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5.4: Particle Density

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  • The particle density of a soil = dry mass (g) of soil / volume of soil particles.

    \[D_p = \frac{ODwt}{V_s} \nonumber \]

    Where Dp is particle density; ODwt is oven-dry weight of soil; Vs is volume of solids. Particle density takes into account the mass and volume occupied by the solid particles only. It excludes the volume occupied by air and water. Since a majority of soils is composed of particles derived from minerals containing 70% or more silica and oxygen, the particle density of most soils is approximately 2.65 g/cm3 . This particle density is nearly the same as the density of quartz.

    Variations in the particle density of soils are due to the presence of heavier minerals (such as iron oxides) or organic matter (which has a low density and reduces the overall particle density value). Kaolinite and illite clays have particle densities of about 2.6 g/cm3 , while colloidal montmorillonite clays have particle densities of about 2.4 g/cm3.

    Although tillage results in a change in both bulk density and porosity, it does not affect particle density. The particle density remains constant because tillage and other short-term changes do not alter the total amount or the chemical composition of the soil mineral particles.

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