2.7 Terminology

As you continue to study soil physical properties and processes, you will need to know and use the following key terms related to soil structure and texture:

• Bulk density (ρ_b) is the mass of oven-dry soil solids per unit volume of soil. In this context, soil is oven-dry when it has been dried at 105°C to a constant weight. Bulk density has dimensions of mass over volume and units of Mg m^-3, kg m^-3, or g cm^-3. For mineral soils it ranges from approximately 0.80 to 2.0 g cm^-3. Organic soils may have lower values. Bulk density is distinct from wet bulk density which is sometimes used in engineering contexts.
• Particle density (ρ_s) is the mass of oven-dry soil particles per unit volume of soil particles. The volume of soil particles is smaller than the bulk soil volume, which includes pore spaces. Therefore, particle density is greater than bulk density. Particle density has dimensions of mass over volume and units of Mg m^-3, kg m^-3, or g cm^-3. For mineral soils a value of 2.65 g cm^-3 is often assumed if the true particle density is unknown.
• Porosity (ε) is the volume of pore spaces per unit volume of soil. Porosity has dimensions of volume over volume and may be written as a unitless decimal, as a percentage, or as a decimal with units of m³ m^-3 or cm³ cm^-3. We will use the latter in this book. If bulk density is known, then porosity can be estimated using the following equation. To better understand the equation, watch this brief video.

 (Eq. 2-3)