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Silt flow in soils
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Silt flow in soils W.D. Nettleton', B.R. Brasher', O.W. Baumerl, and R.G. Darmody2 lUSDA-SCS, NSSC, Soil Survey Laboratory, Federal Building, Lincoln, NE, USA 2Department of Agronomy, University of Illinois, Urbana, IL, USA
ABSTRACT Nettleton, W.D., Brasher, B.R., Baumer, O.W. and Darmody, R.G., 1994. Silt flow in soils. In: A.J. RingroseVoase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 361-371.
Silts and very fine sands, although considered to be skeleton grains, behave as soil plasma when they are moved and reorganized or concentrated in soil crusts, fragipans, and buried paleosols. We investigated the sizes of the voids through which the silt and very fine sand have moved and infer conditions responsible for formation of silty pedogenic features such as pedotubules and silt and very fine sand cutans. Soils which have both argillic horizons with illuviation argillans and underlying horizons with the silty pedogenic features were selected from Indiana, Illinois, Missouri, and Idaho. The average minimum diameters of these features, measured by image analysis, were 0.074 mm for voids with illuviation argillans and 0.402 mm for the silty pedogenic features. Most of the silty pedogenic features are striotubules. The voids, and former voids, are all < 3 mm in diameter and water flow in them theoretically has been mostly laminar. Under special cases, such as rapidly wetting of dry soil, the draining of saturated soil, or thawing of frozen soil, silty pedogenic features form as a result of detachment, transport, and deposition of silt and very fine sand. The deposition process is favoured by pore size discontinuities, low Ca and Mg content, low Fe content of prism coatings, high silt content, low aggregate stability, and by low organic carbon content. Because of the accumulated, silty pedogenic features most of the horizons have the lowest total porosity and the highest ratio of pores fded to those drained at 0.03 MPa. Besides producing root restricting horizons in soils and impeding drainage in tile lines and septic tank drain fields, silt and very fine sand movement and accumulation are involved in subsurface tunnel erosion or piping. INTRODUCTION Plasma as defined by Brewer and Sleeman (1960) includes colloidal-size mineral and organic material, as well as relatively soluble material not bound up in skeleton grains. Plasma is capable of being moved, or has been moved and reorganized or concentrated by soil forming processes. Silt, although not of colloidal size and subsequently excluded from plasma by Brewer (1976, pp. 11-12), is reported to have moved in the formation of surface crusts in soils (Falayi and Bouma, 1975; Boiffin and Bresson, 1987; Arshad and Mermut, 1988; and West et al., 1990), in vesicular layers (Sullivan and Koppi, 1991), in cryoturbated features such as silt
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cappings and distorted horizons (Tarnocai and Valentine, 1989; Romans et al., 1966; Catt, 1989; Bronger, 1969/1970), in fragipan horizons (Carlisle, 1954; Nettleton et al., 1968; Miller et al., 1971; De Kimpe and McKeague, 1974; Fitzpatrick, 1974; Van W e t and Langohr, 1981; Anderson and Darmody, 1989; and Habecker et al., 1990), and in buried paleosols (Ransom, 1984; Ransom et al., 1987; and Fedoroff et al., 1990; Thompson, 1986; Thompson and Smeck, 1983). In this paper we investigate the size of the voids through which clay and silt move in soils and infer the kind of water movement responsible for formation of the silty pedogenic features. METHODS AND MATERIALS Fifteen soils with argillic horizons and underlying Btx horizons with pedotubules, or other evidence of silt and very fine sand movement were selected. Fragipans were recognized in some of them and all of them formed in loess over paleosols. The soils occur in Indiana, Illinois, Missouri and Idaho in association with other loessial soils that have argillic horizons, but do not have Btx horizons or fragipans (see the review by Franzmeier et al., 1989). These associated soils formed in deeper loess or in loess over calcareous till. The soils selected in Indiana and Illinois formed in Peorian loess over Illinoian paleosols which formed in drift or pedisediment. The soils in Missouri were formed in loess over paleosols which had developed in moderately weathered local colluvium from limestone and dolomite. The soil from Idaho was formed in loess over a paleosol which had developed in loess also. Eluvial horizons formed at the loess paleosol boundaries in each of the soils. Unoriented thin sections of pairs of horizons from each soil were mounted on 27 x 46 mm glass slides. The pairs included a Bt horizon with mostly illuviation argillans in voids in the overlying loess and a Btx horizon with mostly pedotubules in the underlying paleosol. These features were studied with the Cue-2 Image Analysis System'. The smallest Martin's radius from the shape analysis was used to estimate the minimum diameter of the voids and pedotubules. The smallest radius was recorded because it is a measurement not influenced by the orientation of the thin section. Ten voids with argillans or 10 pedotubules were selected by transecting and were analyzed for each of the two horizons. In all 300 observations were made, 150 in each kind of horizon. The soil characterization data were obtained by methods described in Soil Survey Investigations Report No. 42 (USDA Soil Conservation Service, 1992) and identified herein by codes. The pore volume ratio was calculated from the volume of pores filled at 0.033 MPa divided by those drained at this suction. Statistics were performed using SAS (1988). RESULTS The minimum diameter of the voids and pedotubules ranged in size from a few micrometers up to slightly more than 1 mm (Fig. l a and Table 1). Two diameters dominated, one at 0.080 mm and the other at 0.320 mm. About half of the voids with illuviation argillans had minimum diameters of <0.065 mm (Fig. lb).
'Olympus Corporation, 4 Nevada Drive, Lake Success, NY 11042-1179, U.S.A. Name and address of manufacturer is given for the convenience of the reader and does not imply endorsement by the USDA Soil Conservation Service.
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SILT FLOW IN SOILS
a
3% (Cumulative Percent) h
6 6 v
a5
0.02 0.04
25%
:
l
r
r
86%
0.64 1.28
100%
300 observations
0
b
20
40
21% (Cumulative
0.01
2
80
60
h
0.05
W
sa a
.I
0.08
.z d
0.12
v)
-
x = 0.073 mm SD = 0.054 m m N = 150 observations
Cj
0
10
0.1
C
aa
20
40
30
50
60
11% (Cumulative Percent)
h
W
0.2
Ll
2 0.3
ii
a
0.4
v)
2 0.7
Cj
1.0
0
10
20
SD = 0.238 mm N = 150 observations 30 40 50
Vreq uency
Fig. 1. Frequency of the minimum diameter o f a) voids with illuviation argillans and pedotubules; b) voids with illuviation argillans and c) pedotubules.
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Table 1. Some statistics for the minimum diameter of voids with illuviation argillans and those with pedotubules in the horizon-pairs of the fifteen soils. Diameter of the features, mm Minimum Maximum Mean
Variable
N
Std.dev.
Voids with illuviation argillans Pedotubules
150
0.004
0.332
0.0735
0.0541
150
0.030
1.035
0.4032
0.2378
Difference Std. error of of means difference Difference
0.3288*
0.0209
*A t-test shows this difference has a t-ratio of 15.72 and is significant at the 0.01% level (SAS Institute, 1988). The average minimum diameter, 0.074 mm (Table l), shows that the distribution is skewed toward the smaller diameters. About half of the pedotubules had silt and very fine sand filled voids, with a minimum diameter of < 0.350 mm (Fig. lc). The average minimum diameter, 0.402 mm, showed that this distribution was also skewed towards the smaller diameters. Selected characterization data for the horizon-pairs of the fifteen soils are shown in Table 2. Some of the silt flows are coatings along void channels or on coarse fragments and consist of finely stratified silts and clays with a few very fine and fine sands like the one in Fig. 2. Most, however, are striotubules. Figs 3a and b are of a silty pedogenic feature like a pedotubule except that this one has a void. Some are outlined by well oriented, ansiotropic clays but consist of silts without apparent order (Figs 3c and d). DISCUSSION The minimum diameters of the former voids filled with silt and clay (Figs 2-3), are significantly larger than those outlined with illuviation argillans (Table 1). Some overlap of diameter sizes occur as shown in Fig. la. Because the microscope and image analyzer system we used is limited to widths of c 2 mm, our study was limited to channel voids and pedotubules of this size range. Furthermore, the program could not cope with complex shapes encountered and hence, no measure of tortuosity of the channel voids was attempted. The voids observed in this study were mostly in the micro to meso range (Brewer, 1964), which is the range for water storage and transmission (Greenland, 1977) and for gravitation (Luxmoore, 1981). At normal temperature and pressure these are the ranges for capillary pores, c 3 mm diameter (Hamblin, 1985), in which flow is assumed to be laminar. Theory
The pedotubules are evidence that silts and very fine sands move in soils. This requires detachment, transport, and then deposition in a new location. Transport occurs because gravity acts on the particles once they are in suspension. The particles drop out of suspension
Table 2. Comparison of physical and chemical data and the minimum pore diameters of horizons with mostly illuviation argillans (Bt) and those with mostly pedotubules (Btx) from fifteen Alfisols, Mollisols, and Udults (Soil Survey Staff, 1992). (Sand, 2 - 0.05 mm; silt, 0.05 - 0.002mm; clay, <0.002mm). Horizon Volume Particle Size Analysis % Analysis of <2mm soil Min.Pore A1 Aggregate >2mm e2mm soil by weight Organic C pb (30kPa) COLE Diameter Saturation Stability %
Bt Btx Typic Natraqualf Bt Btx Mollic Albaqualf Bt BtX Typic Albaqualf Bt Btx Bt Typic Hapludalf Btx Bt TypicFragiudalfs Btx Bt Typic Fragiudalfs Btx Bt Aquic Fragiudalf BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragixeralf Btx Bt Argic Cryoboroll BtX Typic Fragiudults in fineBt loamy, siliceous mesic families B k Typic Fragiudults in fineBt loamy, siliceousmesic families Bw Horizon Me& S.D. Bt BtX Aeric Ochraqualf
0 2 0 0 0 0 0
0 1 1 1 1 0 1 0 3 9 81 8 82 1 7 1
0 62 70 20 28 12 13 8f16 19f31
Sand 4 31 4 8 7 10 4 12 26 63 23 31 13 30 16 28 5 11 6 21 4 6 7 6 67 73 10 18 7 10 14H6
24SO
silt 70 42 72 69 69 66 53 60 39 24 52 45 61 52 59 50 56 57 51 40 55 59 75 65 21 17 54 49 59 66 56514 51f15
clay 26 27 24 23 24 24 43 28 35 14 25 24 26 18 25 22 39 32 43 39 41 35 18 29 12 10 36 33 34 24 3of9 25f8
dg kg-' 0.11 0.08
g cm-3
cm cm-1 mm 1.47 0.05 0.090 1.57 0.03 0.272 0.44 1S O 0.02 0.092 0.09 1.51 0.04 0.209 0.37 1.56 0.01 0.114 0.10 1.59 0.00 0.302 0.43 0.02 0.082 1S O 0.13 1.52 0.03 0.414 0.15 1.56 0.05 0.097 0.06 1.89 0.01 0.315 0.20 1.49 0.070 0.03 0.14 1.61 0.04 0.387 0.21 0.080 1S O 0.01 0.06 1.75 0.01 0.263 0.13 0.02 0.103 1.54 0.06 1.63 0.02 0.476 1.02 1.33 0.04 0.030 0.55 1.70 0.00 0.728 0.67 1.36 0.04 0.021 0.26 0.640 1.68 0.01 0.91 0.01 0.038 1.37 0.23 0.02 0.484 1.59 0.85 0.01 1.36 0.086 0.34 0.05 1.55 0.218 0.33 1.49 0.01 0.086 0.01 0.18 1.66 0.342 0.046 0.31 1.51 0.02 0.18 1.41 0.00 0.561 0.02 1s o 0.21 0.043 0.06 1.51 0.01 0.418 0.421to.30 1.471to.08 0.02fl.01 0.072M.03 0.17M.14 1.61fl.12 0.02M.01 0.402M.15
%
35 0 0 0 41 0 46 9 7 0 41 19 20 29 42 1 31 52 57 49 34 41 0 0 0 0 49 75 57 80 31B0 24i29
%
4.7 0.3 5.8 0.7 20.1 2.0 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 19.9 26.9 23.4 27.9 48.6 15.1 20.4f15.9 12.2f13.0
F+I F
2 2
P
W A
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Fig. 2. Photomicrograph of a thin section of the BC horizon of the Argic Cryoboroll in plane polarized light. The feature consists of finely stratified silt and clay bridging between two coarse fragments.
when the water flow slows upon reaching a pore size discontinuity or where the water infiltrates ped interiors leaving the particles at the interface. The size of the voids with illuviation argillans (0.007 cm, see Table 1) suggests that for turbulent flow to be responsible for detachment of particles, the discharge velocity would need to be at the high rate of 1.4 cm sec-l. We arrived at the rate by rearranging the equation for Reynold's number (Harr, 1962) and solving for the discharge velocity, v in cm s-l:
v = (Rq)/(dD)
Eqn. 1
where R is the Reynolds number, here set to its minimum of 1 (Muskat, 1946), q the viscosity of water, 0.01 g s cm-2, d the average minimum diameter of voids with illuviation argillans, 0.007 cm, and D the density of water, 1 g cm-3. However, if we accept that only laminar flow occurs through the fine pores just mentioned, detachment can still be explained by the stress on the pore walls, z , created by shear. Thus: 2
= F/A =q (dv/dr)
Eqn. 2
where F is the force, A the area, r the pore radius and dv/dr the change of velocity in the direction of the increasing radius. Changes in pore water velocity will cause corresponding changes in stress. If stress exceeds a critical value, particles will be detached from the pore wall. Sufficient stress to detach particles may be produced during wetting. The upper, slower conducting layer will become saturated, and as the water breaks into preferred passages of the faster conducting, lower layer, pore water velocity in the larger pores of the upper layer increases. This increases the stress on the pores walls so that detachment of particles occurs. In the drying cycle, stress increases as water from the larger pores drains first and water from the soil matrix enters larger pores through the pore walls. We have noted clays and silts in the
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Fig. 3. Photomicrographs of thin sections of: a) the 2Btg5 horizon, 140 - 160 cm depth, of the Moltic Albaqualf in plane polarized light., showing layers of silt and clay around an elliptical void; b) as a) but in crossed polarized light; c) the 2Btx horizon, 94 - 125 cm depth, of the Aeric Ochraqualf in plane polarized light showing the clay layers outlining the mostly silt filled voids; d) as c) but in crossed polarized light, showing the orientation of the clays that outline the island of silt and the small void in the upper part.
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water draining from soil cores at the end of hydraulic conductivity studies. Measurement of stress, however, is beyond the scope of our study. Supporting Observations
Silt and very fine sand have been translocated and deposited as silty pedogenic features in Btx horizons of each of the 15 soils. Physical and chemical data show that the overlying Bt horizons have some features in common that help explain the silt movement. All except the silt (Table 2) so material was available Argic Cryoboroll have at least one horizon with ~ 5 0 % to be moved if conditions were favourable. Each of the soils has a particle size discontinuity with finer-textured material overlying coarser-textured material and hence smaller diameter voids overlying larger diameter silt-fied voids (Table 2). We infer that the silty pedogenic features formed below the contact occupy former voids. The larger diameter voids in these lower horizons would have favoured formation of near saturated conditions in overlying Bt horizons before the wetting front could break through to the lower horizons. As the wetting front passes across the pore-size discontinuity occurring at the contact, strain develops in the pores at the base of the overlying horizon. Clay, silt, and very fine sand particles are detached from the walls of these channel voids, and are carried into the lower horizons to form pedotubules (see Figs 3a and b for an incomplete pedotubule) and other silty pedogenic features. Particles in weak soil aggregates are more easily detached from pore walls than particles in well aggregated material. These particles then, may be moved by processes described herein into lower horizons. In our study, most of the Bt and Btx horizons have weak soil aggregates, i.e. aggregate stabilities are <30% (Table 2 and R. Grossman, pers. commun., 1993). Several of their attributes help explain this weak aggregation. Low Ca and Mg saturation of some of the soils and high Na percentages in others (Table 2) tend to produce weak aggregation (Hamblin, 1985). Low organic carbon content of these horizons tends to produce weak aggregation (Table 2) (Emerson, et al., 1986; McKeague et al., 1986; Mbagwu and piccolo, 1989). Dithionite-citrate Fe contents of <0.2 dg kg-l in ped coatings and prism faces in and above the fragipans and other horizons in which the silty pedogenic features occur (Unpublished data from the Soil Survey Laboratory) are believed to be too low to stabilize soil aggregates (Panayiotopoulos and Kostoponlou, 1989; Columbo and Torrent, 1991). All 15 soils are subject to freezing and thawing, an action also known to weaken soil aggregates (Bisal and Nielsen, 1967; Hinman and Bisal, 1968; Sillanpaa and Webber, 1961; and Mbagwu and Bazzoffi, 1989). Application
The understanding of the process of silt flow and deposition developed herein helps explain the high bulk density of soil horizons such as fragipans and aids in the design of agricultural subsurface drainage systems, lined irrigation ditches, earthen dams, and structures for the control of gully erosion. The design of drainage lines should include encasement by envelopes of material to receive all incoming water without developing flow velocities high enough to detach and move particles into the drains (Willardson, 1974). Our results suggest that one way to control the flow velocity would be to change the particle size of the envelope material gradually from the soil to the coarse material at the drain so as to avoid pore-size
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discontinuities. To avoid piping and subsurface erosion the design of structures should avoid development of strong hydraulic gradients in soil adjacent to the structures. CONCLUSIONS The occurrence of pedotubules and other silty pedogenic features in soils is evidence that silt and very fine sand move downward. This requires particle detachment, transport, and deposition in a new location. Particles are detached when shear stress forces developed by laminar flow in a pore or by the surge of suspended water across a pore size discontinuity exceed the forces that retain a particle in the soil mass. Detachment can also occur as water in the soil matrix drains through the pore wall under suction induced by drying. Once in suspension, a particle moves with the water until the flow slows enough for deposition to occur. This happens where the water is suspended above a discontinuity or where it slows d t e r flowing across a discontinuity. Deposition also occurs as water infiltrates ped interiors, and the particles are sieved out of suspension. These deposits are in the form of pedotubules, coarse cutans, and related features. REFERENCES Anderson, E.R. and Darmody, R.G., 1989. Origin of silt coatings in a Typic Fragiudalf. Agronomy Abstracts. 1989 Annual Meeting of the Soil Science of America. Las Vegas, NV, USA, pp. 4. Arshad, M.A. and Mermut, A.R., 1988. Micromorphological and physio-chemical characteristics of soil crust types in northwestern Alberta, Canada. Soil Sci. SOC.Am. J., 52: 724-729. Bisal, F. and Nielsen, K.F., 1967. Effect of frost action on the size of soil aggregates. Soil Sci., 104 268-272. Boiffi, J. and Bresson, L.M., 1987. Dynamique de formation des croutes superfkielles: apport de l'analyse microscopique. In: N. Fedoroff, L.M. Bresson and M.A. Coutry (Editors), Soil Micromorphology. Proc. VII Int. Working Meeting of Soil Micromorphology, Paris, July 1985. Association FranGaise pour 1'Etude du Sol, Plaisir, France, pp. 393-399. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. John Wiley and Sons, New York, 470 pp. Brewer, R., 1976. Fabric and Mineral Analysis of Soils. Krieger, New York, 482 pp. Brewer, R. and Sleeman, J.R., 1960. Soil structure and fabric: their definition and description. J. Soil Sci., 11: 172-185. Bronger, A., 1969/1970. Zur mikromorphogenese und zum tonmineralbestad quartarer lossboden in sudbaden. Geoderma, 3: 281-320. Carlisle, F.J., 1954. Characteristics of Soils with Fragipans in a Podzol Region. Ph.D. diss. Cornell Univ., Ithaca, New York (Diss. Abstr. 14: 1861-1862). Catt, J.A., 1989. Relict properties in soils of the central and north-west European temperate region. In: A. Bronger and J.A. Catt (Editors), Paleopedology, nature and application of paleosols. Catena Supplement 16: 4 1-58. Columbo, C. and Torrent, J., 1991. Relationships between aggregation and iron oxides in Terra Rossa soils from Southern Italy. Catena, 18: 51-59.
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DeKimpe, C.R. and McKeague, J.A., 1974. Micromorphological, physical, and chemical properties of a podzolic soil with a fragipan. Can. J. Soil Sci., 54: 29-38. Emerson, W.W., Foster, R.C. and Oades, J.M., 1986. Organo-mineral complexes in relation to soil aggregation and structure. In: P.M. Huang and M. Schnitzer (Editors), Interactions of soil minerals with natural organics and microbes. Soil Sci. SOC. Am. Spec. Publ. 17., Madison, Wisconsin, pp. 521-548. Falayi, 0. and Bouma, J., 1975. Relationships between the hydraulic conductance of surface crusts and soil management in a Typic Hapludalf. Soil Sci. SOC.Am. J., 39: 957-963. Fedoroff, N., Courty, M.A. and Thompson, M.L., 1990. Micromorphological evidence of paleoenvironmental change in Pleistocene and Holocene paleosols. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 653-665. Fitzpatrick, E.A., 1974. Cryons and isons. Proc. of the North of England Soils Discussion Group, No. 11, Penrith, pp. 31-43. Franzmeier, D.P., Norton, L.D. and Steinhardy, G.C., 1989. Fragipan formation in loess of the midwestern United States. In: N.E. Smeck and E.J. Ciolkosz, (Editors), Fragipans: Their Occurrence, Classification and Genesis. pp. 69-97. Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Phil. Trans. R. SOC.Lond. B, 281: 193-208. Habecker, M.A., McSweeney, K., and Madison, F.W., 1990. Identification and genesis of fragipans in Ochrepts of North Central Wisconsin. Soil Sci. SOC.Am. J., 54: 139- 146. Hamblin, A.P., 1985. The influence of soil structure on water movement, crop root growth, and water uptake. In: N.C. Brady (Editor), Advances in Agronomy. Vol. 38. Academic Press, New York, pp. 95-158. Harr, M.E., 1962. Mechanics of Particulate Media, a Probalistic Approach. McGraw-Hill Book, 543 pp. Hinman, W.C. and Bisal, F., 1968. Alterations of soil structure upon freezing and thawing and subsequent drying. Can. J. Soil Sci., 48: 193-197. Luxmoore, R., 1981. Micro-, meso-, and macroporosity of soil. Soil Sci. SOC.Am. J., 45: 67 1-672. Mbagwu, J.S.C. and Bazzoffi, P., 1989. Effect of antecedent matric potential on the stability of soil aggregates subjected to cyclic freezing and thawing as evaluated by three structural indices. Soil Technology, 2: 59-70. Mbagwu, J.S.C. and Piccolo, A., 1989. Changes in soil aggregate stability induced by amendment with humic substances. Soil Technology, 2: 49-57. McKeague, J.A., Cheshire, M.V., Andreux, F. and Berthelin, J., 1986. Organo-mineral complexes in relation to pedogenesis. In: P.M. Huang and M. Schnitzer (Editors), Interactions of soil minerals with natural organics and microbes. Soil Sci. SOC.Am. Spec. Publ. No. 17. ,Madison, Wisconsin, USA, pp. 549-592. Miller, F.P., Wilding, L.P. and Holowaychuk, N. , 1971. Canfield silt loam, a Fragiudalf: I1 Micromorphological, physical, and chemical properties. Soil Sci. SOC.Amer. Proc., 35: 324-33 1. Muskat, M., 1946. The flow of homogeneous fluids through porous media. McGraw-Hill Book, New York,, 1937; reprinted by J.W. Edwards, Publisher, Ann Arbor, Michigan, 737 PP.
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Nettleton, W.D., McCracken, R.J. and Daniels, R.B., 1968. Two North Carolina coastal plah catenas: I1 Micromorphology, Composition, and fragipans genesis. Soil Sci. SOC.Am. Proc., 32: 582-587. Panayiotopoulos, K.P. and Kostopoulou, S., 1989. Aggregate stability dependence on size, cultivation and various soil constituents in Red Mediterranian soils (Alfisols). Soil Technology, 2: 79-89. Ransom, M.D., 1984. Genetic processes in seasonally wet soils on the Illionian tdl plain in southwestern Ohio. Ph.D. dissertation, The Ohio State Univ., Columbus, Ohio, 355 pp. Ransom, M.D., Smeck, N.E. and Bigham, J.M., 1987. Micromorphology of seasonally wet soils on the Illinoian till plain, USA. Geoderma, 4 0 83-99. Romans, J.C.C., Stevens, J.H. and Robertson, L., 1966. Alpine soils of north-east Scotland. J. Soil Sci., 17: 184-199. SAS Institute, 1988. SAS/STAT User's Guide, release 6.03 ed. SAS Inst., Cary, North Carolina, 1027 pp. Sillanpaa, M. and Webber, L.R., 1961. The effect of freezing-thawing and wetting-drying cycles on soil aggregation. Can. J. Soil Sci., 41: 182-187. Soil Survey Staff., 1992. Keys to Soil Taxonomy. SMSS Technical Monograph No. 19. Fifth Ed. Pocahontas Press, Blacksburg, Virginia, 541 pp. Sullivan, L.A. and Koppi, A.J., 1991. Morphology and genesis of silt and clay coatings in the vesicular layer of a desert loam soil. Aust. J. Soil Res., 29: 579-586. Tarnocai, C. and Valentine, K.W.G., 1989. Relict soil properties of the arctic and subarctic regions of Canada. In: A. Bronger and J.A. Catt (Editors), Paleopedology, nature and application of paleosols. Catena Supplement 16. Catena Verlag, Cremlingen-Destedt, Germany, pp. 9-39. Thompson, M.L., 1986. Morphology and mineralogy of a pre-Wisconsinian paleosol in Iowa. Soil Sci. SOC.Am. J., 50: 981-987. Thompson, M.L. and Smeck, N.E., 1983. Micromorphology of polygenetic soils in the Teays River Valley, Ohio. Soil Sci. SOC.Am. J., 47: 734-742. USDA Soil Conservation Service., 1992 (rev). Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 2.0, U.S. Gov. Printing Office, Washington, D.C., 400pp. Van Vliet, B. and Langohr, R., 1981. Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France. Catena, 8: 137-154. West, L.T., Bradford, J.M. and Norton, L.D., 1990. Crust morphology and infiltrability in surface soils from the southeast and midwest U.S.A. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 107-113. Willardson, L.S., 1974. Envelope materials. In: J. Van Schilfgaarde (Editor), Drainage for Agriculture. Agron. Series 17. Am. SOC.Agron. Madison, Wisconsin, pp. 179-200.
Silt flow in soils W.D. Nettleton', B.R. Brasher', O.W. Baumerl, and R.G. Darmody2 lUSDA-SCS, NSSC, Soil Survey Laboratory, Federal Building, Lincoln, NE, USA 2Department of Agronomy, University of Illinois, Urbana, IL, USA
ABSTRACT Nettleton, W.D., Brasher, B.R., Baumer, O.W. and Darmody, R.G., 1994. Silt flow in soils. In: A.J. RingroseVoase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 361-371.
Silts and very fine sands, although considered to be skeleton grains, behave as soil plasma when they are moved and reorganized or concentrated in soil crusts, fragipans, and buried paleosols. We investigated the sizes of the voids through which the silt and very fine sand have moved and infer conditions responsible for formation of silty pedogenic features such as pedotubules and silt and very fine sand cutans. Soils which have both argillic horizons with illuviation argillans and underlying horizons with the silty pedogenic features were selected from Indiana, Illinois, Missouri, and Idaho. The average minimum diameters of these features, measured by image analysis, were 0.074 mm for voids with illuviation argillans and 0.402 mm for the silty pedogenic features. Most of the silty pedogenic features are striotubules. The voids, and former voids, are all < 3 mm in diameter and water flow in them theoretically has been mostly laminar. Under special cases, such as rapidly wetting of dry soil, the draining of saturated soil, or thawing of frozen soil, silty pedogenic features form as a result of detachment, transport, and deposition of silt and very fine sand. The deposition process is favoured by pore size discontinuities, low Ca and Mg content, low Fe content of prism coatings, high silt content, low aggregate stability, and by low organic carbon content. Because of the accumulated, silty pedogenic features most of the horizons have the lowest total porosity and the highest ratio of pores fded to those drained at 0.03 MPa. Besides producing root restricting horizons in soils and impeding drainage in tile lines and septic tank drain fields, silt and very fine sand movement and accumulation are involved in subsurface tunnel erosion or piping. INTRODUCTION Plasma as defined by Brewer and Sleeman (1960) includes colloidal-size mineral and organic material, as well as relatively soluble material not bound up in skeleton grains. Plasma is capable of being moved, or has been moved and reorganized or concentrated by soil forming processes. Silt, although not of colloidal size and subsequently excluded from plasma by Brewer (1976, pp. 11-12), is reported to have moved in the formation of surface crusts in soils (Falayi and Bouma, 1975; Boiffin and Bresson, 1987; Arshad and Mermut, 1988; and West et al., 1990), in vesicular layers (Sullivan and Koppi, 1991), in cryoturbated features such as silt
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cappings and distorted horizons (Tarnocai and Valentine, 1989; Romans et al., 1966; Catt, 1989; Bronger, 1969/1970), in fragipan horizons (Carlisle, 1954; Nettleton et al., 1968; Miller et al., 1971; De Kimpe and McKeague, 1974; Fitzpatrick, 1974; Van W e t and Langohr, 1981; Anderson and Darmody, 1989; and Habecker et al., 1990), and in buried paleosols (Ransom, 1984; Ransom et al., 1987; and Fedoroff et al., 1990; Thompson, 1986; Thompson and Smeck, 1983). In this paper we investigate the size of the voids through which clay and silt move in soils and infer the kind of water movement responsible for formation of the silty pedogenic features. METHODS AND MATERIALS Fifteen soils with argillic horizons and underlying Btx horizons with pedotubules, or other evidence of silt and very fine sand movement were selected. Fragipans were recognized in some of them and all of them formed in loess over paleosols. The soils occur in Indiana, Illinois, Missouri and Idaho in association with other loessial soils that have argillic horizons, but do not have Btx horizons or fragipans (see the review by Franzmeier et al., 1989). These associated soils formed in deeper loess or in loess over calcareous till. The soils selected in Indiana and Illinois formed in Peorian loess over Illinoian paleosols which formed in drift or pedisediment. The soils in Missouri were formed in loess over paleosols which had developed in moderately weathered local colluvium from limestone and dolomite. The soil from Idaho was formed in loess over a paleosol which had developed in loess also. Eluvial horizons formed at the loess paleosol boundaries in each of the soils. Unoriented thin sections of pairs of horizons from each soil were mounted on 27 x 46 mm glass slides. The pairs included a Bt horizon with mostly illuviation argillans in voids in the overlying loess and a Btx horizon with mostly pedotubules in the underlying paleosol. These features were studied with the Cue-2 Image Analysis System'. The smallest Martin's radius from the shape analysis was used to estimate the minimum diameter of the voids and pedotubules. The smallest radius was recorded because it is a measurement not influenced by the orientation of the thin section. Ten voids with argillans or 10 pedotubules were selected by transecting and were analyzed for each of the two horizons. In all 300 observations were made, 150 in each kind of horizon. The soil characterization data were obtained by methods described in Soil Survey Investigations Report No. 42 (USDA Soil Conservation Service, 1992) and identified herein by codes. The pore volume ratio was calculated from the volume of pores filled at 0.033 MPa divided by those drained at this suction. Statistics were performed using SAS (1988). RESULTS The minimum diameter of the voids and pedotubules ranged in size from a few micrometers up to slightly more than 1 mm (Fig. l a and Table 1). Two diameters dominated, one at 0.080 mm and the other at 0.320 mm. About half of the voids with illuviation argillans had minimum diameters of <0.065 mm (Fig. lb).
'Olympus Corporation, 4 Nevada Drive, Lake Success, NY 11042-1179, U.S.A. Name and address of manufacturer is given for the convenience of the reader and does not imply endorsement by the USDA Soil Conservation Service.
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SILT FLOW IN SOILS
a
3% (Cumulative Percent) h
6 6 v
a5
0.02 0.04
25%
:
l
r
r
86%
0.64 1.28
100%
300 observations
0
b
20
40
21% (Cumulative
0.01
2
80
60
h
0.05
W
sa a
.I
0.08
.z d
0.12
v)
-
x = 0.073 mm SD = 0.054 m m N = 150 observations
Cj
0
10
0.1
C
aa
20
40
30
50
60
11% (Cumulative Percent)
h
W
0.2
Ll
2 0.3
ii
a
0.4
v)
2 0.7
Cj
1.0
0
10
20
SD = 0.238 mm N = 150 observations 30 40 50
Vreq uency
Fig. 1. Frequency of the minimum diameter o f a) voids with illuviation argillans and pedotubules; b) voids with illuviation argillans and c) pedotubules.
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Table 1. Some statistics for the minimum diameter of voids with illuviation argillans and those with pedotubules in the horizon-pairs of the fifteen soils. Diameter of the features, mm Minimum Maximum Mean
Variable
N
Std.dev.
Voids with illuviation argillans Pedotubules
150
0.004
0.332
0.0735
0.0541
150
0.030
1.035
0.4032
0.2378
Difference Std. error of of means difference Difference
0.3288*
0.0209
*A t-test shows this difference has a t-ratio of 15.72 and is significant at the 0.01% level (SAS Institute, 1988). The average minimum diameter, 0.074 mm (Table l), shows that the distribution is skewed toward the smaller diameters. About half of the pedotubules had silt and very fine sand filled voids, with a minimum diameter of < 0.350 mm (Fig. lc). The average minimum diameter, 0.402 mm, showed that this distribution was also skewed towards the smaller diameters. Selected characterization data for the horizon-pairs of the fifteen soils are shown in Table 2. Some of the silt flows are coatings along void channels or on coarse fragments and consist of finely stratified silts and clays with a few very fine and fine sands like the one in Fig. 2. Most, however, are striotubules. Figs 3a and b are of a silty pedogenic feature like a pedotubule except that this one has a void. Some are outlined by well oriented, ansiotropic clays but consist of silts without apparent order (Figs 3c and d). DISCUSSION The minimum diameters of the former voids filled with silt and clay (Figs 2-3), are significantly larger than those outlined with illuviation argillans (Table 1). Some overlap of diameter sizes occur as shown in Fig. la. Because the microscope and image analyzer system we used is limited to widths of c 2 mm, our study was limited to channel voids and pedotubules of this size range. Furthermore, the program could not cope with complex shapes encountered and hence, no measure of tortuosity of the channel voids was attempted. The voids observed in this study were mostly in the micro to meso range (Brewer, 1964), which is the range for water storage and transmission (Greenland, 1977) and for gravitation (Luxmoore, 1981). At normal temperature and pressure these are the ranges for capillary pores, c 3 mm diameter (Hamblin, 1985), in which flow is assumed to be laminar. Theory
The pedotubules are evidence that silts and very fine sands move in soils. This requires detachment, transport, and then deposition in a new location. Transport occurs because gravity acts on the particles once they are in suspension. The particles drop out of suspension
Table 2. Comparison of physical and chemical data and the minimum pore diameters of horizons with mostly illuviation argillans (Bt) and those with mostly pedotubules (Btx) from fifteen Alfisols, Mollisols, and Udults (Soil Survey Staff, 1992). (Sand, 2 - 0.05 mm; silt, 0.05 - 0.002mm; clay, <0.002mm). Horizon Volume Particle Size Analysis % Analysis of <2mm soil Min.Pore A1 Aggregate >2mm e2mm soil by weight Organic C pb (30kPa) COLE Diameter Saturation Stability %
Bt Btx Typic Natraqualf Bt Btx Mollic Albaqualf Bt BtX Typic Albaqualf Bt Btx Bt Typic Hapludalf Btx Bt TypicFragiudalfs Btx Bt Typic Fragiudalfs Btx Bt Aquic Fragiudalf BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragiudalfs BtX Bt Mollic Fragixeralf Btx Bt Argic Cryoboroll BtX Typic Fragiudults in fineBt loamy, siliceous mesic families B k Typic Fragiudults in fineBt loamy, siliceousmesic families Bw Horizon Me& S.D. Bt BtX Aeric Ochraqualf
0 2 0 0 0 0 0
0 1 1 1 1 0 1 0 3 9 81 8 82 1 7 1
0 62 70 20 28 12 13 8f16 19f31
Sand 4 31 4 8 7 10 4 12 26 63 23 31 13 30 16 28 5 11 6 21 4 6 7 6 67 73 10 18 7 10 14H6
24SO
silt 70 42 72 69 69 66 53 60 39 24 52 45 61 52 59 50 56 57 51 40 55 59 75 65 21 17 54 49 59 66 56514 51f15
clay 26 27 24 23 24 24 43 28 35 14 25 24 26 18 25 22 39 32 43 39 41 35 18 29 12 10 36 33 34 24 3of9 25f8
dg kg-' 0.11 0.08
g cm-3
cm cm-1 mm 1.47 0.05 0.090 1.57 0.03 0.272 0.44 1S O 0.02 0.092 0.09 1.51 0.04 0.209 0.37 1.56 0.01 0.114 0.10 1.59 0.00 0.302 0.43 0.02 0.082 1S O 0.13 1.52 0.03 0.414 0.15 1.56 0.05 0.097 0.06 1.89 0.01 0.315 0.20 1.49 0.070 0.03 0.14 1.61 0.04 0.387 0.21 0.080 1S O 0.01 0.06 1.75 0.01 0.263 0.13 0.02 0.103 1.54 0.06 1.63 0.02 0.476 1.02 1.33 0.04 0.030 0.55 1.70 0.00 0.728 0.67 1.36 0.04 0.021 0.26 0.640 1.68 0.01 0.91 0.01 0.038 1.37 0.23 0.02 0.484 1.59 0.85 0.01 1.36 0.086 0.34 0.05 1.55 0.218 0.33 1.49 0.01 0.086 0.01 0.18 1.66 0.342 0.046 0.31 1.51 0.02 0.18 1.41 0.00 0.561 0.02 1s o 0.21 0.043 0.06 1.51 0.01 0.418 0.421to.30 1.471to.08 0.02fl.01 0.072M.03 0.17M.14 1.61fl.12 0.02M.01 0.402M.15
%
35 0 0 0 41 0 46 9 7 0 41 19 20 29 42 1 31 52 57 49 34 41 0 0 0 0 49 75 57 80 31B0 24i29
%
4.7 0.3 5.8 0.7 20.1 2.0 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 19.9 26.9 23.4 27.9 48.6 15.1 20.4f15.9 12.2f13.0
F+I F
2 2
P
W A
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Fig. 2. Photomicrograph of a thin section of the BC horizon of the Argic Cryoboroll in plane polarized light. The feature consists of finely stratified silt and clay bridging between two coarse fragments.
when the water flow slows upon reaching a pore size discontinuity or where the water infiltrates ped interiors leaving the particles at the interface. The size of the voids with illuviation argillans (0.007 cm, see Table 1) suggests that for turbulent flow to be responsible for detachment of particles, the discharge velocity would need to be at the high rate of 1.4 cm sec-l. We arrived at the rate by rearranging the equation for Reynold's number (Harr, 1962) and solving for the discharge velocity, v in cm s-l:
v = (Rq)/(dD)
Eqn. 1
where R is the Reynolds number, here set to its minimum of 1 (Muskat, 1946), q the viscosity of water, 0.01 g s cm-2, d the average minimum diameter of voids with illuviation argillans, 0.007 cm, and D the density of water, 1 g cm-3. However, if we accept that only laminar flow occurs through the fine pores just mentioned, detachment can still be explained by the stress on the pore walls, z , created by shear. Thus: 2
= F/A =q (dv/dr)
Eqn. 2
where F is the force, A the area, r the pore radius and dv/dr the change of velocity in the direction of the increasing radius. Changes in pore water velocity will cause corresponding changes in stress. If stress exceeds a critical value, particles will be detached from the pore wall. Sufficient stress to detach particles may be produced during wetting. The upper, slower conducting layer will become saturated, and as the water breaks into preferred passages of the faster conducting, lower layer, pore water velocity in the larger pores of the upper layer increases. This increases the stress on the pores walls so that detachment of particles occurs. In the drying cycle, stress increases as water from the larger pores drains first and water from the soil matrix enters larger pores through the pore walls. We have noted clays and silts in the
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Fig. 3. Photomicrographs of thin sections of: a) the 2Btg5 horizon, 140 - 160 cm depth, of the Moltic Albaqualf in plane polarized light., showing layers of silt and clay around an elliptical void; b) as a) but in crossed polarized light; c) the 2Btx horizon, 94 - 125 cm depth, of the Aeric Ochraqualf in plane polarized light showing the clay layers outlining the mostly silt filled voids; d) as c) but in crossed polarized light, showing the orientation of the clays that outline the island of silt and the small void in the upper part.
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water draining from soil cores at the end of hydraulic conductivity studies. Measurement of stress, however, is beyond the scope of our study. Supporting Observations
Silt and very fine sand have been translocated and deposited as silty pedogenic features in Btx horizons of each of the 15 soils. Physical and chemical data show that the overlying Bt horizons have some features in common that help explain the silt movement. All except the silt (Table 2) so material was available Argic Cryoboroll have at least one horizon with ~ 5 0 % to be moved if conditions were favourable. Each of the soils has a particle size discontinuity with finer-textured material overlying coarser-textured material and hence smaller diameter voids overlying larger diameter silt-fied voids (Table 2). We infer that the silty pedogenic features formed below the contact occupy former voids. The larger diameter voids in these lower horizons would have favoured formation of near saturated conditions in overlying Bt horizons before the wetting front could break through to the lower horizons. As the wetting front passes across the pore-size discontinuity occurring at the contact, strain develops in the pores at the base of the overlying horizon. Clay, silt, and very fine sand particles are detached from the walls of these channel voids, and are carried into the lower horizons to form pedotubules (see Figs 3a and b for an incomplete pedotubule) and other silty pedogenic features. Particles in weak soil aggregates are more easily detached from pore walls than particles in well aggregated material. These particles then, may be moved by processes described herein into lower horizons. In our study, most of the Bt and Btx horizons have weak soil aggregates, i.e. aggregate stabilities are <30% (Table 2 and R. Grossman, pers. commun., 1993). Several of their attributes help explain this weak aggregation. Low Ca and Mg saturation of some of the soils and high Na percentages in others (Table 2) tend to produce weak aggregation (Hamblin, 1985). Low organic carbon content of these horizons tends to produce weak aggregation (Table 2) (Emerson, et al., 1986; McKeague et al., 1986; Mbagwu and piccolo, 1989). Dithionite-citrate Fe contents of <0.2 dg kg-l in ped coatings and prism faces in and above the fragipans and other horizons in which the silty pedogenic features occur (Unpublished data from the Soil Survey Laboratory) are believed to be too low to stabilize soil aggregates (Panayiotopoulos and Kostoponlou, 1989; Columbo and Torrent, 1991). All 15 soils are subject to freezing and thawing, an action also known to weaken soil aggregates (Bisal and Nielsen, 1967; Hinman and Bisal, 1968; Sillanpaa and Webber, 1961; and Mbagwu and Bazzoffi, 1989). Application
The understanding of the process of silt flow and deposition developed herein helps explain the high bulk density of soil horizons such as fragipans and aids in the design of agricultural subsurface drainage systems, lined irrigation ditches, earthen dams, and structures for the control of gully erosion. The design of drainage lines should include encasement by envelopes of material to receive all incoming water without developing flow velocities high enough to detach and move particles into the drains (Willardson, 1974). Our results suggest that one way to control the flow velocity would be to change the particle size of the envelope material gradually from the soil to the coarse material at the drain so as to avoid pore-size
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discontinuities. To avoid piping and subsurface erosion the design of structures should avoid development of strong hydraulic gradients in soil adjacent to the structures. CONCLUSIONS The occurrence of pedotubules and other silty pedogenic features in soils is evidence that silt and very fine sand move downward. This requires particle detachment, transport, and deposition in a new location. Particles are detached when shear stress forces developed by laminar flow in a pore or by the surge of suspended water across a pore size discontinuity exceed the forces that retain a particle in the soil mass. Detachment can also occur as water in the soil matrix drains through the pore wall under suction induced by drying. Once in suspension, a particle moves with the water until the flow slows enough for deposition to occur. This happens where the water is suspended above a discontinuity or where it slows d t e r flowing across a discontinuity. Deposition also occurs as water infiltrates ped interiors, and the particles are sieved out of suspension. These deposits are in the form of pedotubules, coarse cutans, and related features. REFERENCES Anderson, E.R. and Darmody, R.G., 1989. Origin of silt coatings in a Typic Fragiudalf. Agronomy Abstracts. 1989 Annual Meeting of the Soil Science of America. Las Vegas, NV, USA, pp. 4. Arshad, M.A. and Mermut, A.R., 1988. Micromorphological and physio-chemical characteristics of soil crust types in northwestern Alberta, Canada. Soil Sci. SOC.Am. J., 52: 724-729. Bisal, F. and Nielsen, K.F., 1967. Effect of frost action on the size of soil aggregates. Soil Sci., 104 268-272. Boiffi, J. and Bresson, L.M., 1987. Dynamique de formation des croutes superfkielles: apport de l'analyse microscopique. In: N. Fedoroff, L.M. Bresson and M.A. Coutry (Editors), Soil Micromorphology. Proc. VII Int. Working Meeting of Soil Micromorphology, Paris, July 1985. Association FranGaise pour 1'Etude du Sol, Plaisir, France, pp. 393-399. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. John Wiley and Sons, New York, 470 pp. Brewer, R., 1976. Fabric and Mineral Analysis of Soils. Krieger, New York, 482 pp. Brewer, R. and Sleeman, J.R., 1960. Soil structure and fabric: their definition and description. J. Soil Sci., 11: 172-185. Bronger, A., 1969/1970. Zur mikromorphogenese und zum tonmineralbestad quartarer lossboden in sudbaden. Geoderma, 3: 281-320. Carlisle, F.J., 1954. Characteristics of Soils with Fragipans in a Podzol Region. Ph.D. diss. Cornell Univ., Ithaca, New York (Diss. Abstr. 14: 1861-1862). Catt, J.A., 1989. Relict properties in soils of the central and north-west European temperate region. In: A. Bronger and J.A. Catt (Editors), Paleopedology, nature and application of paleosols. Catena Supplement 16: 4 1-58. Columbo, C. and Torrent, J., 1991. Relationships between aggregation and iron oxides in Terra Rossa soils from Southern Italy. Catena, 18: 51-59.
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DeKimpe, C.R. and McKeague, J.A., 1974. Micromorphological, physical, and chemical properties of a podzolic soil with a fragipan. Can. J. Soil Sci., 54: 29-38. Emerson, W.W., Foster, R.C. and Oades, J.M., 1986. Organo-mineral complexes in relation to soil aggregation and structure. In: P.M. Huang and M. Schnitzer (Editors), Interactions of soil minerals with natural organics and microbes. Soil Sci. SOC. Am. Spec. Publ. 17., Madison, Wisconsin, pp. 521-548. Falayi, 0. and Bouma, J., 1975. Relationships between the hydraulic conductance of surface crusts and soil management in a Typic Hapludalf. Soil Sci. SOC.Am. J., 39: 957-963. Fedoroff, N., Courty, M.A. and Thompson, M.L., 1990. Micromorphological evidence of paleoenvironmental change in Pleistocene and Holocene paleosols. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 653-665. Fitzpatrick, E.A., 1974. Cryons and isons. Proc. of the North of England Soils Discussion Group, No. 11, Penrith, pp. 31-43. Franzmeier, D.P., Norton, L.D. and Steinhardy, G.C., 1989. Fragipan formation in loess of the midwestern United States. In: N.E. Smeck and E.J. Ciolkosz, (Editors), Fragipans: Their Occurrence, Classification and Genesis. pp. 69-97. Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Phil. Trans. R. SOC.Lond. B, 281: 193-208. Habecker, M.A., McSweeney, K., and Madison, F.W., 1990. Identification and genesis of fragipans in Ochrepts of North Central Wisconsin. Soil Sci. SOC.Am. J., 54: 139- 146. Hamblin, A.P., 1985. The influence of soil structure on water movement, crop root growth, and water uptake. In: N.C. Brady (Editor), Advances in Agronomy. Vol. 38. Academic Press, New York, pp. 95-158. Harr, M.E., 1962. Mechanics of Particulate Media, a Probalistic Approach. McGraw-Hill Book, 543 pp. Hinman, W.C. and Bisal, F., 1968. Alterations of soil structure upon freezing and thawing and subsequent drying. Can. J. Soil Sci., 48: 193-197. Luxmoore, R., 1981. Micro-, meso-, and macroporosity of soil. Soil Sci. SOC.Am. J., 45: 67 1-672. Mbagwu, J.S.C. and Bazzoffi, P., 1989. Effect of antecedent matric potential on the stability of soil aggregates subjected to cyclic freezing and thawing as evaluated by three structural indices. Soil Technology, 2: 59-70. Mbagwu, J.S.C. and Piccolo, A., 1989. Changes in soil aggregate stability induced by amendment with humic substances. Soil Technology, 2: 49-57. McKeague, J.A., Cheshire, M.V., Andreux, F. and Berthelin, J., 1986. Organo-mineral complexes in relation to pedogenesis. In: P.M. Huang and M. Schnitzer (Editors), Interactions of soil minerals with natural organics and microbes. Soil Sci. SOC.Am. Spec. Publ. No. 17. ,Madison, Wisconsin, USA, pp. 549-592. Miller, F.P., Wilding, L.P. and Holowaychuk, N. , 1971. Canfield silt loam, a Fragiudalf: I1 Micromorphological, physical, and chemical properties. Soil Sci. SOC.Amer. Proc., 35: 324-33 1. Muskat, M., 1946. The flow of homogeneous fluids through porous media. McGraw-Hill Book, New York,, 1937; reprinted by J.W. Edwards, Publisher, Ann Arbor, Michigan, 737 PP.
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Nettleton, W.D., McCracken, R.J. and Daniels, R.B., 1968. Two North Carolina coastal plah catenas: I1 Micromorphology, Composition, and fragipans genesis. Soil Sci. SOC.Am. Proc., 32: 582-587. Panayiotopoulos, K.P. and Kostopoulou, S., 1989. Aggregate stability dependence on size, cultivation and various soil constituents in Red Mediterranian soils (Alfisols). Soil Technology, 2: 79-89. Ransom, M.D., 1984. Genetic processes in seasonally wet soils on the Illionian tdl plain in southwestern Ohio. Ph.D. dissertation, The Ohio State Univ., Columbus, Ohio, 355 pp. Ransom, M.D., Smeck, N.E. and Bigham, J.M., 1987. Micromorphology of seasonally wet soils on the Illinoian till plain, USA. Geoderma, 4 0 83-99. Romans, J.C.C., Stevens, J.H. and Robertson, L., 1966. Alpine soils of north-east Scotland. J. Soil Sci., 17: 184-199. SAS Institute, 1988. SAS/STAT User's Guide, release 6.03 ed. SAS Inst., Cary, North Carolina, 1027 pp. Sillanpaa, M. and Webber, L.R., 1961. The effect of freezing-thawing and wetting-drying cycles on soil aggregation. Can. J. Soil Sci., 41: 182-187. Soil Survey Staff., 1992. Keys to Soil Taxonomy. SMSS Technical Monograph No. 19. Fifth Ed. Pocahontas Press, Blacksburg, Virginia, 541 pp. Sullivan, L.A. and Koppi, A.J., 1991. Morphology and genesis of silt and clay coatings in the vesicular layer of a desert loam soil. Aust. J. Soil Res., 29: 579-586. Tarnocai, C. and Valentine, K.W.G., 1989. Relict soil properties of the arctic and subarctic regions of Canada. In: A. Bronger and J.A. Catt (Editors), Paleopedology, nature and application of paleosols. Catena Supplement 16. Catena Verlag, Cremlingen-Destedt, Germany, pp. 9-39. Thompson, M.L., 1986. Morphology and mineralogy of a pre-Wisconsinian paleosol in Iowa. Soil Sci. SOC.Am. J., 50: 981-987. Thompson, M.L. and Smeck, N.E., 1983. Micromorphology of polygenetic soils in the Teays River Valley, Ohio. Soil Sci. SOC.Am. J., 47: 734-742. USDA Soil Conservation Service., 1992 (rev). Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 2.0, U.S. Gov. Printing Office, Washington, D.C., 400pp. Van Vliet, B. and Langohr, R., 1981. Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France. Catena, 8: 137-154. West, L.T., Bradford, J.M. and Norton, L.D., 1990. Crust morphology and infiltrability in surface soils from the southeast and midwest U.S.A. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 107-113. Willardson, L.S., 1974. Envelope materials. In: J. Van Schilfgaarde (Editor), Drainage for Agriculture. Agron. Series 17. Am. SOC.Agron. Madison, Wisconsin, pp. 179-200.