To measure the acidity of a substance, scientists use the pH test. The abbreviation pH stands for parts Hydrogen, as the ability to release Hydrogen to a chemical reaction is one characteristic of an acid. The pH scale runs logarithmically from 0 to 14 - where 0 is a highly acidic value, 14 is highly alkaline, and 7 is perfectly neutral.
Generally, soils have a pH in the slightly acidic range (the upper 6's), with only soils that have a high lime content running into the alkaline end of the scale. Most plants require a soil pH between 6.6 and 7.0, with a few exceptions preferring more extreme conditions. Wet soils tend to be more acidic than dry soils, as chemicals which would otherwise neutralize the pH level get washed away before they have a chance to react.
IGBP-DIS (1998) SoilData(V.0) A program for creating global soil-property databases, IGBP Global Soils Data Task, France.
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