Soil pH Soil pH Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions ( or, more precisely, ) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution, such as 0.01 M ), 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)
Acid dissociation constant the added complication that the neutral molecule is subject to an internal acid–base equilibrium in which the basic amino group attracts and binds the proton from the acidic carboxyl group, forming a zwitterion. At pH less than about 5 both the carboxylate group and the amino group are protonated. As pH increases the acid dissociates according to At high pH a second dissociation may take place. Thus the zwitterion, , is amphoteric because it may either be protonated or deprotonated. The equilibrium constant "K" for a base is usually defined as the "association" constant for protonation of the base, B,