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III. Occurrence of Fragipan Soils
FRAGIPAN SOILS OF THE EASTERN UNITED STATES
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
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
R. B. GROSSMAN A N D F. J. CARLISLE
obtain a sense of the pattern of occurrence of fragipan soils on this scale,
one should mentally walk across the detailed map observing the pattern
of mapping unit delineations in relation to slope, topography, and other
Practical considerations do not permit delineation on the usual soil
map of the pattern of soil occurrence within distances measured in tens of
feet. This scale of observation, however, may be very fruitful in understanding the genesis of the soils. Daniels and associates, for example,
have worked at this scale to study certain soils with fragipans of North
Carolina that occur on the less dissected part of a pre-Wisconsin geomorphic surface formed in Coastal Plain sediments (among other papers,
Daniels et af., 1966; Daniels and Gamble, 1967; Nettleton et af., 1968a,b).
Lyford et al. (1963) have reported a study at a similar scale on soils
with fragipans in central Massachusetts that are developed in Wisconsinage glacial till. Less extensive studies on such a very local scale have
been reported by Bornstein et al. ( I 9 6 3 , Carlisle (1954), Lyford and
MacLean ( 1 966), Olson ( 19621, and Denny and Lyford ( I 963).
IV. Properties of Fragipans
1 . Texture
Most fragipans are loamy as the term is defined by the Soil Survey
Staff (1 967). They may be skeletal, but not fragmental. Few strong fragipans are sandy. Although some fragipans exceed 35 percent clay, no
fragipans have been reported with over 60 percent clay. Fragipans in
weakly illuvial horizons (IIa, IIc, IIe, Fig. 21, such as are common in
the northeastern United States, contain less than 35 percent clay, and
most contain less than 25 percent clay (Jha and Cline, 1963). Maximum
clay percentage does not appear to be strongly related to the kind and
proportions of clay minerals. Fragipans with montmorillonite predominating may exceed 35 percent clay (for example, Hutcheson et al., 1959).
The fine earth of fragipans, after allowance for the clay percentage,
usually is high in material from 0.2 10 0.02 mm. and low in the range
above 0.25 mm. A high content of silt and very fine sand may promote
development and expression of fragipans (Jha and Cline, 1963; Carlisle,
1958; Grossman and Cline, 1957).
Chemical properties of fragipans do not seem unique. Fragipans are
low in organic matter, have low or moderate levels of extractable iron
FRAGIPAN SOILS O F THE EASTERN UNITED STATES
relative to the clay percentage, are rarely if ever calcareous, lack appreciable soluble salts, and have at most moderate exchangeable-sodium
levels. Exchange capacity ranges widely, depending on the amount and
kind of clays. Base saturation ranges from low to high. Fragipans in
strongly illuvial horizons of soils weakly influenced by the Wisconsin
glacial advances may have very low base saturation. Base saturation
usually rises with increasing wetness. Fragipans in soils with low base
saturation in their lower parts may have more extractable magnesium
than calcium, and some extractable sodium may be present. Hutcheson
et a f . (1 959) and Pettiet ( 1 964) studied fragipans in such soils. They suggest that high extractable magnesium may make clay more susceptible to
movement and rearrangement, thereby fostering fragipan formation.
Presence of magnesium may be a factor in some soils with fragipans but
many fragipans do not have particularly high extractable magnesium.
Soil pH values range from 4 to 7. Aluminum extractable with a neutral
salt follows the common pattern. Values commonly increase rapidly as
the soil pH drops below 5. Soluble silica and aluminum determinations
have been reported by a number of workers, either in connection with
studies on bonding or incidental to characterization of. the clay mineralogy (Alexander, 1955; Comerma, 1964; Jha and Cline, 1963; Knox,
1957; Olson and Hole, 1967-1 968; Pettiet, 1964; Yassoglou and Whiteside, 1960). No consistent pattern has been observed.
Fragipan expression seems largely unrelated to the mineralogy of the
sand and silt other than for the control that presence of carbonate may
impose. Daniels er af. (1966) found no relationship between fragipan
expression and the percentage of feldspar in the very fine sand fraction
for certain soils of North Carolina having strongly eluvial fragipans.
In some landscapes fragipans are restricted to the older geomorphic
surfaces. These soils may have lower weatherable minerals than the
associated soils on the younger geomorphic surfaces. T h e difference in
mineralogy, however, would not appear responsible for the pattern of
fragipan occurrence. Fragipans are common on the Ozark Plateau and
on the Coastal Plain. Parent material sources high in quartz and low in
weatherable minerals are common to these areas. The high proportion of
quartz does not appear to have directly contributed to the prevalence of
fragipans in these areas (see Section VI, A).
The clay mineralogy of fragipans is similar to the clay mineralogy of
horizons in comparable positions in associated soils without fragipans.
In the northeastern United States, related to the prevalence of paleozoic
R. B. GROSSMAN AND F. J. CARLiSLE
sedimentary rocks as sources, 2:1 lattice clays other than montmorillonite predominate, with mica (illite) a quantitatively important component (among others, Knox, 1957; Jha and Cline, 1963; F. P. Miller,
Moving westward into the northern Middle West, Yassoglou and
Whiteside ( 1960) have reported illite, chlorite, and interstratified minerals with some interlayered montmorillonite in the clay fraction of several soils with fragipans in Michigan. Olson ( 1 962) indicated a somewhat
similar suite of clay minerals in some soils with fragipans of northeastern Wisconsin. Moving southward, the fragipan in Lebanon-like soils
of Missouri studied by Scrivner (1 960) contains randomly stratified
montmorillonite, illite, and Vermiculite with some kaolinite; the mineralogy is allied with the surficial loess rather than the limestone residuum
beneath the loess.
Soils with fragipans of the middle and lower Mississippi Valley developed in loess contain appreciable montmorillonite (Anderson and
White, 1958; Glenn, 1960; Grossman et af., 1959b; Hutcheson et af.,
1959; Pettiet, 1964). In these soils the fragipans occur in horizons that
have undergone moderate or strong illuviation (sequences IIb, IId, IIf,
IIg of Fig. 2). The montmorillonite shows evidence of interlayers that
do not extend fully on treatment with glycols. Hutcheson et al. (1959)
suggested that in the fragipan vermiculite forms at the expense of montmorillonite. Glenn ( 1 960) and Pettiet ( 1964) reported appreciable
In the middle Atlantic states, Nikiforoff et al. ( 1948) found significant
amounts of kaolinite in the fragipan of a soil that occurs in Maryland.
Nettleton et af. (1968b) studied the clay mineralogy of soils in North
Carolina with fragipans in highly eluvial horizons (sequence IIg of Fig.
2). Kaolinite or a vermiculite-chlorite intergrade were the most abundant
components, with lesser amounts of montmorillonite. Evidence was presented for considerable amorphous material, and the kaolinite had imperfect crystallinity.
4 . Bulk Density
Bulk densities of the moist fine-earth fabric usually exceed 1.4 g./cc.
and mostly exceed 1.6 g.lcc. Differences between the moist and dry
natural-clod bulk densities are not large. The coefficient of linear extensibility (Grossman er al., 1968) usually does not exceed 0.04. The fragipan
rarely has a lower bulk density than overlying horizons. Differences
from horizons below are variable. In soils with a strong influence of parent
material on the fragipan (horizon sequences Ila, IIc, IIe of Fig. 2 ) and a
FRAGIPAN SOILS OF THE EASTERN UNITED STATES
parent material with a high bulk density, the fragipan may differ little in
bulk density from the parent material (examples in Soil Survey Staff,
1968b). If the bulk density of the parent material is moderate, the fragipan horizon may have the highest bulk density in the soil (for example,
Jha and Cline, 1963). Fragipans in eluvial horizons that immediately
overlie strongly expressed illuvial horizons (horizon sequence IIh)
often have the maximum bulk density of the soil (examples in Nettleton
et al., 1968a; Scrivner, 1960; Yassoglou and Whitesite, 1960).
Rutledge and Horn (1965), Yassoglou and Whiteside (1960), and
Pettiet ( 1964) have discussed estimates of the pore-size distribution
of fragipans based on water retention determinations. A large body of
information is available for the computation of pore-size distribution in
the Soil Survey Investigations Reports series published by the U.S.
Department of Agriculture. Medium- and fine-textured fragipans can be
shown to have a high proportion of the total porosity filled with water
held at energies above 15 bar; this porosity must consist of very small
pores. The air-filled porosity at 1/3 bar can be shown to be small, but
this is also common to many medium-textured soil materials.
I . Macroscopic
Many fragipans have roughly vertical planes that delineate large prisms
or blocks. Nikiforoff (1 955) vividly described the feature in the Beltsville
soil of Maryland: “Throughout its thickness the pan is split into large
irregular blocks ranging from about 1 f.i to 2 feet in horizontal diameter.
Planes of cleavage are marked by strong bleaching of the walls of fissures
which produces conspicuous streaks on the exposures of the hardpan.
In vertical planes, these irregular light colored streaks are roughly
parallel, whereas in horizontal planes, they form a striking polygonal network . . . . Walls of the cracks are bleached laterally for . . . a few millimeters to more than an inch . . . . Beyond these bleached zones there are
yellow to orange oxidized zones so that on cuts the bleached streaks are
enclosed between rust-colored bands.”
The pattern may be much less distinct in other soils, particularly if the
fragipan is coarse textured. The fragipans studied by Daniels et at. ( I 966)
and Nettleton et al. (1 968a) lack the polygonal pattern. Information on
the composition and organization of the periphery of the large prisms and
blocks relative to the interiors was reported by Carlisle (1954), Gile
(1958), Jha and Cline (1963), F. P. Miller (1965), Pettiet (1964), and
Vanderford and Shaffer (1966). The study by F. P. Miller (1965) is
R. B. GROSSMAN A N D F. J. CARLISLE
Structure of the large prisms or blocks is variable. Trends follow those
for horizons other than fragipans: eluvial horizons usually are massive or
platy; illuvial horizons usually are blocky. For example, the fragipans
described by Yassoglou and Whiteside (1960) and by Nettleton et al.
(1968a) occur in horizons that overall are eluvial; much of the fabric has
massive or platy structure. In contrast stand the more clayey, strongly
illuvial fragipans (Bailey, 1964; Rutledge and Horn, 1965). They have
subangular or angular blocky structure of moderate expression and
medium size. Fragipans that have undergone weak alteration may be
strongly influenced by the structure of the parent material. If the parent
material is platy (for example, Lyford et al., 1963), the fragipan may be
platy. If it is crudely blocky (for example, Neeley, 1965; Carlisle, 1954),
the fragipan usually has a crude blocky structure. If massive (for example,
Jha and Cline, 1963), the fragipan may tend to massive structure.
Large pores that are continuous vertically over the thickness of the
fragipan usually are widely spaced. Vertical planar surfaces within the
large structural units extend over distances that are a small fraction of
the thickness of the fragipan, usually having dimensions of a few centimeters. Some fragipans have platy structure. The plates usually overlap,
and consequently vertical pores between peds are tortuous. Fragipans
are often described as vesicular, implying that the pores within peds are
not interconnected. Lyford et ad. (1 963) have provided an apt description:
“The pores in the peds are seldom continuous. They branch and rebranch but tend to end up inside the ped rather than continue from one
side to the other. Many of the pores observed in broken peds are short and
bulbous; they do not empty at the surface.”
Evidence for translocation of silt or clay is common in fragipans.
Field descriptions of fragipans generally refer to clay films on ped surfaces and within pores. The clay films may be thin to thick. Continuous
clay films on ped surfaces are not reported in fragipans. Clay films,
however, may be strongly expressed in pores. Rearrangement of silty
material has been observed in many fragipans. Carlisle ( 1958) described
the feature as follows: “Gray silty material, which does not occur in the
layers above the fragipan, coats the upper surfaces of rock fragments and
impregnates the uppermost part of the peds within the fragipan.” Descriptions of fragipans from outside New York and New England, including fragipans developed in Wisconsin glacial till, do not place as
much emphasis on rearrangement of silty material.
2 . Thin-Section Observations
A partial list of observations include Jha and Cline (1963), Carlisle
(1954), Calhoun (19681, F. P. Miller (196.9, Nettleton et at. (1968b),
FRAGIPAN SOILS OF THE EASTERN UNITED STATES
Olson ( I 9621, Pettiet ( I964), Yassoglou and Whiteside (1 960), Grossman et al. (1 959c), Horn and Rutledge ( 1965), Nikiforoff et al. (1 948),
and McCracken and Weed ( 1 963). The descriptions have one unifying
feature. Some of the clay connects between sand and silt grains, and
some of it shows optical evidence of preferred orientation; the fabrics
would be plasmic and sepic (Brewer, 1964). The clay may be scarce and
strongly concentrated at contact points of sand and silt grains with interstices only partially filled; or the clay may be abundant and form a continuous medium within which the sand and silt grains are set. A significant
portion of the clay apparently has moved over distances measured at least
in millimeters; some may have come from horizons above. Bodies of
moved clay are present in strongly eluvial fragipans as well as in fragipans
that are illuvial B horizons. Amounts of moved clay are reported by
Calhoun (1968), Carlisle (1 954), Horn and Rutledge (1 9 6 3 , F. P. Miller
(1965), Nettleton et ai. ( 1968b), and Soil Survey Staff (1968a).
Horn and Rutledge ( 1965) attach particular importance to the sepic
micromorphology of fragipans in determining brittleness. The fragipan is
viewed as “cellular.” Plasma separations that impart rigidity delimit
small volumes with lower strength. Several workers have emphasized
the importance of closeness of packing of the sand and silt grains. Some
fragipans, however, do not show particularly close packing (Horn and
Rutledge, 1965). Packing is generally closer in coarser-textured fragipans and may be a more important factor in determining the rigidity in
the coarser textures. Horizons above the fragipan may show closer packing of the sand and silt grains and yet have markedly lesser rigidity than
the fragipan (Grossman et al., 1959~).
Consistence is the principal defining property of fragipans. But the
classes of soil consistence are defined qualitatively and evaluation of
consistence is subjective. This results in vagueness in the definition of
fragipan and leads to difficulty in achieving uniform application of the
The soil consistence test (Soil Survey Staff, 195 1) involves compressing a piece of soil about 2 to 4 cm. across between thumb and forefinger
until failure occurs. It is a kind of unconfined compression test, a subject reviewed recently by Gill and Vanden Berg (1967). If the piece of
soil is uniform with no predetermined planes of weakness and the cracks
that form at failure are near the center of the bearing surface and parallel
the direction the force is applied, then cohesional forces largely determine the resistance to rupture. Tensile strength is therefore measured. Usually, however, the cracks form at an angle to the axis of corn-
R. B. GROSSMAN A N D F. J. CARLISLE
pression; hence some frictional force contributes to the resistance to
rupture. Moreover, the area of contact between finger and soil is appreciable relative to the dimensions of the piece of soil. This tends to cause
shearing stresses. Rearrangement may occur before rupture. The rearrangements commonly are such as to blunt the point of the crack, leading to an increase in strength of the soil material. Often the piece of soil
has predetermined planes of weakness. Those cracks which most nearly
parallel the axis of compression probably determine the strength.
1 . Induration
Fragipans appear cemented when dry, but soften when wetted. Cementation implies little reduction in hardness on moistening (Soil Survey
Staff, 195 1). Fragipans, therefore, are not cemented. The expression
“reversibly indurated” has been used, but it is not very satisfactory as it
seems to be a contradiction in terms. Most fragipans slake when dry
pieces are placed in water (Anderson and White, 1958; Knox, 1957;
Comerma, 1964; Jha, 1961; Nikiforoff et al., 1948; Olson, 1962; R. M.
Smith and Browning, 1946). One of the fragipans studied by Knox ( 1957)
did not slake; silica was implicated in the bonding. Field observations
of slaking by fragipans are of common occurrence, but few are published. Nettleton et al. (1968b) described the slaking of fragipans in
ditch banks. Olson (1962) studied the breakdown of fragipan material on
2. Resistance to UnconJined Compression
When dry, most fragipans are at least hard-“moderately resistant to
pressure; can be broken in the hands without difficulty but is barely
breakable between thumb and forefinger” (Soil Survey Staff, 195 1).
When moist, fragipans are usually at least firm-“crushes under moderate pressure between thumb and forefinger, but resistance is distinctly
noticeable” (1951). Grossman and Bartelli (1957) used a hand dynamometer to determine the force applied at the point of discrimination
between several consistence classes by a group of soil scientists. The
lower limit was 5 kg. for hard consistence and 3 kg. for firm consistence.
Grossman and Cline ( 1 957) report a median value of 17 kg./cm.2 and a
range of 4 to 25 kg./cm.2 for the resistance to rupture when dry of a number of fragipan horizons from soils of New York. Grossman (1954)
measured a resistance to rupture of 4 kg./cm.2 at a water content near
1/3 bar retention for a fragipan having a resistance of 20 kg./cm.2 when
dry. Knox ( I 954) assembled information on the resistance to rupture of
FRAGIPAN SOILS OF THE EASTERN UNITED STATES
various particulate materials. Molding sands with 5 to 10 percent bentonite have crushing strength similar to the median value for the fragipans
from New York studied by Grossman and Cline ( 1 957).
3 . Miscellaneous Tests
Yassoglou and Whiteside ( 1 960) used a cone penetration test to characterize the hardness of moist fragipan material from certain soils of Michigan. The fragipan had higher resistance than other horizons, and the
fragipan subhorizon judged hardest by field examination offered the
highest resistance. Rutledge and Horn ( 1 965) employed a needle penetrometer to study fragipans in soils of Arkansas. Penetration at several
moisture tensions was somewhat lower for the fragipan than for the
horizon immediately above. Grossman et al. (1959a) used a “dropshatter” test to characterize a fragipan soil from Illinois developed in
loess. The fragipan resisted shatter more than horizons above, but less
than the C horizon beneath.
4 . Brittleness
There are very few laboratory measurements of brittleness. Grossman ( 1954) compared the abruptness of rupture under unconfined compression of two fragipan materials. At a moisture content near that retained against 5 bars, the fragipan with less than 5 percent clay failed
abruptly; the one with 15 percent clay showed some plastic deformation.
The concept of brittleness as applied to fragipan soil material has not
been clearly formulated. The reference state with respect to moisture
tension for medium textures probably should be about 1/3 bar. Many
moist, coarse-textured soil materials exhibit brittleness. Even some
moist clayey materials exhibit brittleness if there are numerous well
expressed planes of weakness in the test specimen; an example would be
the B horizon of some Oxisols. Most fragipans, however, have incomplete, weak, and commonly widely spaced structural planes. Their brittleness is usually not the result of rapid failure along structural planes.
Rather, it would appear to be related to the bonding by clay-size material of the sand and silt grains (the s-matrix as defined by Brewer,
1964). For coarse-textured fragipans, much of the clay occurs as braces
between sand and silt grains. Amounts of clay are insufficient for local
plastic movement. The leading point of a crack is not subject to blunting. Rather, the stress remains concentrated, leading to rapid propagation of the crack. Such an explanation is not applicable to medium and
fine textures. In some of these instances, the highly sepic micromorphology may contribute to brittleness (Rutledge and Horn, 1965).
B. GROSSMAN A N D F. J. CARLISLE
V. Fragipans and the Soil Water Regime
Several investigators have reported on the water tables of soils with
fragipans (Gile, 1958; Nettleton, 1965; Spaeth and Diebold, 1938; Lyford, 1964; J. H. Huddleston and Olson, 1967). Many data exist in mimeographed reports; these often are available from state soil survey organizations. Some of the studies employ lined wells placed at depths above,
within, and below the fragipans. In principle, this arrangement permits
detection of a perched water table above the fragipan. Other studies
employ unlined wells which do not permit detection of a perched water
table. Interiors of the large structural units may be considerably below
saturation while free water is present between them. Lined wells that
terminate within these structural units may not indicate the presence of
this free water.
Much of the area of fragipan soils now supports trees. Removal of
water by transpiration by trees usually is greater than when the land is
cleared. The water table regime under trees may underestimate the wetness of the soil when it is cultivated or used as sites for construction.
On the other hand, measurements in cleared areas may indicate shallower
water tables than are actually present over most of the area.
Soils with fragipans are subject to the rather well defined seasonal
pattern in water table depth found over much of the eastern United
States. Water tables rise in late fall and remain high until transpiration
by plants becomes appreciable. Often the drop in water table occurs
about when the trees leaf. Water-holding capacity of the fragipan is
usually low (Section V, B), and small additions of water may raise the
water table markedly. Soils are placed in wetness classes mainly on inference based on the depth to and expression of mottles or low chroma
parts. A significant practical question is the extent to which the water
table regime of soils with fragipans differs from associated soils in the
same wetness class.
Laboratory estimates of field capacity or maximum water retention
must be viewed cautiously. The determinations may be on fragmented
samples resulting in a significant overestimate of water retained against
low tension. As previously indicated, under field conditions the interior
of the large structural units of the fragipan may not contain free water,
even though free water is present in the cracks between the structural
units (Nikiforoff et al., 1948; Carlisle, 1954; Alexander, 1955). Conse-
FRAGIPAN SOILS OF THE EASTERN UNITED STATES
quently, maximum water retention in place would be below that calculated from the total porosity of the moist fabric. R. M. Smith and Browning ( 1946) commented on the low water content of fragipan material after
wetting in the laboratory, even under vacuum. They suggested that the
fragipan material may have appreciable porosity that does not fill readily
Comer and Zimmerman ( 1 969) reported that for a 3-year period the
water content in the fragipan of a wet soil in Vermont ranged only from
19 to 23 percent by volume. The soil is not subject to recharge by upward water movement from a regional water table; recharge is by downward moving water with perhaps a lateral component of movement. The
constancy of the water content over this period suggests a high degree
of isolation from both withdrawal of water by plants and additions from
precipitation. The effective contribution of the fragipan to the water holding capacity of the soil at any point in time over this period would appear
to have been small.
Laboratory measurements of the saturated hydraulic conductivity
have been reported by R. M. Smith and Browning ( 1 946), Grossman et al.
( 1 959a), Yassoglou and Whiteside ( 1 960), and Pettiet (1 964). Values
range from 0.01 to 1 inch per hour. Large numbers of field percolation
determinations have been made. In some areas, these are required by law
in planning small-scale sewage disposal systems. Hill ( 1966), Alexander
(1955), and J . H. Huddleston and Olson (1967) have discussed procedures and present values for soils with fragipans.
Horizons above the fragipan are usually quite pervious unless altered
by man’s activities. Infiltration usually is not limited by a horizon above
the fragipan until near-saturated conditions prevail. Fragipans have a
lower saturated hydraulic conductivity than do horizons above. Consequently, low-tension water accumulates at the top of the fragipan and
moves laterally. Fragipans do not necessarily have lower saturated
hydraulic conductivity than the horizons beneath (for example, Yassoglou and Whiteside, 1960; Alexander, 1955). If the underlying soil
materials are pervious, then the saturated hydraulic conductivity of the
fragipan may be a minimum for the profile.
There may be several reasons for the low saturated hydraulic conductivity of fragipans. Lack of vertical continuity of interped pores and
isolation of pores within peds may have importance. O’Neal (1952)
discussed the relationship between perviousness and vertical continuity