 Soil-particle density

Soil-particle density compares dry particle mass with the volume assigned to the particles themselves:Spaces between particles are excluded from $V_s$. By contrast, the denominator of soil bulk density includes those spaces. A measurement must specify how particle volume is determined, especially when the particles themselves contain pores.^[ref]

One measurement principle uses a pycnometer: determine the dry sample mass, then infer its volume from the displaced liquid mass and the liquid's density. Trapped gas, access to internal pores and the chosen volume convention matter when interpreting the displacement. The standard distinguishes fine soil from gravel and stones, and its stated particle-volume convention includes internal particle pores while excluding spaces between particles.^[ref]

A constructed mixture shows why particle composition matters. Assume additive volumes, no internal pore complication, and two kinds of particles with densities $2.60$ and $1.30\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. Equal masses of $13\mathrm{g}$ occupy $5$ and $10\mathrm{c}{\mathrm{m}}^3$, respectively. The combined soil-particle density israther than the arithmetic mean $1.95\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. For mass fractions $w_i$ and constituent densities ${\rho }_i$, the same additive-volume model gives ${\rho }_s^{-1}={\sum }_i{w}_i/{\rho }_i$.

For compatible volume conventions, a particle-and-pore model relates the two densities to porosity:This identity requires the same dry mass in both density definitions. An assumed soil-particle density therefore makes the derived porosity an estimate under that assumption, rather than an independent measurement of pore volume.^[ref]