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The space occupied by bulk soil can be categorized as volumes of solids (mineral and organic matter), liquids (water and solutes), and gases (soil air and water vapor). The sizes of particles composing the solid phase are classed by the USDA as gravel (< 2mm), sand (2mm - 50 micron), silt (50 - 2 micron), and clay (< 2 micron). The particle size distribution of a soil is termed its texture. |
Shown above is the schematic volumetric composition of a medium-textured soil at a condition considered optimum for plant growth. Note that the solid matter constitutes 50% and the pore space 50% of the soil volume, with the latter divided equally between air and water. The arrow indicates that the proportions of air and water can vary interdependently, with an increase in one being associated with a decrease in the other.
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