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II. Horizons of Fragipan Soils

II. Horizons of Fragipan Soils

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containing discontinuous eluvial parts. The Be notation may find use as a

designation for lower eluvial horizons that cannot be designated A'2 or

B & A.


The uppermost panel of Fig. 2 illustrates the most general statements

that may be made about the position of the fragipan. All fragipans occur

beneath an eluvial horizon unless the soil has been eroded severely. In



A'2 Lxl

ond /or










and/or 1





FIG.2. Horizon sequences of soils with fragipans in the eastern United States. Elements

within brackets indicate characteristics that are not essential to the definition of the sequence. The fragipan horizons are indicated by the suffix x: the suffix t denotes silicate

clay sufficient for recognition of an argillic horizon.

some soils, more often the wetter ones, the fragipan occurs immediately

beneath the eluvial horizon. In many soils, however, there is an intervening B horizon, which may be a spodic horizon, a cambic horizon, or

an argillic horizon. Some soils with fragipans are bisequal. The fragipan

may occur in the lower eluvial horizon and not in the underlying B

horizon, in the lower B horizon but not in the second eluvial horizon,

or in both. These variations form the basis for the four classes of horizon

sequences shown in the set of panels designated Ia through Id; these are

illustrated by soil descriptions in the Appendix. These four classes are




subdivided to give the eight classes, IIa through IIh. Accumulation of

clay and occurrence of the fragipan in the lower eluvial horizon are the

criteria for these subdivisions. Further subdivisions could be made. Panel

IIf, for example, could be subdivided into soils with argillic, spodic, or

cambic horizons above the fragipan. Brackets are placed around the

horizon designations or elements of the horizon designations that are not

necessary to the definition.

In Fig. 2 the suffix t denotes an accumulation of silicate clay sufficient

for recognition of an argillic horizon. This is a more limiting use of the t

suffix than is specified by the Soil Survey Staff ( 1 962). The horizon symbols in Fig. 2 are only for convenience in discussing this subject; it is

not suggested that any of these symbols should replace horizon designations in current use.

Soils with horizon sequences Ha, Ilc, and IIe have fragipans in

horizons where the parent material has been weakly altered. Many of

the soils with fragipans in the northeastern United States developed in

Wisconsin glacial till belong in one of these three classes. In contrast

are fragipans in soils that have horizon sequences IIb, IId, IIf, IIg, and

IIh. In these soils, alteration of the parent material of the fragipan by

eluviation, illuviation, or both, has been strong. Hence, influence of the

parent material in determining the properties of the fragipan is less important. Fragipans in soils south of the Wisconsin glacial advance tend

to exhibit stronger alteration of the parent material.



Fragipan expression involves thickness and depth to its upper boundary. These are considered in subsequent paragraphs of this section. It

also involves properties, such as consistence, structure, and pore

arrangement, that determine mechanical impedance to root penetration

and rate of movement of low-tension water. Objective measurement and

integration of these properties has not been achieved. Within a local

association of soils, however, evaluation of relative degree of fragipan

expression has validity. Maximum degree of expression of the fragipan is

commonly reported for the soils of intermediate wetness (Neeley, 1965;

A. E. Thomas, 1967; Redmond and Engberg, 1967; Grossman et al.,

1959a; Nettleton et al., 1968a). This intermediate wetness is described

approximately by the terms somewhat poorly and poorly drained (Soil

Survey Staff, 195 1) and by the Aquic subgroups of the soil classification

system (Soil Survey Staff, 1967). The relationship is widespread enough

to suggest implication in the genesis of the fragipan.



Excluding severely eroded soils, the depth to the fragipan ranges from

about 10 to 150 cm., with the upper boundary for most between 25 and

100 cm. In landscapes that have been affected by severe erosion, depth

to the fragipan often is closely related to past land use and to the slope.

The area of occurrence of Grenada soils in Grenada County, Mississippi

(A. E. Thomas, 1967) is an example (see Appendix for description).

Concepts have centered on the less eroded soils. Consequently, in many

areas fragipans are shallower than much of the literature would suggest.

On a given landscape the depth is frequently relatable to wetness of the

soil. Two of many examples have been selected. Neeley ( 1965), studying

fragipans in soils of New York developed in medium-textured glacial till,

reported a range of 75 to 30 cm. to the top of the fragipan from the drier

to the wetter soils. The soils range from sequence IIe through IIc to I Ia of

Fig. 2. A. E. Thomas ( 1 967), working with soils developed in loess in

Mississippi, reports a range from 50 to 10 cm. Sequences involved are

IIf and IIb.

In some associations, the fragipan is not shallower in the wetter soils.

Soils of the Lebanon series and related soils in the Missouri Ozarks

(Krusekopf, 1942; Scrivner, 1960) have a fragipan immediately beneath

and perhaps partly within the loess that covers a buried soil (sequence

1Ig or JIh). Fragipans in this soil association are no shallower in the somewhat wetter soils that occur in slight depressions than in the associated

better drained soils.

Thickness ranges from about I5 to 125 cm. In some local associations

in the northeastern states, thickness remains fairly constant over a considerable range in wetness (Carlisle, 1954; Neeley, 1965). These fragipans are weakly altered and bear a strong imprint of the compact glacial

till parent material. The drier soils have sequences IIc and IIe of Fig. 2;

the wetter, sequence IIa. Constant thickness over a wide range in wetness is not restricted to fragipans with strong parent material influence.

Bailey (1964) reports rather constant thickness for the fragipans in

strongly illuvial horizons (sequence Ild, Fig. 2 ) of some soils developed

from limestone residuum and mixed sedimentary rocks in Kentucky. In

certain loess-derived soils of the middle and lower Mississippi Valley,

the fragipans become thicker in the wetter soils of the association (A. E.

Thomas, 1967; Bailey, 1964). The fragipans in these soils are strongly

affected by illuviation, eluviation, or both. The drier of these soils have

sequences IId, IIf, and IIg; the wetter soils, sequence IIb. In other

associations of loess-derived soils in the same general area, the fragipan

thins with increasing wetness (Grossman el al., 1959a). Nettleton et al.

( 1968a), in their study of fragipans in certain soils of North Carolina de-



veloped from Coastal Plain sediments (sequence IIh, Fig. 2), also report

thinning of the fragipan with increasing wetness.

111. Occurrence of Fragipan Soils

The occurrence of soils with fragipans may be considered at several

scales. Looking at the country as a whole, fragipans have been recognized in some soils in all states east of the Mississippi River and in adjacent Minnesota, Missouri, Arkansas, Oklahoma, Louisiana, and eastern

Texas. Soils with fragipans are the principal soils in some parts of that

area, and they are of minor extent in other parts. They have been reported

in the western states (Cline, 1952; Whittig et al., 1957) but seem to be

of very minor extent and importance there. Figure 3 shows areas in the

United States where one or more of the principal kinds of soils have

FIG. 3. Areas in the United States where one or more of the principal kinds of soils have

fragipans. The most extensive soils in the areas delineated are: A, Altisols: I , Inceptisols;

S , Spodosols: U and U 2 , Ultisols (generalized from Soil Conservation Service, 1969, except U 2 delineations, which are from Bartelli, 1968). Dashed line shows the approximate

eastern boundary of the prairie (generalized from Shantz and Zon, 1914).



fragipans. They are extensive and may be among the principal soils in

areas where the dominant soils are Spodosols, Inceptisols, Alfisols, or

Ultisols in the warm humid and the cold humid, central and eastern parts

of the country. Fragipans apparently do not occur in soils of the humid

prairies, the Great Plains, or the semiarid and arid areas of the west.

The following relationships are evident in the broad-scale occurrence

of fragipan soils: ( 1 ) Fragipans are restricted to areas where the excess

of precipitation over evapotranspiration is sufficient at some time of the

year for movement of water down through the soil. (2) They occur in

both warm and cold climates. (3) Fragipans seemingly are absent in soils

of the extensive natural grasslands of the humid prairies and the Great

Plains. (4) Fragipans occur in Spodosols, lnceptisols, Alfisols, and

Ultisols. Spodosols with fragipans are so common as to suggest a genetic


Other relationships may be evident at the larger scale of soil association maps for counties where fragipans are important. The occurrence

of fragipan soils in relation to composition of soil parent materials

apparently is complex when viewed generally. But local relationships

may be evident, especially if the soils are not old. The surveys of Tompkins County, New York (Neeley, 1965) and of Franklin County, New

York (Carlisle, 1958) illustrate the influence of lime content of parent

material on occurrence of soils with fragipans. In these areas soils with

well expressed fragipans are prevalent in areas of low-lime glacial till

and absent in areas of high-lime till. The Franklin County, New York,

survey illustrates the influence of texture of soil materials on the occurrence of soils with fragipans. I n that area fragipans are present in Spodosols developed in relatively well-graded glacial till and are absent in

Spodosols developed in poorly graded glaciofluvial materials of roughly

comparable mineralogical composition. In Tate County, Mississippi (J. S.

Huddleston, 1967) the distribution of soils with fragipans on the countywide scale is related in part to the loess distribution pattern and to the

pattern of geological erosion. Other illustrations of these kinds of local

relationships may be found in published soil surveys of areas shown in

Fig. 3.

For a multicounty area in southern New York and northern Pennsylvania, Denny and Lyford ( 1963) showed the distribution of soil associations in which soils with fragipans are extensive. They concluded that

within the region studied many of the soil differences are primarily related to kind of parent material and its hydrologic characteristics.

A third and still larger scale is the detailed soil map showing delineations dominated by a particular kind of soil. This is the scale commonly

used in planning farming practices for individual fields and farms. To

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II. Horizons of Fragipan Soils

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