Tải bản đầy đủ - 0 (trang)
III. Occurrence of Fragipan Soils

III. Occurrence of Fragipan Soils

Tải bản đầy đủ - 0trang



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



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

landscape features.

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).

2. Chemical

Chemical properties of fragipans do not seem unique. Fragipans are

low in organic matter, have low or moderate levels of extractable iron



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.

3. Mineralogy

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



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

amorphous material.

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



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

particularly detailed.



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),


25 1

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

fragipan concept.

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-



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



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).




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-



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

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

III. Occurrence of Fragipan Soils

Tải bản đầy đủ ngay(0 tr)