Soil Science

Particle size analysis

Particle size analysis involves two main steps: dispersion of the soil into its primary particles and fractionation of the different particle sizes based on their rate of fall through water.

Fractionation is an application of Stokes' Law.

Start

About 50 g of soil is accurately weighed and then added to a high-speed mixer cup. Take care to ensure that the sample is representative of the soil you wish to characterize. The weight of the soil sample will also need to be corrected for its moisture content in later calculations.

Chemical dispersant and sufficient water are added to aid mixing.

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Dispersion

The soil is thoroughly dispersed by removing cementing agents such as organic matter, calcium carbonate, and iron oxides using a chemical dispersant, along with mechanical mixing.

Without dispersion, some particles may remain aggregated and behave as if they were a much larger diameter particle.

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Fractionation

Start Timing - Start timing after thoroughly mixing the contents of the sedimentation cylinder.

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Silt Sampling - According to Stoke's Law, after 4.1 minutes, particles larger than 0.02 mm in diameter will have fallen more than 10 cm (assuming a soil particle density of 2.65 g/cm³ and a water temperature of 25°C). Therefore, the density of the solution at 10 cm is influenced by the presence of silt and clay particles. This density can be determined using a hydrometer or by oven-drying a known volume sampled by pipette from this depth.

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Clay Sampling - After 6 hours and 52 minutes, all particles larger than 2 µm in diameter will have fallen more than 10 cm. Therefore, the density of the solution at 10 cm is influenced by the presence of clay particles in the soil. This density can be determined using a hydrometer or by pipette sampling.

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Sand Sieving - The quantity of fine sand (20-200 µm) can also be determined. However, in practical classes, we often estimate the proportion of fine sand by difference, which involves subtracting the mass of the clay, silt, and coarse sand fractions from the mass of the original oven-dried sample.

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