 Soil bulk density

Soil bulk density is dry soil mass divided by the total sampled volume, including the spaces between particles:Here $M_s$ is the dry-solid mass. The denominator includes pore space, which distinguishes soil bulk density from soil-particle density. Reporting the dry-solid mass also distinguishes this quantity from the mass of wet soil per sampled volume.^[ref]

A constructed core measurement starts with a cylinder containing $100\mathrm{c}{\mathrm{m}}^3$ of soil. If its wet mass is $150\mathrm{g}$ and its dry-solid mass is $130\mathrm{g}$, then soil bulk density is $130/100=1.30\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. Using the wet mass would give $1.50\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, a different quantity. The relevant sampled volume must be retained even if the removed material changes shape during drying.

The basic measurement connects a known-volume core with drying and weighing. The USDA guide describes this cylindrical-core approach and identifies a drying device and a weighing scale as needed equipment.^[ref] Sample disturbance, omitted coarse fragments or inconsistent volume conventions can change what the result represents.

Soil bulk density supplies a useful conversion between amounts per mass and amounts per volume. For example, suppose a measured substance contributes $10\mathrm{mg}$ per kilogram of dry soil. With the stipulated soil bulk density $1300\mathrm{kg}{\mathrm{m}}^{-3}$, its amount per bulk volume isThus identical mass-based measurements need not imply identical amounts in an equal soil volume. Packing and particle composition both affect soil bulk density; it is not a universal numerical constant.^[ref]