Soil Particle Size Analysis Methods

There are three basic classifications of soil particle size. These 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. Methods may be mechanical or Two mechanical methods involve sieving, however, a possibly more accurate method uses a hydrometer. Please redirect to the Loss on Ignition test, for a method which determines the amount of organic matter in the soil.

Soil Particle Size Analysis Methods

US Standard Sieve

Sieving

To begin, the soil sample needs to be dried to a constant weight, for 24 hrs at 120˚F this will eliminate all the moisture held in the soil. Once the soil is dried to constant weight, the sieving can begin. The sieves come with different size screens and they should be chosen according to the particles that are to be isolated.

Stacked Sieve: largest screen size at the top, smallest screen sieve at the bottom

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 that has been 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. The proportions can then 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 accurately separating clay particles from the soil would be to use a hydrometer.

Hydrometer

Clay particles are made up of either three or four charged ions, because of this they tend to cling to one another [3], this tendency is called flocculation[4]. This can sometimes pose a problem when trying to accurately determine the proportion of clay particles in a sample of soil. When measuring soil samples using a hydrometer, sodium hexametaphospahte is added to the water, this acts as a defloccuant, meaning the clay particle ions will now repulse each other instead of clinging to one another. [4,5] This will result in a more accurate reading of sand, silt, and clay particles than could be achieved by sieving alone.

To measure using a hydrometer, a solution of water mixed with sodium hexametaphosphate is prepared and poured into a 1000ml graudated cylinder. The solution must be at room temperature for accurate readings[6].

The soil sample (for example 50g soil) is then poured into the solution and shaken or stirred until evenly distributed. Historically, the use of a milk shake, or malt machine was recommended to shake the solution! This was to ensure even distribution of the soil throughout the water solution.[7]

The hydrometer is then placed in the graduated cylinder, and a measurement is read off of it after 40 seconds.

The hydrometer is then taken out and the solution of soil, water, and sodium hexmetaphosphate it stirred or shaken again.

Another 40 second reading will be taken and then an average of the two readings will be calculated to determine the amount of sand (and gravel) in the sample.

To obtain an accurate reading of clay and silt percentages the solution should sit for 6.5-8 hours at least, 12 at the most[6], however if 100% accuracy isn't need, an hour will suffice.

The solution, with the hydrometer in it still, will then sit for at least 1-8 hours depending on the accuracy desired. During this time the silt will settle to the bottom on the cylinder and the clay particles will remain suspended in the water[6], this suspension will allow the hydrometer to stay suspended as well and the water line will correlate to a reading which will allow the reader to determine the amount of clay in the sample. The remaining part of the proportion of sand + clay subtracted from the total soil sample will determine the amount of silt in the sample.

Conclusion

In conclusion, performing both experiments is a good way to compare and contrast results. The gravel/sand, and silt proportions should resemble one another between the two tests, however a more accurate proportion of clay particle may be obtained from a hydrometer reading.

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.

4. Tozzi, Nilo. “Deflocculants: A Detailed Overview.” Deflocculants: A Detailed Overview, digitalfire.com/4sight/education/deflocculants_a_detailed_overview_324.html.

5. Andreola, Fernanda, et al. “The Role of Sodium Hexametaphosphate in the Dissolution Process of Kaolinite and Kaolin.” Journal of the European Ceramic Society, Elsevier, 24 Sept. 2003, www.sciencedirect.com/science/article/pii/S0955221903003662.

6. Carolyn, C. B., and G. Karl. 1989. Comparison of Hydrometer Settling Times in Soil Particle Size Analysis. Journal of Range Management 42:81-83.

7. Bouyoucos, G. J. 1927. Directions for Determining the Colloidal Material of Soils by the Hydrometer Method. Science 66:16-17.