2.6 Scale of Primary Soil Particles
When we break down the soil aggregates completely, we are left with the individual soil particles, and the size distribution of these primary particles defines the soil texture. Typically, we classify these particles as either sand, silt, or clay based on their size. In the widely-used classification system of the United States Department of Agriculture, particles < 2.0 and ≥ 0.05 mm are considered sand, particles between < 0.05 mm and ≥ 0.002 mm are considered silt, and particles < 0.002 mm are considered clay. The particle size distribution or soil texture is one of the most significant soil physical properties, and it has a major influence on the chemical, physical, and biological processes that occur in the soil.
To measure the particle size distribution, we typically disperse, or separate, the soil particles in a liquid to create a suspension using either chemical or physical dispersion methods, or both. Sometimes pre-treatment of the sample is necessary to remove binding agents such as soil organic matter. Once the soil particles are completely dispersed and well-mixed in the suspension, the particle size distribution can be determined based on the rate at which particles settle out of the suspension using sedimentation theory. Alternatively, the particle size distribution can be determined using laser diffraction methods, which can also provide information about the particle shape [7].
Sedimentation, or sediment deposition, is the process of particles settling out of suspension, and it is the process most commonly used to determine soil particle sizes and soil texture. You have probably observed that solid particles in a fluid settle at different rates depending on their size and density. In soil science, we often use Stokes’ Law to represent the relationship between a particle’s size, its density, and its settling velocity. For a spherical particle a conceptual diagram of the situation looks like Fig. 2‑12. The symbol, Fg, represents the downward force due to the difference between the weight of the sphere and the buoyancy of the sphere:
where r is the radius of the particle, ρs is the particle density, ρf is the fluid density, and g is the acceleration due to gravity.
When the falling particle is no longer accelerating, it has reached terminal (or maximum) velocity, and the downward force due to gravity is perfectly balanced by an upward drag force, Fd, due to the particle’s motion through the fluid:
where h is the dynamic viscosity of the fluid and u is the terminal velocity. The dynamic viscosity is basically a fluid’s resistance to flow and is similar to what might commonly be referred to as the “thickness” of the fluid. Setting Fg = Fd and rearranging, we can solve for the terminal velocity:
(Eq. 2-2)
where d is the particle diameter. Once we know the settling velocity for particles of a given diameter, we can easily calculate the time required for those particles to fall a specified distance throu