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Professor Emeritus Plant Science Department (Soils) South Dakota State University Brookings, South Dakota 57007-1096
The paper, "Using surface crack spacing to predict crack network geometry in swelling soils", by Chertkov (2000) uses a mathematical approach to estimate soil-crack-network geometry. His approach may be suitable for random cracking, but random cracking is not natural in soils. The cracking geometry of soils can be deduced from soil structure characteristics (White, 1967).
The boundaries of soil peds are planes of weakness, and the peds separate at the same locations with each drying event. Visible soil cracks follow the boundaries of peds. Ped boundaries may be somewhat indistinct in very wet and very dry soils. At intermediate moisture contents, ped boundaries may be more easily observed. As the soil dries, very narrow cracks can form between the peds. With additional drying, cracks close at some locations and widen at others. The arrangement and distribution of the crack space is moisture dependent.
In soils with some clay content, the larger subsoil peds are prism shaped and their size tends to increase with depth. Cracks form because the desiccation contraction forces exceed the tensile strength of the soil (White, 1972). Soil particles are packed more closely with increasing depth because of the weight of the overlying soil. Thus, a greater distance of subsoil is needed before the contraction force can exceed the tensile strength to cause a crack to form. This relationship causes several small prisms to be joined together at their base to form a larger prism (White, 1966; 1970).
Cracks are at locations where the energy used in the average dehydration-contraction and hydration-expansion cycle is at minimum values in a ped and group of peds. In other words, the average soil particle moves the least possible distance. This quasi equilibrium occurs first in the smallest peds and subsequently in successively larger peds and eventually includes both the upper and lower layers of the soil if they have been moistened and then dried.
The diameter of the larger prisms decreases as the soil dries so that a crack is formed around the prism and smaller overlying prisms. Surface soil may fall into the crack so the outline of the larger prism may be easily seen (White, 1989, p. 154 picture). The larger prisms often have surfaces coated with some dark-colored clay-rich material likely washed down from the overlying soil. The cracks between the smaller overlying prisms will decrease in width as the basal-prism diameter decreases. Successively larger prisms form with increasing depth as the soil dries so that the crack space in overlying layers is rearranged into a larger crack at the edge of the larger prism that formed at that depth. Thus the width of the upper part of the crack is a function of the depth of drying.
If the lower part of the soil is infrequently moistened, open cracks in the layer will act as a template for the formation of a crack when the upper part of the soil dries. This process also causes the crack space between smaller prisms to move to the margin of the larger prisms. Giant desiccation cracks, some 20- to 30-cm wide, likely form by this process (White, 1970). The giant cracks form where regional or local conditions have occasionally caused the lower subsoil to be moistened while the upper soil is moistened and dried annually. Giant cracks are most frequently found in fields where alfalfa (Medicago sativa L.) is being or has been grown. Alfalfa has extensive, deep, perennial roots. The cracks may form where snowdrifts or runoff accumulated to moisten the lower soil to a greater depth than in the adjacent areas. As the cracks in the lower subsoil become wider, the smaller cracks between prisms in the upper soil decrease in width and the space will move to the larger crack.
When the soil dries and contracts to form cracks, the shrinking also decreases the elevation of the soil surface (White, 1962). Calculations of crack volume from wet and dry clod volumes need to be adjusted for this decrease in surface elevation that accompanies soil drying.
Soil areas with wavy gilgai have prisms but the largest desiccation cracks occur in the microvalleys. The prisms are often not well developed because parallelepiped structure distorts the prism boundaries (White, 1967). As in most soils, the smaller blocky structure in parallelepiped and prismatic structure is surrounded by narrow cracks that are not extensive. The microvalley cracks are oriented in the slope direction so that crack widths should be measured along transects on the contour. In normal gilgai, the largests desiccation crack forms in the moat-like circular depression that surrounds each microhigh. The crack space around the microhighs in an area could be used to calculate the total crack amount. As with prisms, the crack depths increase as the distance from one microhigh to the next increases. The microhighs in wavy gilgai are generally from about 1 m to 5 m apart (White, 1997) but in one small area they were found to be 6.6 m apart. This small area may have collected windblown snow from a nearly level area to the north. When the added snow melted, the soil would have been moistened to a greater depth than is typical.
Soil structure can be used to determine the location of cracks and cracking depth. Parallelepiped structure should be present in the subsoil of swelling clays if the layer has been moistened and dried frequently. If structure boundaries are not present it seems obvious that the soil has not had cracks at that depth, which could be a factor in hydraulic conductivity. The hydraulic conductivity would then be controlled by the pore characteristics of the material.
An objective of Chertkov's study was to measure crack space at the soil surface to study hydraulic conductivity. Presumably, water would enter the surface crack. Is this infiltration (White, 1986) rather than conductivity? Hydraulic conductivity has been defined in the geologic literature as flow under saturated conditions; but, in most soils, water movement is more by unsaturated flow.
Received for publication February 19, 2001.
REFERENCES
This article has been cited by other articles:
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  R. R. Wells, D. A. DiCarlo, T. S. Steenhuis, J.-Y. Parlange, M. J. M. Romkens, and S. N. Prasad Infiltration and Surface Geometry Features of a Swelling Soil following Successive Simulated Rainstorms Soil Sci. Soc. Am. J., September 1, 2003; 67(5): 1344 - 1351. [Abstract] [Full Text] [PDF]
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