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•530 AIR-TO-WATER PERMEABILITY RATIO <br /> 41-6 LITERATURE 5 3 1 <br /> The permeability of soil to water is not constant for most soils but varies <br /> with time or with the amount of water passing through the soil. Therefore, 41-6 LITERATURE CITED <br /> measurements of time and volume of percolate should be continued until <br /> the changes dictated by the purpose for which the tests are being made American Petroleum Institute. 1956. Recommended practice for determining per- <br /> have taken place. The total depth of water passed through the soil may meability of porous media.Code No. 27,third issue.Am. Petrol. Inst.,Dallas, Tex. <br /> correspond to the depth of water applied in an irrigation, which may be Brooks, R. H., and Reeve, R. C. 1959. Measurement of air and water permeability of <br /> from 3 to 6 inches. For comparing one soil with another, or for determin- soils.Am.Soc.Agr.Eng.Trans.2:125-126, 128. <br /> in the effect of various treatments on a given soil, it is frequently desirable Carmen, P. C. 1956. Flow of Gases through Porous Media. Academic Press, Inc., <br /> g g Q y New York. <br /> to use a value obtained after the hydraulic conductivity has become essen- Corey, A. T. 1957. Measurement of water and air permeability in unsaturated soil. <br /> tially constant. From 70 to 80 hours usually are required before constancy j Soil Sci.Soc.Am.Proc.21:7-10. <br /> of values is obtained. Grover, Ben L. 1955. Simplified air permeameters for soil in place. Soil Sci. Soc. Am. <br /> Proc. 19:414-418. <br /> Kirkham, D. 1947. Field method for determination of air permeability of soil in its <br /> undisturbed state.Soil Sci.Soc.Am.Proc. (1946) 11:93-99. <br /> 41-5 EXPRESSION AND INTERPRETATION OF RESULTS Reeve, R. C. 1953. A method for determining the stability of soil structure based <br /> upon air and water permeability measurements. Soil Sci. Soc. Am. Proc. 17:324— <br /> Both the absolute value of permeability of soil to water k'u and the air- 329. <br /> Reeve, R. C., and Brooks, R. H. 1953. Equipment for subsampling and packing frag- <br /> to-water permeability ratio k'Q/k',, should be reported. The former is an mented soil samples for air and water permeability tests. Soil Sci. Soc. Am. Proc. <br /> absolute measure of the soil physical condition, and the latter is an index 17:333-336. <br /> of stability or the susceptibility of soil structure to deterioration as a result U. S. Salinity Laboratory Staff. 1954. Diagnosis and improvement of saline and <br /> of wetting with water. As an aid in interpretation of results, water perme- alkali soils. U.S.Dept.Agr.Handbook 60. <br /> � Wyckoff, R. D., and Botset, H. G. 1936. The flow of gas-liquid mixtures through <br /> ability values may be converted to hydraulic conductivity at a specified l unconsolidated sands.Physics 7:325-345. <br /> temperature, usually 25`C. This is done by multiplying water permeability <br /> by n,,,g/j. <br /> A value of k',,/k'u, = 1 indicates a stable medium. A value of one has <br /> been measured for carbon cores and glass beads; but, for soils, measured <br /> values always have been greater than one. Values as low as 2.5 have been I <br /> measured for soils of low clay content and low exchangeable sodium. Values <br /> as great as 50,000 have been obtained where distilled water was used with <br /> problem soils having high exchangeable sodium. <br /> In all cases the salt concentration and composition of the water used 1 <br /> should be reported. Salt content may be expressed in milliequivalents per <br /> liter, or the electrical conductivity may be given. The concentration of the <br /> individual ions should be reported. In the absence of a complete analysis, <br /> a minimum report should include the total salt concentration and the so- <br /> dium adsorption ratio (SAR) where SAR = Na+/[(Ca-+ + Mg"+)/2]1/= <br /> (cation concentrations expressed in mill iequivalents per liter). <br /> Individual determinations of k',/k',,. are satisfactory for most purposes. <br /> Seventy-five percent of the time, individual determinations can be expected <br /> to lie within ±10% of the mean, and 80% of the time within -L20%. <br />