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V. Fragipans and the Soil Water Regime

V. Fragipans and the Soil Water Regime

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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

with water.

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



of pores. Some fragipans have low total porosity. Those with moderate

porosity tend to have appreciable clay (for example, Rutledge and Horn,

1965). The area that actually conducts low-tension water in the field may

be quite small, limited to the periphery of the large structural units.

Hydrology studies of watersheds reflect the complexity of the total

natural condition. Minimum discharge rates are indicative of the integrated amounts of water moving through the soils of the area. Comer and

Zimmerman ( 1 969) reported that the minimum discharge rate for a watershed in Vermont where wet soils with fragipans occupy 44 percent of the

area is a magnitude lower than for a contiguous watershed where such

soils occupy 22 percent of the area. They suggested that the combination

of low permeability of the fragipan and the high water-holding capacity

of the horizons above the fragipan is largely responsible for the lower

minimum discharge rate of the watershed with the greater proportion of

wet fragipan soils.

VI. Genesis of Fragipans

This section has been written on the underlying assumption that clay

is the bonding agent; moreover, it is assumed that this clay ranges widely

in mineralogy and surface chemical properties.


1 . Silicate Clay as the Bonding Agent

Several workers have proposed that silicate clay is the principal bonding agent (R. M. Smith and Browning, 1946; Nikiforoff et al., 1948;

Carlisle, 1954; Knox, 1957; Jha and Cline, 1963; Yassoglou and Whiteside, 1960; Comerma, 1964; Hutcheson and Bailey, 1964). Dispersing

agents for clay have been shown by Comerma (1964), Knox (1957), and

Jha and Cline ( 1 963) to disaggregate the fragipans of certain soils more

completely than treatments designed to remove silica, hydrous iron

oxides, or hydroxy aluminum compounds. There is no evidence from the

large number of particle-size analyses using standard dispersing treatments that the clay in fragipans resists disaggregation.

Close packing of the sand and silt is thought to contribute to the effectiveness of the clay as a bonding agent. Nikiforoff et al. ( 1 948) placed

principal emphasis on the close packing and related interlocking of the

sand and silt. Hutcheson and Bailey ( 1 964) also emphasized closeness of

packing. They write, “. . . [we] visualize pan horizons as brick and

mortar structure, i.e., silt particles acting as bricks held in a dense mass

by clay mortar.” The thin-section observations by Jha and Cline ( 1 963) fit



the “brick and mortar” model. Other workers have placed greater emphasis on the clay bridges between sand and silt grains (Knox, 1957;

Yassoglou and Whiteside, 1960; Grossman and Cline, 1957) with proportionately less on a dense, continuous filling of clay in the interstices.

More would seem to be involved than disposition of the clay. In some

horizons of clay accumulation, bridges of clay between sand grains are

prominent (Soil Survey Staff, 1960, p. 41). Yet, when moist, these

horizons do not necessarily exhibit the brittleness and rigidity of fragipans. Several writers (Carlisle et a f . , 1957; Yassoglou and Whiteside,

1960; Grossman and Cline, 1957) have commented on the dual role of

clay. At low clay contents the bridging of clay between sand and silt

grains lends rigidity. At higher clay contents, volume changes with

moisture promote formation of cracks that reduce the rigidity when


2. Other Bonding Agents

a. Silica. The earlier literature contains suggestions that silica is the

bonding agent (Marbut, 1935; Krusekopf, 1942; Winters, 1942). At the

time, total analyses of soils received more emphasis than they do now.

Total analyses of fragipans low in clay, with the sand and silt dominated

by quartz, indicate high proportions of silica, which would be consistent

with the idea of a siliceous bonding agent. Studies were then current on

silica cementation of indurated horizons in certain soils of western

United States (for example, Nikiforoff and Alexander, 1942). These

studies were employed to support bonding by silica in fragipans. There is

similar informal conjecture today. The argument runs that as silica does

cement some soils, perhaps small amounts -less than that detectable by

methods employed to date-may play a similar role in fragipans. Knox

( 1 957) presented the only experimental data for the implication of silica.

Extraction of silica with various reagents (Section IV, A, 2) has not

shown a consistent maximum in the fragipan. Baker (1967) determined

the mineral stability by solubility investigations for the strongly eluvial

fragipan horizon of certain soils developed in loess over cherty limestone

residuum in Missouri. Kaolinite and quartz were stable. The concentration of silica was well below that supportable by opal. McKeague and

Cline ( 1 963) suggested that concentration of silica by evaporation may

be important in the surface adsorption of silica from solution on soil

particles. Fragipans as a rule, however, are not subject to frequent and

pronounced desiccation. R. W. Miller ( 1967) stressed the importance of

small differences among horizons in controlling the translocation and deposition of silica. H e studied soils from Utah that are not subject to high



precipitation, and one of the soils was formed in materials containing

volcanic glass. Such differences from soils with fragipans of the eastern

United States must be considered in the application of research on silica

solubility in soils of the western United States. Calcite apparently adsorbs little or no silica (McKeague and Cline, 1963), and presence of

carbonate appears to increase the adsorption of silica (R. W. Miller,

1967; McKeague and Cline, 1963). The scarcity of fragipans in calcareous soil materials and the influence of carbonates on silica solubility relationships might be related.

b. Aluminum and Iron. Hydrated oxides of iron or aluminum have

been suggested as bonding agents. Knox ( 1 957) and Comerma ( 1 964)

found no evidence for their implication. Alexander (1955) failed to find

a maximum in extractable aluminum in the fragipan. Anderson and White

(1958) presented evidence for iron oxides contributing to the rigidity in

a fragipan. Horn and Rutledge (1965) suggested that segregations of

iron oxides in association with silicate clay are important in determining

rigidity. Nettleton et al. ( I 968b) suggested that amorphous aluminum

compounds may play a role in the hydrogen bonding of fragipans.

c. Water Films. Surface tension effects associated with water films

lend rigidity to moist soil material (Knox, 1954; Fountaine, 1954). For

spherical particles, cohesion increases proportional to the reciprocal

of the radius. As a point of reference, the cohesion for spheres with a

radius of 10 p ranges roughly from 0.1 to 1 kg./cm.2, depending on the

assumptions about packing and whether the pores are partially or entirely

filled (Knox, 1954). This range compares with a crushing strength of 4

kg./cm.2 for a moist fragipan soil material of silt loam texture (Grossman, 1954). Water films may contribute appreciably to the rigidity of

some moist fragipan material. It is doubtful, however, that the greater

rigidity when moist of fragipans than of otherwise comparable soil materials is due to water films.

3 . Mechanisms of Bonding

Knox ( 1 954) concluded from a review of the literature that chemical

forces rather than mechanical interlocking are probably responsible for

the strength of fragipan material. Nettleton et al. (196%) propose that

amorphous clays adhere strongly to the disordered surface of quartz

grains by hydrogen bonding. Acidity of the soil material fosters this

bonding. Surface negative charge of the clay is low and hydroxyl groups

exert a strong influence. Infrared analyses indicate the presence of

hydrogen-bonded hydroxyls. These are lost on heating to 300°C. Heating

to this temperature leaves the fragipan material soft and loose. Brittle-



ness was restored by rehydration under moderate relative humidity,

Knox (1954) studied the effect of heating to as high as 500°C. on the

strength of fragipan material. The strength was not reduced appreciably.

His observations would appear to be at variance with those of Nettleton

et al. ( 1968b). The fragipans investigated, however, differ markedly.

Knox ( 1 954) studied fragipans with strong influence of the parent material

(sequences IIa, IIc of Fig. 2); illite dominates the clays. The fragipan

studied by Nettleton et al. (1968b) occurs in a strongly eluvial horizon

(sequence IIh, Fig. 2) where accumulation of amorphous clay through soil

development would be more likely; kaolinite is a prominent clay mineral.

The relationship between the properties of the exchange complex and

attraction between clay particles has received much attention in the

field of soil physical chemistry. Nettleton et al. (1 968b) apply concepts

from this field to bonding of fragipan material. Many fragipans have low

pH and contain appreciable aluminum extractable with a neutral salt.

Calcareous fragipans, moreover, are rare. Applying the ideas presented

by Emerson and Dettmann ( 1960), both the low pH through the increase

in positive charge and consequently stronger electrostatic attraction, and

the presence of trivalent aluminum, would increase attraction between

clay particles. Carbonate would reduce the attraction because the resulting pH leads to low positive charge and to precipitation of the aluminum.

Such ideas, however, do not provide a general explanation for the

rigidity of fragipans. Many kinds of soil horizons are acid and have

high extractable aluminum. Moreover, some fragipans have pH values

near neutrality.




I. Inheritance of Properties

Fragipans in some areas of the Coastal Plain south of Wisconsin

glaciation occur on the older geomorphic surfaces but not on the younger

surfaces (Daniels et al., 1966; Nikiforoff et al., 1948; Nikiforoff, 1955).

This is evidence that these fragipans may be relicts of an older environment. Restriction of fragipans to the older, more stable parts of the landscape is not limited to the Coastal Plain. This is the pattern of occurrence

in parts of the Ozark Plateau, for example. Investigators of fragipans in

areas of Wisconsin glaciation do not agree on the extent to which fragipans may be relict features. Denny and Lyford ( 1 963) wrote for the area

of Wisconsin glaciation along the southwestern New York-Pennsylvania

border: “The soils are relatively young, are in equilibrium with the

present environment, and contain few, if any, features acquired during

past weathering intervals.” In contrast, Olson and Hole ( 1967- 1968)



placed emphasis on the importance of changes in climate in the development of fragipans in soils in northeastern Wisconsin.

Some fragipans bear a strong influence of the parent material. Not

only composition, but organization of the soil fabric, may be largely

determined by the parent material (Section IV, B, 1). In some soils parent

material has such a strong influence that the distinction between glacial

till and the fragipan may be difficult to establish (Section IX).

2 . Catastrophic Development

Processes common to a periglacial environment have been implicated

in the development of fragipans. Fitzpatrick (1956) offered evidence

that three features common to some fragipans can be produced by freezing wet soil. These features are platy structure, discontinuous spherical

or vesicular pores, and a sheathing of fine material around pebbles.

Yassoglou and Whiteside (1960) and Jha and Cline (1963) presented

evidence against a periglacial origin being applicable generally and the

main agent in fragipan formation. Fragipans occur in areas thought not

to have been subject to periglacial influence. This would seem a convincing argument against a periglacial origin for all fragipans. Some

fragipans, however, may have strong relict influence of a periglacial environment. The fact that the upper surface of fragipans occurs at predictable depths and generally conforms to the land surface does not rule

out relict periglacial influence. As Nikiforoff (1955) and Lyford et al.

(1 963) pointed out, the upper boundary of the fragipan could be controlled largely by the lower limit of obliteration of periglacial influence

by soil development.

Nikiforoff (1955) explored in detail the possibility that the gross

prismatic structure of many fragipans has its origin in a periglacial environment. The large prisms would be formed by frost wedges or would

be the result of dessication and contraction of soil material having an

originally high water content. Jha and Cline (1963), studying a fragipan

in lacustrine sediments, suggested that desiccation cracks which formed

soon after drainage of the lake may have been the origin of the polygonal

pattern. Olson and Hole ( 1 967- 1968) suggested that the large prisms are

related to desiccation cracks formed during a dry period after close of

the Pleistocene.

The top of the fragipan often coincides with discontinuities in composition, organization, or both of the soil material. Evidence for changes

in composition are given in the studies by Nikiforoff et af. (19481,

Scrivner ( 1960), Rutledge and Horn ( 1 9 6 3 , Bailey ( 1964), Vanderford

and Shaffer ( I 966), Beavers ( 1 960), and Calhoun (1968). An example of a


26 1

change in organization would be the shift from ablation till (let down by

ice wastage) to denser till thought to have been compacted by the weight

of ice. Such changes, either in composition or organization, although not

directly responsible for the fragipan themselves, may have lead to conditions favorable to formation of a fragipan.

3. Incremental Development

Fragipans occur beneath eluvial horizons. The overlying B horizon,

if present, commonly is a type of cambic horizon which appears to have

gone through a stage of active clay removal. If an overlying B horizon

is an argillic horizon, it commonly is one in which accumulation of illuvial

clay does not greatly overbalance removal. Tavernier and Smith (1957)

pointed out that Gray-Brown Podzolic soils with fragipans commonly

have either heavy bleached silt coatings on the ped surfaces of the B

horizon above the fragipan or a distinct bleached horizon of eluviation

between the B and the fragipan. Where the latter horizon occurs, it

tongues into or interfingers with the overlying B horizon. Tavernier and

Smith interpreted the tonguing or mingling of eluvial and illuvial horizons

in these soils as evidence of destruction of the B horizon.

Fragipans are therefore subject to the accumulation of substances from

the horizons above. The substances may move as particles or in solution.

Much speculation has centered on whether a precipitated substance is the

bonding agent.

Wetting and drying is probably the principal agent responsible for

mobilization and translocation of silt and clay into the fragipan. Illuvial

silt would reduce porosity and perhaps increase mechanical strength.

The role of illuvial clay is less clear-cut. Many investigators have suggested that illuvial clay reduces porosity. Addition of clay, however, may

increase volume change with change in moisture content and lead to more

large planar pores. Furthermore, illuvial clay coats planar pores and

thereby increases the prominence of structural planes of weakness.

Wetting and drying may also cause translocation of substances within

the fragipan. The mottled color pattern and low-chroma parts of many

fragipans strongly suggest translocation of hydrous iron oxides. Once

the hydrous iron oxide coatings have been removed, the silicate clay may

be more subject to movement. In strongly eluvial horizons, removal of

silicate clay and hydrous iron oxide coatings may have so weakened the

fabric that wetting and drying can lead to extensive rearrangement to

produce a denser fabric (Daniels et al., 1966; Nettleton et al., 1968b;

Yassoglou and Whiteside, 1960; Pettiet, 1964). Strongly eluvial subsoil

horizons tend to be zones of accumulation of free water. This free water



commonly extends throughout the fabric; it is not restricted to vertical

planes. Such high water contents may weaken the fabric and make it

prone to rearrangement.

The subject of incremental development leads to the question of time

required to form a fragipan. Fragipans have formed since the close of the

Pleistocene. Jha and Cline ( 1 963) reported that a fragipan formed in

lacustrine sediments deposited in a lake that drained seven to eight

thousand years ago. On the other hand, fragipans are not found in very

youthful deposits. Lyford er al. ( 1 963) offer evidence that the fragipans

in certain soils of Massachusetts must be atleast 500 years old. Many

fragipans show the marks of illuviation of clay, its removal, or both. Such

alteration does not occur rapidly. Fragipans are not necessarily restricted

to old soils, but neither do they form in a matter of hundreds of years.

4 . Weak Disturbance

Many fragipans have undergone only weak disturbance. Strong influence of the organization of the parent material implies weak disturbance as does strong expression of pedological features such as gross

polygonal structure, a pronounced pattern of accumulation and depletion of hydrous iron oxides, and moved clay bodies.

Fragipans are not subject to frequent and large changes in volume.

Many fragipans occur deep enough and contain so few roots that intense

desiccation is rare. Interiors of the large structural units of some fragipans

resist wetting (Section V, B). Many fragipans have a low potential for

volume change because of moderate or low clay contents. Some fragipans

do have fairly high clay contents. The latter, however, tend to occur at

appreciable depths, where frequent and intense desiccation would be less

likely. Fragipans do not occur in shallow, clayey horizons unless the soil

has been severely eroded.

Disturbance by tree throw and by mass movement is greater above than

within the fragipan. Lyford et al. (1 963) showed for an area of soils developed in glacial till on steep slopes that the base of small gravity movements coincided with the upper boundary of the fragipan. R. M. Smith

and Browning ( 1 946), studying soils with fragipans that occur on steep

slopes in West Virginia, suggested that the base of the small slip scars

common to the area was at the top of the fragipan. The base of the zone

of disturbance due to tree throw commonly coincides roughly with the

top of the fragipans (Lyford and MacLean, 1966; Denny and Lyford,

1963; Olson and Hole, 1967-1 968; Mueller and Cline, 1959). Estimates

of the rapidity of mixing by tree throw point to its importance. Denny and

Lyford ( 1 963) concluded that for soils of the northeastern United States



much of the upper 2 feet has been disturbed by tree throw. The study by

Mueller and Cline (1959) tends to substantiate this conclusion. Milfred

e? al. ( I 967) emphasized the importance of tree throw in northeastern


Fragipans commonly occur at shallower depths in the wetter soils of

a local association. Trees may have rooted less deeply in the wetter soils

because of the shallower depth to free water. Disturbance related to the

presence of the tree roots, such as the volume change resulting from water

withdrawal and recharge, root ramification, tree throw, and root movement resulting from tree sway, would all have extended to shallow depths.

In this view, the upper boundary of the fragipan would be largely determined by the control exercised by the water table regime on the depth

of frequent disturbance.

Presence of a fragipan may increase disturbance of horizons above.

Mixing by tree throw, for example, is probably more frequent above the

fragipan because the fragipan limits the depth of support roots (Section

VII, A). Higher rates of disturbance above the fragipan may increase the

perviousness of these horizons. Low-tension water consquently may

move more rapidly through the horizons above and accumulate at the top

of the fragipan. Accumulation of this low-tension water would affect development of the fragipan. Weak disturbance within the fragipan and

stronger disturbance above the fragipan are two sides of the same coin.

Properties of the soil are a reflection of both.

VII. Fragipans and Soil Use


Fragipans unfavorably influsnce growth by restricting rooting, either

through mechanical impedance or by creating saturated conditions. Soils

with shallow fragipans usually have high water tables over extended

periods. Effects of mechanical impedance and of the shallow water table

on plant growth often are confounded.

Improving the plant nutrient status of soils has become increasingly

feasible. One result has been to increase the relative importance of a

physical limitation such as presence of a fragipan. This is true particularly

in the southeast part of the country. The Coastal Plain of the Southeast

contains much of our potential new land for farming (Bartelli, 1968). In

parts of the Coastal Plain, soils with fragipans are among the principal

soils (Fig. 3).

Depth to the fragipan partially determines its significance to plant

growth. The critical depth depends on the plants. For many plants, the



influence diminishes rapidly at depths greater than 50 cm. Erosion brings

the fragipan closer to the soil surface and thereby increases its significance. Erosion has greatly reduced the average depth to the fragipan

in certain soils, such as the Beltsville and Grenada soils (see Appendix).

Thickness of the fragipan may determine its significance to soil use and

management. The apparently small influence of the fragipan in the

McBride soil (see Appendix) may be largely a reflection of its thinness.

Trees are the principal vegetation on much of the area of soils with

fragipans. The density of tree roots is low in the fragipan (Olson and

Hole, 1967- 1968; Lyford and MacLean, 1966; Mueller and Cline, 1959).

The influence of root distribution on tree throw has received much attention. Olson ( 1 962) suggested that the restriction of roots by fragipans is

greater for the larger roots that give mechanical support to trees than for

the smaller feeder roots. Mueller and Cline ( 1 959) observed that calcareous glacial till of similar bulk density to the fragipan is not as effective

a barrier to rooting. Goodlett ( 1960) discussed the confounding influences

of the water table regime and depth to the fragipan for an area in central




Depth to the fragipan influences the kind of agricultural drainage system needed. If the fragipan is shallow, the system should be designated

to remove surface water. If the fragipan is deep, then ditches or tile lines

may be feasible. Thickness of the fragipan may also affect suitability of a

drainage system. If the fragipan is thin, although shallow, tile or ditches

may be feasible.

Fragipans increase construction costs in several ways. Difficulty in

digging makes small excavations more expensive. The slow movement of

low-tension water in the pan causes several problems. Pervious seepage

beds for septic tanks may be required (J. H. Huddleston and Olson, 1967).

Accumulation of water at the top of the fragipan may incease the time required for drainage after heavy precipitation during earth-moving operations. Fragipans beneath surfaced runways or roads may lead to accumulation of water in the subgrade, which loses strength and is subject to

being pumped out under repeated loading and relaxation. Lateral water

movement above the fragipan causes problems where cuts are made on

sloping land. Road design must provide for the interception of water

moving laterally along the top of the fragipan. Cuts must be deep enough

and fill thick enough so that the road surface is either well below or above

the top of the fragipan.

The location and manner in which people live in the United States

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