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VIII. Classification of Fragipan Soils.

VIII. Classification of Fragipan Soils.

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R. B. GROSSMAN A N D F. J. CARLISLE



Soil Survey Staff of the U.S. Department of Agriculture ( 1 95 1, 1960) and

has been in general use for about 20 years. But that is getting ahead of the

story.

The kinds of horizons that are designated fragipans were recognized as

important characteristics of some soils fairly early in the course of the

soil survey of the country. Early in this century, Carr (1 909) wrote about

the compact “hardpan” in the subsoil of Volusia soils in New York and

its importance to plant roots and water relations in the soil. The concept

of the Volusia series at that time, however, was based in large part on

geology, and the series was not characterized as having a pan (Marbut el

al., 1913). A few years later, Carter and Hull (1916) described the

Leonardtown soils in Maryland as having a compact hardpan in the subsoil, and subsequently the presence of the hardpan became part of the

concept of that series (H. C. Smith and Rose, 1924). A substantial number of soil series were characterized as having a compact, slowly permeable hardpan layer of the same general nature during the two decades

that followed.

As more detailed field studies of soils have been made and as more

kinds of interpretations of soil surveys have been required, the number of

kinds of soils in which fragipans have been recognized has increased. At

this writing, approximately 250 soil series in that part of the country lying

east of the Great Plains are characterized as having fragipans.

The presence or absence of fragipans apparently was not a factor in the

classification of soils into groups broader than series during the first two

decades of the soil survey in this country. During that period groupings

of series were primarily in terms of geography, geology, or physiography

(Marbut et al., 191 3; Marbut, 1935), an approach that slowly faded during

succeeding decades. Concepts of soil development and of “normal soils”

that were to affect strongly soil classification in this country in later years

were gaining ground in the late teens. Describing the soils of Charles

County, Maryland, H. C . Smith and Rose (1 924) wrote that the profile of

the Leonardtown series differs from the normal mature soils of the region

in the presence of the compacted zone (fragipan) in the illuvial horizon

and somewhat lighter than normal color of the overlying horizon. For

the same reasons, Perkins and Bacon ( 1 925) considered Leonardtown

soils to be “postmature soils.”

This was in accord with Marbut’s ideas about “normal,” “mature,” or

“fully developed” soils that strongly influenced his soil classification

(Marbut, 1928). Seemingly, soils with fragipans and other kinds of pan

horizons did not have a clear place in the higher categories of his classification. They were recognized as “overdeveloped soils” in category 111,



FRAGIPAN SOILS OF THE EASTERN UNITED STATES



267



within which groupings were based on stage in development, and in the

lower two categories. Thus, Marbut (1935) discussed the Grenada series

under the heading Red and Yellow soils and the Leonardtown series

under Gray-Brown Podzolic soils, but these series apparently were not

considered bona fide members of those great soil groups because of their

fragipans.

The logical inconsistencies of Marbut’s classification were corrected

to a large extent by the subsequent classification scheme outlined by

Baldwin et al. ( I 938). In this system, soils with fragipans seemed to have

a legitimate place in the great soil group of Planosols, which had a place

in the order of Intrazonal soils. The term, Planosol, was “proposed to

cover those soils with claypans and cemented hardpans not included with

Solonetz, Ground-Water Podzol, and Ground-Water Laterite.” Thorp

and Smith ( I 949) modified the definition as follows: Intrazonal soils

having one or more horizons abruptly separated from and sharply contrasting to an adjacent horizon because of cementation, compaction, or

high clay content. Soils of the Grenada, Leonardtown, and other series

with fragipans were classified as Planosols in this system (for example,

see Fox et al., 1958; Leighty and Wyatt, 1950).

Many soil scientists recognized that the Planosol group as then constituted was very heterogeneous and that most Planosols had important

properties in common with those of the zonal soils with which they were

associated (unpublished working papers of the I 947 National Conference

of the Cooperative Soil Survey). But agreement on an alternative classification was not forthcoming. Thorp and Smith (1949) noted that an

earlier proposal to elevate Planosols to the rank of a suborder of Intrazonal soils and to recognize groups of soils with “silt pans,” clay pan soils,

and certain other soils as subdivisions in the great soil group category had

not been unanimously accepted by the National Conference of the

Cooperative Soil Survey.

During the next dozen or so years, several different approaches were

followed in classifying soils with fragipans. In a number of soil surveys in

the southeastern part of the country soils with fragipans were classified

as Planosols according to the system outlined by Baldwin et al. (1938),as

modified by Thorp and Smith (1949) (for example, McNutt et al., 1959;

Fox et al., 1958; Leighty and Wyatt, 1950). In the latter part of that

period some writers classified only the somewhat poorly drained and the

poorly drained fragipan soils as Planosols and included the better drained

fragipan soils in the zonal great soil groups to which the soils belonged

on the bases of their features other than the fragipan (for example, Love

et al., 1959; Matthews et al., 1961). In the northeastern states soils with



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R. B. GROSSMAN A N D F. J. CARLISLE



fragipans were not classified as Planosols but were placed in the great

soil group to which they would belong on the basis of their other characteristics (for example, Cline, 1955; Taylor, 1960; Pearson and Cline,

1960; Shearin and Hill, 1962). Nevertheless, the presence or absence of

a fragipan was emphasized in the series classification of soils in that part

of the country.

In discussing Fragipan Planosols Cline (1 952) pointed out that, unlike the claypan of the Planosol, which occurs in the position of the B

horizon of the associated normal soil, the fragipan occurs beneath, and

in addition to, all the essential horizons of the solums of the Red-Yellow

Podzolic, Gray-Brown Podzolic, Podzol, Brown Podzolic, and probably

some other zonal great soil groups. He wrote, “In one sense, climate

and vegetation appear to have exerted little control over its [the fragipan] character in comparison with their influence on horizons above.”

Thus, there could be no modal horizon sequence in a great group that included all soils with fragipans. Cline ( 1 952) suggested that a great group

of fragipan soils might be defined for the poorly drained soils with fragipans, as the variation in horizon sequence seemed relatively minor in

those soils throughout the several soil zones. The imperfectly, moderately

well, and well-drained soils with such pans could then be placed in the

appropriate zonal great group and be set apart in the next lower category

from soils lacking fragipans.

The decision to revise the soil classification system used by National

Cooperative Soil Survey and the reviews of early drafts of a new system

continued to focus attention on how soils with fragipans would be best

classified at categorical levels above the soil series (unpublished working

papers of the National Cooperative Soil Survey). Some soil scientists

wanted to make claypans, fragipans, and indurated pans bases for grouping soils at the great soil group level or above; others wanted to bring

them in at the series level. Most reviewers thought the initial approximation of the new classification system overemphasized the importance of

Planosols by giving them a separate place high in the system and advised

that pan horizons be made diagnostic in lower categories.

The idea of a broad group of Planosols, subdivided in the next lower

category into soils with claypans, soils with fragipans, soils with indurated

hardpans, etc., survived in the 2nd Approximation prepared in 1952, but

at a lower level of the system. That was the last time in the development

of the current system that soils with the various kinds of pan horizons

were placed together in a class resembling the old Planosol great soil

group. In subsequent drafts of the system, soils with and without fragipans were placed together in classes that roughly approximated the



FRAGIPAN SOILS OF THE EASTERN UNITED STATES



269



Podzol, Gray-Brown Podzolic, Red-Yellow Podzolic, and a few other

great soil groups. These were subdivided into classes with and without

fragipans, among other criteria in the next lower category, which

approximated the current great group level. With some variations, and

with one exception, this arrangement persisted through the 7th Approximation of the classification system (Soil Survey Staff, 1960) and to the

system that is in use today.

The exception was a trial in the 5th, 6th, and 7th Approximations

( 1 956-1 960) of a proposal to keep Spodosols with and without fragipans

together at the great group and subgroup levels of the system and to make

the distinction on the presence or absence of a fragipan in the family

category. (The class of Spodosols is approximately equivalent to the

Podzol and part of the Brown Podzolic great soil groups.) The rationale

for the proposal was the belief that a fragipan or a comparable horizon

was normal in Spodosols. Those Spodosols that lack the fragipan were

thought to be too coarse textured for a fragipan to form (Soil Survey

Staff, 1960). After a few years’ trial, the classification system was

amended to set apart Spodosols with and without fragipans in the great

group category. Use of the fragipan as a differentia within Spodosols

then paralleled its use in the orders of Inceptisols, Alfisols, and Ultisols.

In the current soil classification system (Soil Survey Staff, 1967) fragipans are used uniformly to set apart one great group in each of the eleven

suborders of the four orders in which soils with fragipans occur. These

are listed in Table I. Note that each of the four aquic suborders has a

great group of soils with fragipans but the dry or seasonally dry suborders

do not have such great groups.

The family category of the current soil classification system is designed

to group series that are similar in certain properties, such as texture and

mineralogy, that have importance in the use and management of soils. The

top of the fragipan is the base of the part of the soil that is considered

in applying the texture and mineralogy criteria for the classification of

series into families. The rationale is that plant roots do not sufficiently

exploit the soil within and below the fragipan.

IX. Unresolved Problems



There are difficulties in the identification of fragipans. High bulk density

and consistence have not been satisfactory criteria for recognition of

fragipans in loess-derived soils of the lower Mississippi Valley (personal

communication from L. J . Bartelli). Brittleness when moist does not

suffice for identification of fragipans and their distinction from soils with

plinthite in North Carolina (personal communication from Forrest



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R. B. GROSSMAN AND F. J. CARLISLE



TABLE I

THEGRE \ T GROUPS OF SOILS HAVINGFRAGIPANS,

SHOWlNG THEIRDISTRIBUTION

OF THE CURRENT

SOILCLASSIFICATION

SYSTEM

AMONGT H I ORDERSA N D SUBORDERS



Order

Entisol.

Vertiso' Inceptisc '



Aridis, I

Molli>,'

Spodo.



1



L,uhorcler



Great groups

having fragipans"



Order



Suborder



-



-



Alfisols



Aqualfs

Boralfs

Udalfs

Ustalfs

Xeralfs

Aquults

Humults

Udults

Ustults

Xerults



_-



Andepts

Aquepts

Ochrepts

Plaggepts

l'ropepts

I I mbrepts

Iquods

kerrods

H timods

Orthods



Fragiaquepts

Fragiochrepts

Fragiurnbrepts



Ultisols



-



Fragiaquods

Fragihumods

Fragiorthods



Oxisols

Histosols



-



Great groups

having fragipans"

Fragiaqualfs

Fragiboralfs

Fragiudalfs

Fragiaquults

Fragiudults

-



Data for illuslrative profiles for seven of these are in ttre 7rh Approximation (Soil Survey

Staff, 1960) as follows: Fragiaquepts-profile Nos. 30, 48; Fragiochrepts-profile No. 24:

Fragiorthods-profile No. 29; Fragiaqualfs-profile No. 82; Fragiudalfs-profile Nos. 84,

98: Fragiaquults-profile No. 94; Fragiudult-profile No. 97.



Steele). These problems concern fragipans showing strong alteration of

the parent material.

There are also problems of identification of fragipans that show weak

alteration of the parent material. Brittle zones occur in the upper substratum of some soils developed from compact glacial till. If many feet

thick, the zone can be eliminated as a fragipan on scale. In some soils the

brittleness decreases in expression within 2 or 3 feet. These zones are

difficult to distinguish from fragipans as presently defined. Modifying the

definition of fragipans to include certain morphological features, such as

bodies of moved clay or expression of a gross polygonal structure, would

provide a basis for excluding certain of these zones of compact glacial

till. Such a definition, however, might exclude some horizons of Spodosols

and Inceptisols that are currently classified as fragipans and thus raise

other problems.

Fragipans are now a criterion for distinguishing great groups in the

Comprehensive Soil Classification System. There is some concern with

the categorical level at which fragipans are recognized. The concern

arises from two somewhat different problems: (1) that of distinguishing



FRAGIPAN SOILS OF THE EASTERN UNITED STATES



27 1



fragipans from other kinds of horizons with properties affecting root

growth and water movement similar to those of fragipans; and (2) that

of determining the lower limit of fragipan expression. From the point of

view of soil use, the first problem may not be too serious. If a horizon has

similar limitations to use and management of soil as a fragipan, its recognition as a fragipan does no great immediate harm. The question of the

lower limit of expression for recognition of a fragipan seems more important. The problem has similarities to an evaluation of engineering

test data that results in overdesign. Fragipans are recognized which

may have marginal importance to use and management. Once a fragipan

is recognized in a soil, however, there is a tendency to change appreciably the interpretive ratings for many kinds of soil use from the ratings

assigned to otherwise similar soils. Moreover, the part of the soil that

determines the family placement may change, and this may affect the

correlation of soil interpretations.

The name fragipan in a sense states and brings into focus an underlying problem that affects both identification and classification. A consistence property, relative fragility or brittleness, is the defining characteristic for a horizon that importantly affects soil use and exhibits a wide

range in kind and degree of soil development. Importance of the fragipan

does not lie in its fragility or any single feature, but rather in a group of

attributes which together make it a pan. These attributes in combination

cannot be readily measured. Separation of fragipans from other kinds of

soil horizons may be improved by modification in the definition and collection of numerical information on consistence. Certainly, if fragipans

were shown to have a unique bonding agent, the distinction from other

kinds of pans would be more definite. But this would not necessarily

resolve the difficulty in specifying the lower limit of expression for the

recognition of a pan.

X. Summary



Fragipans are subsoil horizons that are brittle and rigid when moist,

that restrict root growth, and that transmit low-tension water more slowly

than the overlying horizons. They occur in soils subject to net downward

water movement. Trees rather than grass were the dominant vegetation

a t the time the country was settled. Some fragipans have undergone

strong eluviation, illuviation, or both. Others occur in soil materials

that exhibit only weak alteration of the parent material. Clay is probably

the chief bonding agent. Organization of the clay relative to the sand and

silt grains may be of importance. Fragipans are low in organic matter

and infrequently, if ever, calcareous. Otherwise, their mineralogical and



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R. B. GROSSMAN A N D F. J . CARLISLE



chemical characteristics range widely. Extremes in texture are excluded;

high proportions of particle size separates in the 0.2- to 0.02-mm. range

may be conducive to fragipan formation. A gross polygonal structure is

common. Blocky structure within the large structural units is at most

moderately expressed; some fragipans have platy structure. Vertical

continuity of large voids is restricted. Bodies of moved clay apparently

are found in all fragipans. Most fragipan material is at least firm when

moist; i.e., a piece held between thumb and forefinger offers moderate or

more resistance to rupture. Fragipans occur deep enough to be subject

to weak physical disturbance and below maximum influence of current

soil development. Some fragipans may be largely relict features; others

are not. Periglacial influences may have affected fragipans in the north,

but not those in the south. Depth to the fragipan largely determines its

influence on plant growth. Soils with shallow fragipans commonly have

high water tables which may be a factor in restricting rooting depth. The

importance of fragipans to mechanical manipulation of soils is largely

relatable to their slow transmission of low-tension water. In the Comprehensive Soil Classification System, fragipans are diagnostic at the great

group level and they are recognized in that category in four soil orders. In

those orders they are used at a lower categorical level than the argillic,

cambic, and spodic horizons but at a higher categorical level than features

of similar importance to soil use having less genetic implication, such as

the lithic contact.

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276



R. B. GROSSMAN A N D F. J. CARLISLE



Appendix



BELTSVILLESOIL

The soil has a fragipan in the lower part of an illuvial B horizon and is an example of

sequence Ild, Fig. 2. The fragipan shows evidence of appreciable clay accumulation. The

description is from the report on soils of Montgomery County, Maryland (Matthews et al.,

1961). The information on soil use comes both from this report and the one for Fairfax

County, Virginia (Porter et al., 1963).

Surface soil

AOO- 1 to f.i inch, scattered pine needles.

AO- M to 0 inch, loose but felty decomposed leaf mold.

A 1-0 to 2 inches, very dark gray (IOYR 3/1) silt loam; moderate, fine, crumb structure: loose; roots abundant; strongly acid; abrupt, wavy boundary.

A2-2 to 13 inches, yellow (2.5Y 8/6) silt loam; weak, fine, subangular blocky structure; friable when moist and slightly sticky when wet; roots abundant; strongly

acid; gradual, smooth boundary.

t

Subsoil

B I - 13 to 2 1 inches, brownish-yellow (IOYR 6/6), light silty clay loam; few, medium,

faint mottles of strong brown (7.5YR S/8);moderate, medium, and coarse,

subangular blocky structure; firm when moist; roots fairly abundant; a few

rounded pebbles; thin, distinct clayskins on some faces: strongly acid; gradual

to clear, wavy boundary.

B21-21 to 31 inches, brownish-yellow (IOYR 618) silty clay loam: few, fine, distinct

mottles of red (2.5YR 5/6); weak, coarse, platy structure; firm when moist;

roots few, between structural elements only: a few pebbles and a few faint

clayskins; strongly acid; clear, irregular boundary.

B22m-31 to 42 inches, reddish-yellow (7.5YR 6/6) clay loam; many, fine, faint

(B22x) mottles of reddish yellow (5YR 6/8) and many vertical channels and horizontal streaks of gray silt and clay; compound structure-moderate, very

coarse, platy and moderate, medium to coarse, subangular blocky; very compact and dense; very firm when moist; very few roots; some pebbles and a

few clayskins; this is the fragipan, or hardpan; strongly acid: clear, smooth

boundary.

B3m-42 to 48 inches, reddish-yellow (7.5YR 6/6) clay loam; many, medium, distinct

mottles of light yellowish brown (IOYR 6/4), reddish yellow (SYR 6/8), and

(B3x)

white (2.5Y 8/2): compound structure-moderate, very thick, platy and

moderate, coarse, blocky: compact and dense; very firm when moist; practically no roots: some pebbles and some light-gray to white silt coats; strongly

acid; this lower hardpan is a transition between the true subsoil and the

substratum.

Substratum

CD-48 to 54 inches t,very pale brown (IOYR 7/3) silty clay loam; abundant, rnediurn, distinct mottles of reddish yellow (7.5YR 6/6) and light gray (IOYR 7/2);

massive; very firm; strongly acid: underlain at some depth by gravel.

Matthews et a / . (1961) wrote: “Because of the almost impervious fragipan, the Beltsville soils tend to be wet at times. Frequently, they are saturated near the surface, but almost

dry within or below the fragipan. The moisture-supplying capacity is moderate. A few depressed areas in the uplands are ponded for short periods after long heavy rains or quick



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VIII. Classification of Fragipan Soils.

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