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CHAPTER 5. FRAGIPAN SOILS OF THE EASTERN UNITED STATES

CHAPTER 5. FRAGIPAN SOILS OF THE EASTERN UNITED STATES

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238



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



As Figure 1 illustrates, fragipans are prominent features of some soils.

Despite their prominence in the soil profile, the definition of fragipans is

vague. The vagueness arises from the nature of the defining properties.



FIG. I . Block diagram of a soil with a fragipan that occurs in northeastern Wisconsin.

The fragipan horizons are indicated by the suffix x. (Modified from Olson and Hole, 19671968.)



The principal defining property is the type of failure exhibited by moist

soil material when pressure is applied. Yield is abrupt; a moist piece of

soil breaks into fragments rather than deforming. This has been described

as brittle consistence. The failure by some fragipans has been likened

(Daniels et al., 1966) to the crumbling of a dry graham cracker. This defining property does not lend itself to laboratory measurement. The

natural organization of the soil fabric must be kept, which raises difficulties with the measurement. Also, the failure observed depends sensitively on the water content. Resultantly, field observations are subject to

large variability. A body of standardized measurements on the defining

property of fragipans does not exist. Many descriptions of fragipans give

no indication of the degree of brittleness.

Winters and Simonson ( 1 95 1) listed some of the early publications on

soils with fragipans. Olson (1962) presented a comprehensive review. As

indicated in Section VIII, fragipans were recognized as pans long before they were given their current name. The concept of a pan in soil

morphology involves restricted root penetration, occurrence below the



FRAGIPAN SOILS OF THE EASTERN UNITED STATES



239



soil surface, and for most kinds of pans-although fragipans are an exception- the pronounced accumulation of a substance, such as silicate

clay, iron oxides, silica, or carbonates. Historically, the concept also

has involved the notion of an unusual or extreme expression of a feature

or attribute and something apart from the normal or expected sequence

of horizons.

Reasons for considering fragipans to be genetic soil horizons have been

discussed by the Soil Survey Staff ( 1960) and by Carlisle et al. (1 957).

Fragipans parallel the soil surface, have upper boundaries at moderate

depths (20 to 100 cm.), have a constant spatial relationship to other soil

horizons in a given kind of soil, occur in geological materials of widely

different origins, and may transgress differing parent materials in a local

area. Their range in thickness (20 to 200 cm.) is within generally accepted

limits for soil horizons. Cline (1952) states, “The consistency of depth

at which it [the fragipan] occurs . . . indicates considerable control by

some thing or things related to the land surface, and this more than any

one thing is probably responsible for its designation as a genetic horizon.”

Fragipans are subsoil horizons unless the soil has been truncated. As

subsoil horizons they are subject to accumulation of illuviated substances. They are beneath the depth of maximum root concentration;

organic matter accumulation is not high. Mechanical disturbance of the

soil fabric is weak. Consequently, fragipans retain the marks of translocation of substances and reorganization of the fabric.

Carlisle et al. (1957) have defined fragipans as follows: “Compact

horizons (of high bulk density) which are hard to extremely hard when

dry and firm to very firm when moist and display the property of ‘brittleness’ when both dry and moist. The term ‘brittleness,’ as used here,

embodies a type of physical behavior that is characteristic of but not exclusively associated with fragipan materials. The term is used to characterize a condition in which a fragment of the material sustains increasing

pressure without detectable deformation until a critical pressure is

reached, at which point the material suddenly shatters.” This definition

differs from that in the Soil Survey Manual (Soil Survey Staff, 195 1) by

putting more emphasis on the retention of brittleness when moist. The

definition in the 7rh Approximation (Soil Survey Staff, 1960) closely

parallels that of Carlisle et al. ( 1 957).

Fragipans have a number of characteristics, which, though not necessary to the definition, form part of the general description applicable

to most but not all fragipans. They include high silt, very fine sand, or

fine sand contents: moderate or low clay content: low organic matter content: medium to high bulk density when moist; slow or very slow satu-



240



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



rated hydraulic conductivity; well expressed mottling; presence of

bleached cracks or fracture planes that form a coarse polygonal pattern

on a horizontal plane; weak pedological structural expression within the

polyhedrons outlined by the bleached cracks; clearly identifiable and

planar upper boundaries; presence of bodies of moved clay; and few

roots, with those present largely restricted to the cracks between large

polyhedrons.

11. Horizons of Fragipan Soils



A. TERMINOLOGY

A N D HORIZONNOMENCLATURE

Fragipans occur in soils with argillic, cambic, or spodic horizons. The

cambic and spodic horizons, if present, occur above the fragipan. The

argillic horizon may occur partly or wholely above or be coextensive

with the fragipan. An argillic horizon is “an illuvial horizon in which

layer-lattice silicate clays have accumulated by illuviation to a significant

extent” (Soil Survey Staff, 1967). Cambic horizons may show several

kinds of alteration, but domination by accumulation of mineral substances is excluded. The marks of cambic horizons in soils with fragipans include the mottling indicative of translocation of iron and associated

with periodic saturation, higher chroma and redder hue associated with

coatings of iron oxides released by mineral weathering, and sufficient development of soil structure largely to obliterate the original organization

(stratification, for example) of the parent material (Soil Survey Staff,

1967). Cambic horizons may contain illuvial clay but do not meet the

requirements for an argillic horizon. Spodic horizons contain appreciable

amounts of precipitated amorphous materials “composed of organic

matter and aluminum, with or without iron . . .” (Soil Survey Staff, 1967).

Figure 1 depicts a soil with a spodic horizon above the fragipan.

The suffix x is currently used to designate the fragipan (Soil Survey

Staff, 1962); an example would be Bx. The suffix rn has been used earlier

(Soil Survey Staff, 1951) but is now restricted to strongly cemented or

indurated horizons having a consistence that is not appreciably affected

by moistening. The proposals by the International Society of Soil Science ( 1 968) on horizon nomenclature use x and rn in these senses. Many

soils with fragipans have two pairs of A and B horizons. Each pair of A

and B horizons is referred to as a sequum. Soils with two pairs are referred to as bisequal. The soil shown in Fig. 1 is bisequal. The lower pair

of horizons is designated with a prime accent. An example would be B’x.

The fragipan, if present, commonly occurs in the lower sequum if the soil

is bisequal.

Daniels el al. (1968) have suggested the notation Be for a B horizon



FRAGIPAN SOILS OF THE EASTERN UNITED STATES



24 1



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.



B. HORIZONSEQUENCES

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

Wetter



Eluvial



A'2 Lxl



ond /or

I



Ilc



-



IId



A2



IA2



B



IB



Bx



and/or 1



cx



1



Btx



I



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



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CHAPTER 5. FRAGIPAN SOILS OF THE EASTERN UNITED STATES

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