Particle size distribution

There are two main methods to determine the particle size distribution (psd) of different soil types and soil compositions; The particles larger than 63 µm can be separated by mechanically sieving the sub-sample over a set of sieves with different dimensions. Particles smaller than 63 µm are separated based on the differences in sedimentation speed of the different fractions in a solution. This method is based on Stokes’ Law, which describes the sedimentation rate of solid particles with a predetermined radius and specific weight in a motionless liquid with a known viscosity at a specific temperature; a particle falling under its own weight in a solution will reach a terminal velocity when an equilibrium is reached between gravity, buoyancy and the drag forces on the particle Using a low-energy X-ray beam the sedigraph detects changes in the sediment concentration during settling at different vertical distances and at specific times. The concentration measured at each specific time and distance (equals velocity) will represent the concentration of particles equal or smaller than the predicted particle size in Stokes’ Law at a given velocity. Eventually leading to a complete distribution of all particles in the sample. The smallest particle size that can be measured using a sedigraph is 0.1 micrometre.

The final psd-diagram can be presented relative to the dry matter (of the total sample) or only to the mineral fraction of the sample (after removal of all and any organic matter). 

If only the coarse fractions are of interest, the PSD is determined only by sieving; For this method the (sub-) sample is air-dried and weight (dry) after which a peptisator (solution to prevent flocculation) is added. The solution is than left to soak for sixteen hours before it is sieved over a 63 µm sieve. The remainder is than mechanically sieved over a set of sieves ranging from 63mm to 63 µm. The material left on each sieve is, subsequently, weight and cumulatively processed to determine the distribution.

If all fractions are of interest, the PSD is determined by sieving as well as through sedimentation; In this case the (sub-) sample is air-dried and weight (dry) after which a peptisator (solution to prevent flocculation) is added. The solution is than left to soak for sixteen hours before it is sieved over a 63 µm sieve. The portion larger than 63 µm is than mechanically sieved, as is described above, and the material passing through the 63 µm sieve will go into the sedigraph. The results of from both methods are eventually combined into a particle size distribution showing both coarse and the fine fractions.