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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Particle Size Analysis  /  Particle Size Analysis (for soils/sediments)

Particle Size Analysis (for soils/sediments)

The following methods and protocols are all related to the chemical and physical properties of soil. Unless stated, the methods are currently being used in the Laboratory and have Risk Assessments associated with them. If you are planning on carrying out any work in the lab you must read and sign the Risk Assessment first. Please contact the Laboratory Supervisor for details.

Not all of the mineral particles in soils or sediments are the same size. Sediment/soil particles are usually sized on the basis of their diameter, and can be classified as a specific sedimentology using scales such as Wentworth (1922). The distribution of particle size determines the texture of soil/sediment - whether it is predominantly sandy, loamy or clayey.














The procedure of determining the proportion of mineral particles in each of these classes is called particle size analysis or mechanical analysis of the soil. In soil analysis, the proportion of gravel and larger particles is determined by first grinding the soil to disaggregate it and then passing it through a 2 mm sieve. What remains in the sieve is weighed and it's proportion calculated as a percentage of the whole soil sample. The proportion of the coarse, medium and fine sand are likewise determined by sieving, thus separating them from the clay and silt fractions of the soil. The latter cannot be separated from one another by sieving. Instead they are separated by a process known as sedimentation, which is based upon the fact that larger particles fall more quickly through water than smaller ones. Thus, if a suspension is made up of clay and silt in water, the silt will settle out more quickly than the clay and by measuring the speed at which the suspension as a whole settles out, it is possible to determine the distribution of particle sizes.

The particle size analysis methods covered here are...