Soils of the Mall in Washington, DC: II. Genesis, Classification, and Mapping1

Soils of the Mall in Washington, DC: II. Genesis, Classification, and Mapping1

J. R. SHORT, D. S. FANNING, J. E. Foss, AND J. C. PATTERSON2

ABSTRACTThe soils on the Mall in Washington, DC have formed in about

6 m of miscellaneous fill deposited in repeated applications to thesite during filling operations. Some limited pedogenesis has oc-curred; cambic, or cambic-like B horizons were recognized in 26%of the profiles examined. Buried A horizons were found in 42% ofthe profiles. Classification of these soils .using Soil Taxonomy alonewas not satisfactory, as variations in organic matter with depth causedmany of the soils to be placed in Fluv-classes that connoted depo-sition of the parent material as alluvium and did not show uniqueaccessory characteristics such as high bulk density. Proposed Urbicand Spolic subgroups of Udorthents and Ochrepts, with criteria toexclude them from Fluv-classes, were utilized to classify these soilsin a manner more consistent with their genesis and characteristics.A detailed soil map was prepared for the Mall using the proposedclassification, and recommendations are made for mapping suchhighly manipulated areas.

Additional Index Words: soil classification, soil genesis, soil map-ping, urban soils, highly man-influenced soils.

Short, J.R., D.S. Fanning, J.E. Foss, and J.C. Patterson. 1986. Soilsof the Mall in Washington, DC: II. Genesis, classification, and map-ping. Soil Sci. Soc. Am. J. 50:705-710.

SOILS IN URBAN AREAS are often unique. Their phys-ical and chemical characteristics, like those of

other soils, arise from the interaction of climate, or-ganisms, relief, parent material, and time. The for-mation of highly man-influenced soils, however, isdominated by the effect of one factor: organisms, spe-cifically man (Bidwell and Hole, 1965). The activitiesof man alter soils, and in some instances new parentmaterial is deposited through earth moving activities.

The often unique properties of soils in urban areasand the value of the real estate associated with such

soils, make an understanding of their properties im-perative; however, only a limited amount of researchhas been accomplished on such soils owing to theirpresumed variability, urban location, and lack of ag-ricultural importance. Pressures to use soils for non-agricultural purposes, the need to provide greenspacein urban environments, and the rapidity with whichhighly man-influenced soils are created, provide theincentive for research into their properties (Short etal., 1986).

Simple characterization of manipulated soils, how-ever, is not sufficient. The soils must be classified ina manner that will enable an individual to accuratelyand readily discern properties important to their useand management. The development of a system of soilclassification organizes knowledge about soils and per-mits an understanding of the relationship of one kindof soil to another; therefore, a system of classificationshould permit the inclusion of all soils (Smith, 1963).Such a system of classification should allow placementof highly man-influenced soils into the system in sucha manner that their properties would be easily under-stood. They should not be forced to fit into unsatis-factory niches.

Attempts to classify highly man-influenced soils havebeen limited, and have met with little success whenusing So/7 Taxonomy alone (Soil Survey Staff, 1975).

1 Contribution no. 7148 and Scientific Paper no. A-4163 of Mary-land Agricultural Experiment Station, Dep. of Agronomy and of theNational Park Service, National Capital Region. This paper repre-sents a portion of the M.S. thesis of the senior author. Received 25Apr. 1985.2 Soil Scientist, National Park Service, National Capital Region,1100 Ohio Dr., S.W., Washington, DC 20242 (also Graduate Stu-dent, Univ. of Maryland), Professor, Univ. of Maryland, CollegePark; Head, Dep. of Plant and Soil Science, Univ. of Tennessee,Knoxville; and Research Agronomist, National Park Service, re-spectively.

706 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Fig. 1. Filling of Mall, ca. 1920.

Sencindiver (1977) used a proposed suborder of En-tisols (Spolents) to classify minesoils. Nine propertieswere recognized as being important for identificationof Spolents, including presence of artifacts (glass, metal,tires, etc.), irregular distribution of organic matter withdepth, and random orientation of coarse fragments.Four subgroups of Entisols and Inceptisols have beenproposed for classification of highly man-influencedsoils in the Washington, DC-Baltimore, MD area(Fanning et al, 1983). These include Urbic (mineralsoil material containing object artifacts, e.g., bricks andglass), Spolic (moved earthy, mineral soils, but with-out artifacts), Garbic (with organic soil material con-taining trash), and Dredgic (soils in dredged materials)subgroups. Stein (1978) and Short (1983) used theseproposed subgroups to classify highly man-influencedsoils in Washington, DC. Many of these soils con-tained object artifacts in their particle size control sec-tion, and displayed an irregular distribution of organicmatter with depth. Without revisions, many of thesesoils would be classified into Fluv-classes (e.g., Udi-fluvents, Fluventic Eutrochrepts) by strict applicationof 5*0/7 Taxonomy (Soil Survey Staff, 1975), whichwould give false implications regarding their genesis(by implying water deposition) and properties. For thisreason, such soils were "forced" into Udorthents inthe soil survey of Washington, DC (Smith, 1976), eventhough this was an incorrect placement by strict tax-onomic criteria.

The objectives of this paper are to: (i) develop amodel for the genesis of the soils of the Mall; (ii) dis-cuss the morphology of these soils; (iii) classify highlyman-influenced soils on the Mall using observable soilproperties; and (iv) discuss the use of a set of criteriafor classifying such unique soils.

MATERIALS AND METHODSThe transect sampling method described in Short et al.

(1986) was chosen to ensure random sampling of the Mall.One-hundred randomly located profiles were examined, 20in each of five transects, and profile descriptions were pre-pared for each profile using standard soil survey terminology(Soil Survey Staff, 1951). Samples were obtained from mor-phological horizons for analysis of particle density, particlesize, soil reaction, soluble salts, organic matter content, cat-ion exchange capacity (CEC), heavy metals (Pb, Ni, Zn, Cu,and Cd), percent coarse fragments, and double acid extract-able Mg, P2O5, K2O, and Ca content (Short et al., 1986).

RESULTS AND DISCUSSIONGenesis of the Mall Soils

Historical records (National Archives, 1981) indi-cate that the Mall was originally a marshy land areaadjacent to the Potomac River. Subsequent applica-tions of locally derived fill materials have raised thesoil surface by approximately 6 m. The source of thefill material apparently is from local excavations forFederal buildings constructed in Washington, DC, andincludes rubble from demolition of older buildings.This fill material is presumed to be the parent materialfor the soils presently existing on the Mall. Applica-tion of the fill material may have occurred in stages,with enough time elapsing between stages for a limitedamount of pedogenesis to occur. Photographs ob-tained from the National Archives (Fig. 1) support thehypothesis that filling occurred in stages. This photo,taken ca. 1920, shows that the soil surface is belowstreet level, and much lower than the present elevation(Fig. 2). An A horizon may have formed at each soilsurface as organic matter accumulated from the veg-etative cover or from soil renovation activities. Sub-

SHORT ET AL.: SOILS OF THE MALL IN WASHINGTON, DC: II. 707

sequent applications of soil material would have re-sulted in formation of a buried A horizon. A model,such as that shown in Fig. 3, would account for thepresence of buried A and B horizons in many of theprofiles studied (Short et al, 1986). Inclusion of arti-facts of human endeavors, such as brick, concrete, cin-ders, glass, slag, and similar materials would be ex-pected during the manipulation of the soil materialsduring deposition, especially because much of the soilmaterial is a by-product of local construction. Addi-tions such as lime, organic matter, fertilizers, heavymetals from vehicle exhaust, tire wear, industrialemissions, and deicing salts, would occur to any in-terim soil surface, and would continue at the presentsoil surface. Subsequent application of additional fillmaterial would result in formation of soil profiles con-taining varying amounts of organic matter, metals, andnutrients, as was found on the Mall (Short, 1983; Shortet al., 1986).

Morphology of Selected ProfilesThe profiles illustrated in Table 1 are typical of the

soils examined on the Mall. Most profiles exhibitedonly limited development, with formation of an A ho-rizon at the soil surface and little additional pedogenicdevelopment. Profiles 1-6 and 3-7 are typical of suchweakly developed soils found on the Mall. Subsoil (B)horizons were recognized, however, in 26% of the pro-files studied. These horizons were distinguished by thepresence of soil structure and by redder hues orstronger chromas than that of the presumed parentmaterial. Although somewhat speculative, structurewas interpreted to have developed in place if therewere differences in appearance between the ped sur-face and the ped interior. Ped faces often appeared tohave a coating of Fe oxides, or small amounts of siltand clay. Lithologic discontinuities in many of theprofiles tended to obscure the relationship of B hori-zons with their presumed parent material, furthercomplicating identification.

Subangular blocky structure was most often ob-served for the B horizons, although other structuresdid exist. The B horizons of profiles 1-9 and 1-14exhibit this type of structure, while the subsoil hori-zons of profile 1-19 exhibit both platy and subangularblocky structure. Formation of these B horizons is in-terpreted to have occurred since deposition. It mustbe recognized that the transported material may havecontained soil structure of its own; however, manip-ulation by land moving equipment would tend to de-stroy such structure, and the presence of platy struc-ture with plate faces parallel to the soil surface supportsthe hypothesis that the structure has developed inplace.

Classification of Highly Man-influenced SoilsDifficulties in classifying highly man-influenced soils

abound. Often, characteristics important for the useof such soils or for understanding their genesis are notconsidered in Soil Taxonomy (Soil Survey Staff, 1975).Properties resulting from manipulation of soil mate-rials, such as variable organic matter content withdepth, are diagnostic for inclusion in classes not rep-resentative of the manipulated nature of these soils.

Fig. 2. Present configuration of Mall.

Therefore, criteria should be developed that will en-able classification of such "disturbed" soils in relationto their genesis and properties.

The soils of the Mall were classified in the Districtof Columbia Soil Survey as Udorthents (Smith, 1976).Such a classification informs the reader that the soilstend to have little soil development and, therefore, ismore informative than classification as "made land".However, the reader is still not provided with insightinto the unique genesis or properties of these soils. Asdiscussed above, classification as Udorthents was ac-complished by ignoring the irregular distribution oforganic matter with depth in the profile.

708 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Profile 1-1

1870 1900 1920

TIME

1980

Fig. 3. Idealized model of genesis of highly man-influenced soils of the Mall, as shown by Profile 1-19. Other profiles exhibit C horizons.

Classification of the soils of the Mall by strict ap-plication of Soil Taxonomy did not satisfactorily con-note their genesis and characteristics. The results of

this effort may be found in Table 2. Fourteen differentfamilies were recognized. Seventy-four of the profileswere classified as Entisols, 28 of which were classified

Table 1. Profile descriptions of typical Mall soils.

Profile

1-6

3-7

Horizon

AC2C

ApC2C

3C

Depth (cm)

0-88-38

38-100

0-1919-3030-53

53-100

Moist colort

10YR4/210YR4/310YR4/3

10YR4/210YR5/37.5YR5/6

10YR5/6

Texture Structure Consistence Artifacts!

Typic111

Typic1silscl

sil

Udorthents (Urbic Udorthents)§2msbkf mfr +2msbk mfr2msbk mfi +

Udifluvents (Urbic Udorthents)2msbk mfr +2msbk mfr +2msbk mfr +

2msbk mfi

Boundary

cscs

csasas

Special features

Many rootsCommon rootsFew roots

Many rootsCommon rootsFew roots, Mn

concretionsFew roots, Mn

concretionsTypic Eutrochrepts (Urbic Eutrochrepts)

1-9 AB

C2C

1-14 AB

C2C3Ab

1-19 AlA2

B2Ab

2Bb

3Ab'

0-88-18

18-3838-100

0-99-20

20-3232-6060-100

0-1515-27

27-5050-74

74-91

91-100

10YR4/35YR4/6

10YR3/210YR4/3

10YR3/210YR4/4

10YR4/410YR5/410YR4/3

10YR3/310YR3/37.5YR5/67.5YR5/810YR4/47.5YR5/610YR5/610YR6/610YR4/4

11

11

Fluventic11

cl1si

Fluventic11

11

1

1

2msbk2msbk

2mplOm

Eutrochrepts (Urbic2cp->2mbk2msbk

2msbkImsbkOm

Eutrochrepts (Urbic2fsbkIfsbk

IfplImsbk

lcpl->lfsbk

Ifgr

mfr +mfi +

mfi +mvfr +

Eutrochrepts)mfr +mfi 4-

mfi +mfr +mfr +

Eutrochrepts)mfr +mfr +

mfi +mfr +

mfi +

mfr +

cwcw

aw

cscs

asas

awas

asaw

as

Many rootsCommon roots,

krotovinasFew rootsFew roots

Many rootsCommon roots,

krotovinasCommon rootsFew rootsFew roots

Many rootsCommon roots

Few rootsFew roots

Few roots

many

many

Few roots, many wormcasts

t All abbreviations after Soil Survey Manual, 1951, p. 139.t Presence of artifacts signified by +. Artifacts include glass, brick, cinders, slag, concrete, etc.§ Names not in parentheses are by Soil Taxonomy, names in parentheses are by proposed subgroups.

SHORT ET AL.: SOILS OF THE MALL IN WASHINGTON, DC: II. 709

as Typic Udorthents. The remaining 46 Entisols wereclassified as Typic Udifluyents. The 26 profiles notclassified as Entisols were identified as Inceptisols be-cause of the presence of cambic B horizons as de-scribed above. The relatively high base saturation ofmost profiles resulted in 21 of the 26 Inceptisols beingclassified as Eutrochrepts, with the remaining five asDystrochrepts. Eight Inceptisols fit into Typicsubgroups, while the remaining 18 profiles fit into theFluventic subgroups. Clearly, the variable organicmatter content with depth of such soils strongly affectstheir classification using Soil Taxonomy (Soil SurveyStaff, 1975). The use of Soil Taxonomy alone gives afalse impression of their genesis, implying that the soilsdeveloped in recent alluvium, rather than miscella-neous fill material. The high bulk density of the Mallsoils (Short et al., 1986) would not be expected in Fluv-soils.

The central concept of Fluvents is that of soilsformed in water deposited sediments. The variableorganic matter content, diagnostic for placement intoFluvents and Fluventic subgroups, is a result of re-peated deposition of alluvium containing varyingamounts of organic matter. Fine stratification of thesediments may also be found in such profiles. TheMall soils formed in miscellaneous fill materials de-posited over a period of time by man; they were notformed in water-deposited alluvium. They also do notexhibit the fine stratification of water deposited sedi-ments. Classification of such soils into Fluv-subordersand Fluventic subgroups is clearly unsatisfactory be-cause a false impression of their genesis is connoted,and little information regarding the influence of manupon their properties is afforded.

Reclassification of the Mall soils was accomplishedusing the subgroups proposed by Fanning et al. (1983).These subgroups were used in addition to thesubgroups of Soil Taxonomy (Soil Survey Staff, 1975),and should be considered as a supplement to Soil Tax-onomy, not as a replacement. It is necessary to modifythe definition of Fluvents to exclude the presence ofurbic or spolic soil materials in the particle size con-trol section (Fanning et al., 1983). The two proposedsubgroups used for reclassification of the Mall soilswere the Urbic and Spolic subgroups, defined as fol-lows:

Urbic subgroup: those soils consisting of miscella-neous urban fill materials (urbic materials) thatcontain inorganic, manufactured artifacts, suchas brick, glass, concrete, pottery, and nails, withinthe particle size control section of 25 to 100 cm;and

Spolic subgroup: those soils consisting of locally de-rived fill material, but with no manufactured in-organic artifacts in the particle size control sec-tion.

Percent coarse fragments may vary greatly amonghorizons, and irregular organic matter content is a su-bordinant characteristic in both subgroups. Historicalrecords are often important aids in recognizing suchsoils.

The results of this reclassification are shown in Ta-ble 2. Fewer subgroups were distinguished when usingthe proposed subgroups than when using Soil Tax-

2. Classification of Mall soils.

Classification(by Soil Taxonomy)

A. Typic Udorthent,fine-loamy, mixed,mesic

B. Typic Udifluvent,fine-loamy, mixed,mesic

C. Typic Udorthent,coarse-loamy, mix-ed, mesic

D. Typic Udifluvent,coarse-loamy, mix-ed, mesic

E. Typic Udorthent,sandy, mixed,mesic

F. Typic Udorthent,fine-loamy, mixed,mesic

G. Typic Udorthent,coarse-loamy, mix-ed, mesic

H. Typic Udifluvent,coarse-loamy, mix-ed, mesic

I. TypicDystrochrept, fine-loamy, mixed,mesic

J. Fluventic Dystro-chrept, fine-loamy,mixed, mesic

K. Typic Eutrochrept,fine-loamy, mixed,mesic

L. Fluventic Eutro-chrept, fine-loamy,mixed, mesic

M. Fluventic Eutro-chrept, coarse-loamy, mixed,mesic

N. Typic Eutrochrept,fine-loamy, mixed,mesic

Total

ClassificationN^ (by proposed criteria)

19 A. Urbic Udorthent, fine-loamy, mixed, mesic

31

4 B. Urbic Udorthent, coarse-loamy, mixed, mesic

14

1 C. Urbic Udorthent, sandy,mixed, mesic

2 D. Spolic Udorthent, fine-loamy, mixed, mesic

2 E. Spolic Udorthent,coarse-loamy, mixed,mesic

1

2 F. Urbic Dystrochrept, fine-loamy, mixed, mesic

3

5 G. Urbic Eutrochrept, fine-loamy, mixed, mesic

10

5 H. Urbic Eutrochrept,coarse-loamy, mixed,mesic

1 I. Spolic Eutrochrept, fine-loamy, mixed, mesic

100

Art50

18

1

2

3

5

15

5

1

100

t Number of profiles examined.

onomy (Soil Survey Staff, 1975) alone. Comparison ofclassifications using Soil Taxonomy and the proposedsubgroups shows the effect that variable organic mat-ter has upon the classification of these soils. Propertiesindicative of the genesis and classification, such as thepresence of artifacts within the soil profile or the for-mation of these soils from locally derived fill material,are readily apparent using the proposed criteria, butare obscured when using Soil Taxonomy (Soil SurveyStaff, 1975) alone.

Mapping Highly Man-influenced SoilsOnce reclassification of these soils was accom-

plished, a map was prepared for the Mall, and is shownin Fig. 4. As would be expected from the results ofreclassification (Table 2), Urbic Udorthents were thedominant soils on the Mall, accounting for nearly 70%of the land area. Each of the other subgroups recog-nized after reclassification are also delineated on thesoil map, but often as a single profile. For example,although five profiles were classified as Spolic Udor-thents, they were widely scattered in the field. The soil

710 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Urbic Udorthent

Q Urbic Eutrochrept

EBSpolic Eutrochrept

Urbic Dystrochrept

Spolic Udorthent

Fig. 4. Detailed soil map of the Mall, showing classification at the subgroup level using proposed subgroups. Mapping units are defined atthe subgroup level.

map, therefore, shows the five profiles individually lo-cated.

The boundaries shown on the map are somewhatarbitrary, as there appeared to be little or no relation-ship between the soils and existing landscape features.This is not surprising because of the manipulated na-ture of these soils and the little time allowed for soildevelopment. A landscape expression has been shownfor roadway berms, however, (Stein, 1978). Historicalrecords show that the elevation of the Mall has in-creased by approximately 6 m; therefore historic rec-ords should be consulted to aid in identifying areas ofhighly man-influenced soils. The location of the In-ceptisols was primarily at the east and west ends ofthe Mall. This distribution may reflect greater soil dis-turbance in the center portion of the Mall throughconstruction activities.

Large scale field mapping of soils in urban environ-ments to this degree of detail is not practical. A map-ping unit for highly man-influenced soils should bedeveloped that will accurately reflect the properties ofsuch soils. The soils of the Mall could be mapped sat-isfactorily in a mapping unit of fine-loamy UrbicUdorthents because that clearly is the dominant soil.The description of the mapping unit should contain adescription of the kind and extent of inclusions, suchas more developed soils (the Urbic Eutrochrepts andUrbic Dystrochrepts), soils without artifacts (the SpolicUdorthents and Spolic Eutrochrepts), and some var-iation in particle size family.

Mapping at this level of accuracy would be con-sistent with guidelines in the Soil Survey Manual (SoilSurvey Staff, 1951). A mapping accuracy of 85% isrecommended, but this is rarely achieved, partly be-cause of the narrowness with which soil series arepresently defined. The proposed mapping unit of Ur-bic Udorthents is 70% accurate, although this map-ping unit is defined at the subgroup rather than theseries level.

SUMMARY AND CONCLUSIONSA model has been developed for the genesis of the

highly man-influenced soils of the Mall. These soilshave formed in soil material obtained from localsources, deposited in increments over a period of timeso that several distinct sequa have formed. Sufficient

time appears to have elapsed for the formation ofcambic or cambic-like B horizons in some profiles.These horizons were identified in the field by the pres-ence of redder hues or stronger chromas than those ofthe presumed parent materials, and/or by the devel-opment of soil structure.

Classification of highly man-influenced soils shouldbe accomplished in such a manner as to provide ac-curate and useful information about these soils to theuser of soil surveys. Classification using present SoilTaxonomy (Soil Survey Staff, 1975) criteria alone doesnot adequately describe the genesis or the propertiesimportant for management of such soils. Use of pro-posed criteria which will allow a more accurate re-flection of the genesis and properties of highly man-influenced soils should be promoted. These criterianeed to be further investigated and refined in con-junction with Soil Taxonomy (Soil Survey Staff, 1975)criteria.

ERRATA

Soils of the Mall in Washington, DC: II. Genesis,Classification, and MappingJ.R. SHORT, D.S. FANNING, I.E. Foss, AND J.C.PATTERSONSoil Sci. Soc. Am. J. 50:705-710 (May-June 1986)

On p. 706, the caption for Fig. 1 should be corrected toread "Fig. 1. Filling of Mall, ca. 1934."