= Volumetric soil water content
{title2=$\theta=V_w/V$}
<Volumetric soil water content> is water <volume> divided by the total <volume> of a <soil> sample:
$$
\theta=\frac{V_w}{V}.
$$
The result is dimensionless and can be written as cubic metres of water per cubic metre of soil. The total sample contains solids and pores, with its fluid phases occupying pore space.<What is Soil? by USDA Natural Resources Conservation Service>[Soil Taxonomy definition, solid, liquid and gaseous phases]{ref}<Effect of Water-Filled Pore Space on Carbon Dioxide and Nitrous Oxide Production in Tilled and Nontilled Soils by D. M. Linn and J. W. Doran>[Abstract, water content and total porosity]{ref}
A constructed measurement example starts with a $100\,\mathrm{cm^3}$ core whose water contributes $25\,\mathrm g$ of mass. With stipulated water <density> $1.00\,\mathrm{g\,cm^{-3}}$, that is $25\,\mathrm{cm^3}$ water and $\theta=0.25$. The conversion from mass to volume needs the chosen water density and an appropriate determination of water mass; total wet mass alone would include the solids.
The same water volume can have different normalizations. If this sample has $50\,\mathrm{cm^3}$ pore volume, its <water-filled pore space> is $25/50=0.50$, despite its volumetric water content being $0.25$. With $150\,\mathrm g$ dry solids, its water-to-dry-solid mass ratio would instead be $25/150=1/6$. The three numbers describe the same stipulated material using three different denominators.
Within a rigid-volume model where water occupies pores and no separate surface layer is included, $0\leq\theta\leq\varepsilon$, where $\varepsilon$ is <porosity>. Filling all pore volume gives $\theta=\varepsilon$, rather than necessarily giving $\theta=1$.
Reporting <volumetric soil water content> therefore requires the relevant sample volume as well as the water amount. Changes in swelling, shrinkage or sample boundaries can affect the ratio. The definition supplies a clear accounting quantity, while interpreting microbial or plant responses requires additional information about the soil and the experimental conditions.
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