Though structure and fabric are often used terms and they are often used interchangeably and inconsistently. In addition, various authors define the terms differently. Thus, there is some confusion concerning these terms. Below are abbreviated definitions from several sources.

Structure:

Soil structure refers to units composed of primary particles. The cohesion within these units is greater than the adhesion among units. As a consequence, the soil mass tends to rupture along predetermined planes or zone. (Soil Survey Division Staff, 1993).

The physical constitution of a soil material as expressed by the size, shape, and arrangement of the solid particles and voids, including both the primary particles to form compound particles and the compound particles themselves; fabric is the element of structure which deals with arrangement (Brewer, 1976).

Structure is concerned with the size, shape, and arrangement of primary particles and voids in both aggregated and non-aggregated material and the size, shape, and arrangement of any aggregates present (Bullock, et al., 1985).

The gradation and arrangement of soil particles, porosity, and pore size distribution, bonding agents, and the specific interactions developed between particles through associated electrical forces (Yong and Warkentin, 1975).

The spatial arrangement and total organization of teh soil system as expressed by the degree and type of aggregation and the nature and distribution of the pores and pore space (FitzPatrick, 1993).

Fabric:

The physical constituion of a soil material as expressed by the spatial arrangement of the solid particles and associated voids (Brewer, 1976).

Soil fabric deals with the total organization of a soil, expressed by the spatial arrangements of the soil constituents (including voids?), their shape, size, and frequency, considered from a cnfigurational, functional, and genetic viewpoint (Bullock, et al., 1985).

The geometric arrangement of the constituent mineral particles, including the void space which can be observed visually or direcctly using optical and electron microscopic techniques. An assessment of soil fabric is required for a proper evaluation of soil structure (Yong and Warkentin, 1975).

The arrangement, size, shape, and frequency of the individual solid soil components within the soil as a whole and within features themselves (FitzPatrick, 1993).

In general, most of these definitions consider fabric to be the spatial arrangement of soil particles and voids while structure is the organization of soil constituents into larger aggregates or compound particles.

these definitions place no limitations on scale. Thus, observations at all magnifications (naked eye, hand lens, optical and electron microscope) must be considered. As magnification is increased, structure and fabric details are more readily observable.

Fabric and structure are often not considered to be measurable quantities. With image analysis techniques, however, quantification can be achieved, especially void size and quantity. Descriptions of fabric and structure can provide valuable information concerning certain processes, i.e. degree of particle sorting, shrink-swell, inferred rates of water movement, etc.

Why worry about structure and Fabric?

Bulk analysis for chemical properties represent "average" values for all constituents without consideration of distribution within soil.

Distribution of components may have an appreciable impact on behavior of soil, i.e. clay or carbonates as ped coatings rather than uniformly disseminated throughout the soil.

Same concept is true for soil physical measurements. Rate of water movement is determined by quantity, shape, and continuity of pore. Water release curve will yield pore size distribution byt says nothing of pore shape or continuity. Ped ocatings may also impact rate of water and solute movement from conductive voids into the soil matrix.

the following are two examples of application to soil structure and fabric description and analysis to developing a better understanding soil processes.

Surface sealing

Surface seal formation is readily observable from infiltration rate measurements as a freshly-tilled soil is subjected to simulated or natural rainfall, i.e. see Table 1.

Table 1. Infiltration rates for a Cecil Ap horizon (packed pan) under 50 mm/hr simulated rainfall (after Miller and Radcliffe, 1992).

These types of data, however, yield little information concerning the mechanisms that have caused the seal to form and that are causing reductions in the rate of water movement. Observations of the morphology and fabric of the surface seals can provide valuable insight into these mechanisms and many have described the fabric of the seals and/or layers within the seals.

Types of seals and layers within surface seals that have been reported include:

Structural Seals - seals formed in place by processes directly related to raindrop impact and associated rapid wetting of the soil surface.

Disruptional layer - layer of variable thickness (generally less than 4-5 mm) resulting from particle and aggregate rearrangment under raindrop impact with fewer and finer pores than subjacent soil (figure 1). Related distribution patterns and coarse/fine rations are normally similar to that in the subjacent soil.

Skin seal - thin (<0.1 mm thick), dense layer of fine particles at the surface of structural crusts. Attributed to aggregate breakdown at the soil surface or to depostion of clays from suspension at the end of the rainfall event.

Washed-Out/Washed-In Layers - Washed-in layers are described as a zone of accumulation of dispersed fine particles from the overlying soil. Washed-out layers are layers depleted in fine particles due to dispersion and downward movement. Washed-out layers have been reproted more often than washed-in layers.

Sedimentary Seals-surface seals that result from flow induced transport and deposition of particles and micro- aggregates from higher to lower topographic positions. Transport may be local as a result of micro-topography (top of clod to base of clod, etc.) or may be over a long distance.

Most often described as mutiple laminated microbeds overlying undisturbed soil or a structural crust. Degree of soring and other characteristics vary with flow regime of transporting water, but often can recognize a "fining-upward" sequence.

Inaddition to observations of structure and fabric of seals at the end of a rainfall event, observations of fabric and structure and measurements of porosity (image analysis or point counting) over a time sequence can yield valuable information concerning genesis of seals for different of soils.

Preferential Flow of Water

Observations of pathways of preferential flow are most often made after conductive voids have been stained by a dye.

Once the flow paths have been identified, observations and measurements at various scales can be made to evaluate factors influencing flow through the soil.

Figure 1. Total visible porosity as determined from point counts along 10 transects from the soil surface to 10 mm. Observaions made at 0.1 mm intervals. Data points are means of 50 observations within the depth increment (from Chiang et al., 1994).

for soils in Georgia, observations and quantification of flow paths and porosity have been made on 15 cm diameter soil columns, impregnated polished blocks, and thin sections.

In upper Bt horizons most flow (dye stained areas) is through regions that have more and larger voids than undyed regions of the soil (Table 3). In lower Bt and BC horizons dye staining was both along strutural (macro structure) faces and in coarser irregularly-shaped areas which again had more and larger voids than undyed soil. The greater porosity within dyed stained regions of the soil was attributed to fabric modifications from biologic activity within the soil.

* Area of pores >0.05 mm equivalent circular diameter determined from impregnated polished blocks with image analysis.
** The BC horizon had insufficient dyed area for evaluation of pore area.

These are only two examples of use of structure and fabric for gaining a better understanding of soil processes. There are numerous other examples. Only the creativity of the researcher limits the application of structure and fabric evaluations to larger questions.

Selected References

Brewer, R. 1976. Fabric and mineral analysis of soils. Krieger Publishing Co., Huntington, N.Y.

Bullock, P., N. Fedoroff, A. Jongerius, G. Stoops, T. Tursina, and U. Babel. 1985. Handbook for soil thin section description. Waine Research Publications, Albrighton, Wolverhampton, U.K.

Chiang, S.C., L.T. West, and D.E. Radcliffe. 1994. Morphological properties of surface seals in Georgia soils. Soil Sci. Soc. Am. J. 58:901-910.

Fitzpatrick, E.A. 1993. Soil microscopy and micromorphology. Wiley, New York.

Franklin, D.H. 1994. Morphological evaluation and quantification of flow paths in a Georgia Piedmont soil. M.S. Thesis, University of Georgia, Athens, GA.

Miller W.P., and D.E. Radcliffe. 1992. soil crusting in the southeastern United States. p.233-266 In M.E. Summer and B.A. Stewart (eds) Soil crusting: Chemical and physical processes. Lewis Publishers, Boca Raton, FL.

Shaw, J.N., L.T. West, and C.C. Truman. 1997. Hydraulic properties of soils with water restrictive horizons in the Georgia Coastal Plain. Soil Sci. (in press).

Yong, R.N., and B.P. Warkentin. Soil properties and behavior. Elsevier, Amsterdam.

This page (http://www.ces.ncsu.edu/plymouth/programs/sap.html) created by
Vera MacConnell, Research Technician, I on February 27, 1998.
Last Updated on March 9, 1998.