= What is Soil? by USDA Natural Resources Conservation Service
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https://www.nrcs.usda.gov/resources/education-and-teaching-materials/what-is-soil
Directly inspected primary agency page. Locators: definition reproduced from Soil Taxonomy, second edition; solids/liquid/gases, horizons and rooted-plant criterion; upper/lower soil boundaries. The page separately attributes an earlier definition to Soil Science Society of America. Original pore-volume example in article is not measured agency data.
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