 Soil (source code)

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= Soil

<Soil> is a natural body at the land surface composed of solid <minerals> and <organic matter>, together with liquids and gases in the spaces between them. Its development reflects interactions among parent material, climate and living <organisms>. <Soil horizons> and the capacity to support rooted <plants> are important in its description; a loose mineral powder alone does not capture the whole concept.<What is Soil? by USDA Natural Resources Conservation Service>[Soil Taxonomy definition; formation and boundary paragraphs]{ref}

An original volume example shows why the pore space matters. In $100\,\mathrm{cm^3}$ of <soil>, suppose solids occupy $60\,\mathrm{cm^3}$, <water> occupies $25\,\mathrm{cm^3}$ and air occupies $15\,\mathrm{cm^3}$. The pore volume is $40\,\mathrm{cm^3}$, giving <porosity> $40/100=0.40$. The <volumetric soil water content> is $25/100=0.25$, while the <water-filled pore space> fraction is $25/40=0.625$. These three fractions use different denominators.

At unchanged pore volume, adding <water> replaces some of the air rather than increasing the solid fraction. Consequently, a description by dry solid composition alone can miss a major change in conditions experienced by <root (botany)>[roots] and <microorganisms>. The example assumes fixed total volume and explicitly specified phase volumes; swelling, compaction or incomplete sampling can require a different model.

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