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Soil Particle Size Analysis Methods

There are three basic classifications of soil particle size. They include clay, silt and sand, from smallest to largest, respectively. There are several different methods to determining how much clay, silt and sand is in a sample of soil, two include sieving, and using a hydrometer. There is another method used in determining the amount of organic mater in soil, to do this one might use the Loss on ignition test, however the purpose of this article will be to focus on clay, silt sand only.

Sieving

To begin the soil sample needs to be dried to a constant weight, eliminating all the moisture held in the soil. This should be done in an airtight oven, for 24 hrs at 120˚C.

Once the soil is dried to constant weight, the sieving can begin. The sieves come with different size screens, they should be chosen according to the particles that are to be isolated.

There should be one sieve on top with larger holes to accommodate for gravel that is in the soil. Gravel includes any particle larger than 2mm and will be considered as "sand". [1]

Course sand particles range from 1.0 - 2.0 mm while very fine sand particles range from 0.05 - 0.10mm.[1] Choosing two or more sieves within that range might be helpful to account for larger or smaller sand particles, for instance include a sieve at 2.0mm and one at 0.05mm. Silt particles ranges from 0.05mm - 0.002 mm [1], therefore a sieve with a corresponding screen hole size of 0.002 mm should suffice.

Lastly clay particles are anything smaller than 0.002mm. [1] At the bottom of the stack of sieves should be placed a bottom container with a solid bottom to collect the clay particle that will sift past the 0.002mm sieve.

Once the sieves are set up, the soil sample, dried to a constant weight, will be sifted through the sieves using a sieve shaker. After about 15-20 min of shaking the soil should be separated into gravel, sand, silt and clay. Then proportions can be calculated to determine how much of each particle is one soil sample. For instance 26g sand/50g total soil = 52% sand.

Soil sieving is a purely mechanical process of determining soil particle size in a sample of soil. [2] A better method to accuratley separating clay particles from the soil would be to a hydrometer.

Hydrometer

Because clay particles are made up of either three or four charged ions they tend to cling to one another [3], this sometimes poses a problem when trying to accurately determine the proportion of clay particles in a sample of soil.

References

1. Whiting, David, et al. Estimating Soil Texture. 2003, Estimating Soil Texture,

       culter.colorado.edu/~kittel/SoilChar(&RibbonTest)_handout.pdf.

2. “Particle Size Analysis (for Soils/Sediments).” UCL Department of Geography, www.geog.ucl.ac.uk/resources/laboratory/laboratory-

       methods/particle-size-analysis/particle-size-analysis-for-soils-sediments.

3. “1.8 Clay Mineral Structure.” Fundamentals of Soil Ecology, by David C. Coleman et al., Academic Press, 2018.