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Concept
Soil pH
Summary
Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+ or, more precisely, H3O+aq) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution, such as 0.01M CaCl2), and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils (pH < 3.5) and very strongly alkaline soils (pH > 9) are rare.
Soil pH is considered a master variable in soils as it affects many chemical processes. It specifically affects plant nutrient availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. The optimum pH range for most plants is between 5.5 and 7.5; however, many plants have adapted to thrive at pH values outside this range.
The United States Department of Agriculture Natural Resources Conservation Service classifies soil pH ranges as follows:
0 to 6=acidic,7=neutral and 8 and above alkalinity
Methods of determining pH include:
Observation of soil profile: certain profile characteristics can be indicators of either acid, saline, or sodic conditions. Examples are:
Poor incorporation of the organic surface layer with the underlying mineral layer – this can indicate strongly acidic soils;
The classic podzol horizon sequence, since podzols are strongly acidic: in these soils, a pale eluvial (E) horizon lies under the organic surface layer and overlies a dark B horizon;
Presence of a caliche layer indicates the presence of calcium carbonates, which are present in alkaline conditions;
Columnar structure can be an indicator of sodic condition.
Observation of predominant flora. Calcifuge plants (those that prefer an acidic soil) include Erica, Rhododendron and nearly all other Ericaceae species, many birch (Betula), foxglove (Digitalis), gorse (Ulex spp.
Official source
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