The link below goes to an Excel file in my google drive:

https://drive.google.com/file/d/1kwK4fmFFdngbu6GJyX0L82q0gq2CCVUn/view?usp=sharing

This Excel file can be used to calculate the centrifugation time required to obtain the clay fraction (< 2 µm) or other desired particle size from a soil sample. The equations used are from four separate resources and all yield the same result (quadruple checked!). The user needs to supply four input parameters to carry out the calculation (green boxes).

– r1, distance from the center of rotation of the centrifuge to the top of the liquid surface (initial settlement radius). This needs to be measured by the user.
– r2, distance from the center of rotation to the decantation level (final settlement radius, above which clays are suspended). This also needs to be measured by the user.
– rpm, revolutions per minute. This is a setting on the centrifuge.
– t, spin time set for the centrifuge

Other parameters such as viscosity of water at room temperature, and particle and liquid density, and constants are already input for common values used in soil research. They can be modified if desired.

This centrifugation method can be coupled with samples that have been dispersed via sonication (see my paper in Geoderma for a sonication method for soil dispersion https://doi.org/10.1016/j.geoderma.2017.03.008)

The four references are below:

-Gee and Or, 2002, Chapter 2 pg 272
-Gimbert et al., 2005
-Jackson, M. L. (1985), pages 113 and 127
-Whittig and Allardice, 1986, Chapter 12, pg 339

(Gee and Or, 2002; Gimbert et al., 2005; Jackson, 1985; Whittig and Allardice, 1986)

Gee, G. W., and Or, D. (2002). Particle-Size Analysis. In “Methods of Soil Analysis Part 4 Physical Methods” (J. H. a. T. Dane, C., ed.). Soil Science Society of America Book Series, Madison, Wisconsin, USA.
Gimbert, L. J., Haygarth, P. M., Beckett, R., and Worsfold, P. J. (2005). Comparison of centrifugation and filtration techniques for the size fractionation of colloidal material in soil suspensions using sedimentation field-flow fractionation. Environmental Science & Technology 39, 1731-1735.
Jackson, M. L. (1985). “Soil chemical analysis – Advanced course,” 2nd/Ed. University of Wisconsin, Madison, WI.
Whittig, L. D., and Allardice, W. R. (1986). X-Ray Diffraction Techniques. In “Methods of Soil Analysis: Part 1—Physical and Mineralogical Methods” (A. Klute, ed.), pp. 331-362. Soil Science Society of America, American Society of Agronomy, Madison, WI.

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Additionally, if a centrifuge is not available, sedimentation tables are available in the following reference, and can sometime be more practical than the centrifugation method (albeit longer sedimentation times):

Soukup, D. A., Buck, B. J., and Harris, W. (2008). Preparing Soils for Mineralogical Analyses. In “Methods of Soil Analysis Part 5—Mineralogical Methods” (A. L. Ulery and L. Richard Drees, eds.), pp. 13-31. Soil Science Society of America.

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Keywords: clay sedimentation, centrifugation, sonication, calculation of clay centrifugation times, Soil Science Society of America Methods, soil chemistry, environmental geochemistry