Witney, Oxford, UK). The soil was smoothedwith a knife prior to making a MS measurement. The probetip (1.5-cm diam.) was placed directly on the soil used fordetermination of field indicators, which was either on soil inan excavated pit or on the center of a sufficiently large horizonsection (>10 cm on a side) of an extracted soil profile. Thesensor in the probe tip emitted a 0.0001-T alternating magneticfield and the operating frequency of the probe was 580 Hz.Each MS measurement was made for a 1-s count time.

For Sites 4 and 5 profile descriptions were made from pitsdug at plot locations. Samples were collected from depths of0 to 15 and 15 to 30 cm. The samples were oven-dried (105°C)and ground to pass a 2-mm-mesh sieve. Because samples werecollected when soils were unsaturated and oxidized, dryingshould not have altered the amounts of magnetic Fe mineralspresent. The ground samples were placed in shallow contain-ers, and MS measurements were made using the same probedescribed above.

Site 6 included a natural deep marsh in Illinois and wasstudied to test the utility of a method to assess hydric soils inthe field by measuring MS directly on the soil surface andwithout identifying field indicators at all plots. Low-lying vege-tation was avoided. Magnetic susceptibility measurementswere made along three transects that contained five to sevenplots. Three volumetric MS measurements, spaced <20 cmapart, were averaged at each location. Hydric soil field indica-tors were identified along two of the transects to compute acritical MS value that could be used to assess the hydric soilboundary around the wetland using additional transects.

Principal soil horizons from all sites were characterized bydetermining textural class, organic C concentration, and pH.

Textural class was determined by hand in the field in order toselect the proper field indicator. In cases where hand-texturingwas inconclusive, samples were brought to the laboratory andthe clay percentage was determined by the pipette method ofGee and Bauder (1986). Organic C content was determinedon samples from surface horizons by dry combustion (Nelsonand Sommers, 1982). Soil pH was determined electrometri-cally using a 1:1 soil/water ratio.

Data AnalysesThe MS data were analyzed using analysis of variance (AN-

OVA) procedures for a randomized complete block design(SAS Institute, 1996). Treatments were considered to be thehydric or nonhydric soil classification, and transects were repli-cates. The triplicate MS measurements made by depth wereaveraged for each plot prior to applying ANOVA procedures.Measurements made at depths of 0 to 15 and 15 to 30 cmwere analyzed separately. The ANOVA procedures were usedto determine whether mean MS values for each site and depthwere significantly different between hydric and nonhydricsoils.

RESULTS AND DISCUSSIONSelected soil properties and hydric soil field indicators

are shown in Table 2. The field indicators are definedin Table 3. Sites 1 through 4 and Site 6 had primarilysilt loam surface horizons in both hydric and nonhydricsoils. The sandy loam textural class found at Site 5 istypical for soil surface horizons in the Atlantic CoastalPlain region. The ranges in pH reflect parent materialand climatic differences. Organic C levels are within the

Table 3. Hydric soil field indicators identified at the sites. Definitions were extracted from USDA-NRCS (1998) and are not necessarilythe complete definition of the indicator, but simply the portion that pertained to the sites. Indicator TF7 is for testing purposes only.

Indicatorsymbol Name Definition

A10

FlF3

F4

F5

F13

TF7

2 cm muck

Loamy mucky mineralDepleted matrix

Depleted below darksurface

Thick dark surface

Umbric surface

Thick dark surface 2/1

A layer of much (sapric material) 2 cm or more thick with value 3 or less and chroma 1 or less startingwithin 15 cm of the soil surface.

A mucky modified mineral layer 10 cm or more thick starting within 15 cm of the soil surface.A layer at least 15 cm thick with a depleted matrix that has 60% or more chroma 2 or less starting

within 25 cm of the soil surface. Two percent or more redox concentrations are required if thevalue/chroma are 4/1, 4/2, or 5/2.

A layer at least 15 cm thick with a depleted matrix that has 60% or more chroma 2 or less startingwithin 30 cm of the surface. The layer(s) above the depleted matrix have value 3 or less andchroma 2 or less.

A layer at least 15 cm thick with a depleted matrix starting below 30 cm of the surface. The layer(s)above the depleted matrix have hue of N and value 3 or less to a depth of 30 cm and value 3or less to a depth of 30 cm and value 3 or less and chroma 1 or less in the remainder of the epipedon.

A layer 15 cm or more thick starting within the upper 15 cm of the soil surface with value 3 or lessand chroma 1 or less immediately underlain by a layer 10 cm or more thick with chroma 2 or less.

A layer at least 15 cm thick with a depleted matrix starting below 30 cm of the soil surface. The layer(s)above the depleted matrix have hue of 10YR or yellower, value 2.5 or less to a depth of 30 cmand value 3 or less and chroma 1 or less in the remainder of the epipedon.

GRIMLEY & VEPRASKAS: MAGNETIC SUSCEPTIBILITY FOR DELINEATING HYDRIC SOILS 2177

Magnetic Susceptibility(x 10'5 SI units)10 20 30 40

Plot PositionIn wetlandEdge of wetlandUpland

Fig. 1. Volumetric magnetic susceptibility (MS) values with depthfrom Site 2 in Lake Co., IL. Each data point is the average of threeMS readings. The distinction between hydric and nonhydric soilsbased on MS values changes with depth, decreasing =20 to 30%in the upper 25 cm of soil. Thus, critical MS values must be mea-sured at a consistent depth for accurate delineation.

ranges expected for soils in the regions studied. Mostfield indicators described are the common ones used todelineate hydric soils in the midwestern and southeast-ern USA. Field indicator TF7 has not been officiallyadopted for routine use by the USDA-NRCS, but it wasfound in some of the hydric soils identified in this study.

As shown in Fig. 1, volumetric MS values varied withdepth and so MS values could only be compared forthe same depth across the landscape. Comparison ofthe MS values between hydric and nonhydric plots areshown in Table 4. Significant differences (P < 0.01) inMS were found between hydric and nonhydric soilswhen sites were grouped by parent material and thetwo depths were analyzed separately. Sites 1 through 3,which contained areas of natural marsh, were analyzedas a group because mean MS values for both hydric andnonhydric soils did not differ significantly among these

Table 4. Statistical analyses comparing magnetic susceptibilityvalues between hydric and nonhydric soils. Hydric soilsmet field indicators whereas nonhydric soils did not.

Volumetric magnetic susceptibilityHydric soils Nonhydric soils

Site(s) Depth Number Mean SD Number Mean SD P > Fx 10-5 SI x 10-

Natural marshes in loessal soils (IL)SI

1-31-3

44

55

0-15 7 10 4 4 58 3215-30 7 8 2 4 4 8 2 8

Created deep marsh in modified loessal soils (IL)0-15 5 13 3 3 19 7

15-30 5 10 3 3 14 2Wet mineral flats in Coastal Plain sediments (NC)

0-15 5 2 1 7 6 415-30 5 1 1 6 6 3

0.00390.007

>0.200.0001

0.170.19

sites. Sites 4 and 5, which contained the created marshand the Coastal Plain landscape, respectively, were ana-lyzed individually because they both had lower MS val-ues than the other sites.

At Sites 1 through 3, significant differences (P <0.01) were found between the MS values of hydric andnonhydric soils. The nonhydric soils had the greatest MSvalues of any group analyzed, indicating the presence ofa larger quantity of strongly magnetic minerals. Thissuggests that these nonhydric soils have not been satu-rated and reduced within 30 cm of the surface for signifi-cant periods in the recent past, but the exact durationof these better drained conditions is not known.

At the created wetland site, Site 4, no significant dif-ference (P > 0.20) in MS was found between the hydricand nonhydric soils at a depth of 0 to 15 cm. All soilshad been capped with the same A horizon material 5yr prior to this study. Insufficient time had elapsed fordifferences in MS to develop between the two soilgroups. For the 15- to 30-cm depth, significant differ-ences (P < 0.01) in MS were found between hydric andnonhydric soil. The nonhydric soils had low MS valuesfound at 15 to 30 cm. The reason for this is that =30 cmof the original soil had been removed during construc-tion. As shown in Fig. 1, MS values of nonhydric soilstend to approach those of hydric soils below depths of30 cm in these landscapes. Results from Site 4 indicatethat separating hydric from nonhydric soils in areas ofconstructed wetlands will be more difficult than in natu-ral soils.

At all depths, soils of the wet mineral flats that wereon the Atlantic Coastal Plain in North Carolina hadlower MS values than their midwestern counterparts(Table 4). Surficial sediments in the Coastal Plain con-tain much less magnetite than Illinois loess because theloess had a fresh glacial source of igneous and metamor-phic minerals (Grimley et al., 1998), and the CoastalPlain sediments had a much longer time for postdeposi-tional weathering (Grimley et al., 1998; Daniels et al.,1978). Coastal Plain sediments were deposited in essen-tially a wetland environment that probably caused somemagnetite originally in the sediments to dissolve (Dan-iels et al., 1975). The nonhydric soils developed onlyafter stream dissection had lowered the water table,which may have further removed dissolved Fe(II) fromthese materials. Furthermore, magnetic mineral neofor-mation, which can greatly increase the MS of nonhydricsoils, is inhibited in soils with pH values <5.5 (Maher,1998), such as those in NC (Table 2). Although therewere small differences in MS values, it is doubtful thatMS measurements will be able to reliably separate hy-dric from nonhydric soils in these soils in this portionof the Atlantic Coastal Plain.

Critical Magnetic Susceptibility Values forHydric Soil Delineation

To identify hydric soil boundaries on site, field indica-tors are examined along transects that extend frompoints of known hydric soil into the upland area. Whenproceeding upslope from a hydric soil, the boundary of

2178 SOIL SCI. SOC. AM. J., VOL. 64, NOVEMBER-DECEMBER 2000

0)

O

OO

O

Hydric

Non - Hydric

10 15 20 25 30 35 40 ' 90 95

Magnetic Susceptibility(x 10-5 SI units)

Fig. 2. Distribution of magnetic susceptibility values collected for the0- to 15-cm depth range in and across the edge of three naturalmarshes formed in glacial deposits (Sites 1-3). Each MS observa-tion is an average of two or three depths (between 0 and 15 cm),with three measurements made at each depth. From this simplehistogram it can be seen that an MS value of 20 X 10 5 SI couldbe used to separate hydric and nonhydric soils in this landscape.This value is approximately the mean MS value of the hydric soilsplus three standard deviations.

the hydric soil is placed at the point where field indica-tors are no longer observed. Magnetic susceptibilitymeasurements can be used in the same way as long asthe critical MS value that separates hydric from nonhy-dric soils in a given landscape is known. Such criticalvalues have to be determined on site. Figure 2 showsthe frequency distribution of MS values (0-15 cm depth)for both hydric and nonhydric soils at Sites 1 to 3. Thissimple plot shows that an MS value of 20 X 10~5 SI atthe 0- to 15-cm depth could be used to separate hydricand nonhydric soils along the transects. This criticalvalue could be computed statistically (mean plus threeSDs), but such accuracy may not be necessary for rou-tine hydric soil delineation in these landscapes.

We tested this concept by identifying hydric soils inan area adjacent to a natural marsh in Illinois (Site 6,Table 1). This site was selected because the changes intopography were small, and the Mollisols present hadfew hydric soil field indicators. The MS values weremeasured along three transects. Along two of the tran-sects (labeled I and II in Fig. 3), MS values were com-pared with the field indicators to determine the criticalMS value that separated hydric and nonhydric soils atthe site. The MS measurements were made by simplyplacing the probe on a natural soil surface that wassmoothed by light scraping. Data collected on both fieldindicators and MS values are shown in Fig. 3 for Tran-sects I and II, and the MS values are plotted in Fig. 4for both hydric and nonhydric plots. A critical volumet-ric MS value for delineation of hydric soils would be30 X 10"5 SI at this site. A slightly higher critical MSvalue here is partially due to measurements being madeon the undisturbed soil surface rather than a compositeof measurements across the 0- to 15-cm depth range.

f

Marsh

LegendI•

F4

22

Nl

TransectPlotField IndicatorMagneticSusceptibility(x 10-s SI )No IndicatorPresent

ScaleI 15 m |

Fig. 3. Location of transects to evaluate use of magnetic susceptibilitymeasurements made at the soil surface to identify hydric soil bound-aries at Site 6. The area is near Site 3 and lies along a deep marsh.The MS values were calibrated to the hydric soil field indicators.Along Transects I and II the number shown for each plot is theMS value, and above it is the symbol for the field indicator identi-fied. The plot identified as NI in Transect I did not meet an indicatorexactly. Transect III was used for testing to show how subsequenttransects of MS measurements alone can be used to identify hydricsoil boundaries after a critical MS value has been identified. Fieldindicators in parentheses were identified after the MS measure-ments were made and the position of the hydric soil boundary es-timated.

Site to site variability may also be a factor, and untilfurther studies are done, critical values should be deter-mined on site by comparison with hydric soil field indica-tors whenever MS measurements are used to delineatehydric soils.

Following selection of a critical value, we measuredMS on Transect III to find the hydric soil boundary.These results were also compared with hydric soil fieldindicators, which are shown in parentheses in Fig. 3.The hydric soil boundary would have been placed inTransect III at the edge of the upland where the MSvalues increased from 20 X 10~5 to 94 X 10~5 SI units.Hydric soil field indicators would not have allowed aprecise placement of the boundary in this transect be-cause no field indicators were found at the two samplingpoints adjacent to the upland (Fig. 3) where MS valueswere 20 X 10~5 and 21 X 10~5 SI units. Indicators werenot met in these plots because the A horizon color

GRIMLEY & VEPRASKAS: MAGNETIC SUSCEPTIBILITY FOR DELINEATING HYDRIC SOILS 2179.

of O

bser

vatio

nsj.

ro w

*k

en a

t

O '

0-

Hydric

^V—

Non-Hydric

H H' "" \ ' ^ '

15 20 25 30 35 40 70 75 80

Magnetic Susceptibility(x 10 -5 SI units)

Fig. 4. Distribution of MS values for hydric and nonhydric soils col-lected at the surface of plots in Transects I and II of Site 6. In thisexample a critical MS value of 30 X 10~s SI would be used toseparate hydric and nonhydric soils using transects as shown inFig. 3.

had a hue of 7.5YR, which was too red to meet fieldindicators F4, F5, and TF7 found in other plots.

CONCLUSIONSWe found that volumetric MS values of samples from

the 0- to 15- and 15- to 30-cm depths were significantlydifferent (P < 0.01) between hydric and nonhydric Mol-lisols and related soils in natural marshes formed inundisturbed, glacial parent materials (primarily loess)in northeastern Illinois. Lower MS values were foundin hydric soils where MS values were <20 X 10~5 SI forsamples from depths of 0 to 15 cm, and <30 X 10~5 SIfor samples directly on the soil surface. No significant(P > 0.15) differences in MS values were found betweenhydric and nonhydric Ultisols formed in sediments onthe Lower Atlantic Coastal Plain. A created marsh con-structed in glacial parent materials had significantlylower MS values in the young (<5-yr-old) hydric soils,but only at depths of 15 to 30 cm, which were below theinfluence of the topsoil used for wetland construction.Within and across natural marshes formed in glacialmaterials, a critical MS value can be determined thatseparates hydric from nonhydric soils. Such critical val-ues will vary by depth, parent material type, and sitehistory, and until further studies are completed, shouldbe determined on site by calibrating the volumetric MSvalues to hydric soil field indicators observed along atleast two transects. We envision that these techniquesfor delineation of hydric soils will have applicability inmost glaciated areas, and in some nonglaciated areaswhere parent materials contain more magnetite andsoils are less acidic than the sediments of the LowerAtlantic Coastal Plain. The use of MS measurements isattractive because it offers a quantitative tool for hydricsoil delineation that is fast, reproducible, precise, andnondestructive.

ACKNOWLEDGMENTSJohn Tandarich and Kathleen Paap, both of Wetlands Re-

search Inc., Wadsworth, IL, helped sample the sites in Illinois.Alien Hayes, Jr., helped collect and analyze samples fromNorth Carolina. The help of these two individuals is greatly

appreciated. Figures were prepared by Beth Haines and Mat-thew Vepraskas. Publication of this article was authorized bythe Chief, Illinois State Geological Survey.

2180 SOIL SCI. SOC. AM. J., VOL. 64, NOVEMBER-DECEMBER 2000

conditions. Tech. Bull 301. North Carolina Agric. Res. Serv., Williams, R.D., and J.R. Cooper. 1990. Locating soil boundaries usingRaleigh. magnetic susceptibility. Soil Sci. 150:889-895.

Vepraskas, M.J., J.L. Richardson, J.P. Tandarich, and S.J. Teets. 1999. Yu, J., S. Zhan, L. Wu, and T. Chu. 1986. The magnetic susceptibilityDynamics of hydric soil formation across the edge of a created of the soils in subtropical and tropical China. Acta Pedol. Sin.deep marsh. Wetlands 19(l):78-89. 23(l):50-56.

Williams, M. 1992. Evidence for the dissolution of magnetite in recentScottish peats. Quat. Res. 37:171-182.