<P> Soil pH is a measure of the acidity or basicity of a soil . pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H + or, more precisely, H 3O + aq) in a solution . In soils, it is measured in a slurry of soil mixed with water (or a salt solution), 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 . </P> <P> 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 . </P> <P> The United States Department of Agriculture Natural Resources Conservation Service classifies soil pH ranges as follows: </P>

Why do farmers need to know the ph level of their soil