= Soil bulk density
{title2=$\rho_b=M_s/V$}
<Soil bulk density> is dry <soil> mass divided by the total sampled <volume>, including the spaces between particles:
$$
\rho_b=\frac{M_s}{V}.
$$
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.<Soil Quality Indicators: Bulk Density by USDA Natural Resources Conservation Service>[Page 1, definition and Relationship to Soil Function]{ref}
A constructed core measurement starts with a cylinder containing $100\,\mathrm{cm^3}$ of <soil>. If its wet mass is $150\,\mathrm g$ and its dry-solid mass is $130\,\mathrm g$, then <soil bulk density> is $130/100=1.30\,\mathrm{g\,cm^{-3}}$. Using the wet mass would give $1.50\,\mathrm{g\,cm^{-3}}$, 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.<Soil Quality Indicators: Bulk Density by USDA Natural Resources Conservation Service>[Page 2, Measuring Bulk Density]{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> $1300\,\mathrm{kg\,m^{-3}}$, its amount per bulk volume is
$$
10\,\frac{\mathrm{mg}}{\mathrm{kg}}\times1300\,\frac{\mathrm{kg}}{\mathrm{m^3}}=13\,\mathrm{g\,m^{-3}}.
$$
Thus 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.<Soil Quality Indicators: Bulk Density by USDA Natural Resources Conservation Service>[Page 1, Factors Affecting and mass-to-volume conversion]{ref}
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