SOIL AND PLANT ANALYSIS (1+1)

pH

The most important characteristic of the soil solution is its reaction. The soil reaction describes the degree of acidity or alkalinity. The acidity or alkalinity is expressed in terms of concentration of H ions or OH ions. pH values are used to express the concentration of H or OH ion. Sorenson (1909) devised the present pH scale.

Water is a neutral substance with equal concentration of H and OH ions. The ionization constant of water is 10-14. That is at neutral, the concentration of products of (H+) and (OH-) is 10-14 g/l. The pH scale ranges from 0-14.
Conc.H+ x Conc OH- = Kw = 10-14
At neutral the concentration of H+ = 10-7 or = 0.0000001 g/l.
The reciprocal of this is 1/0.0000001 = log 10 10000000 the log of which is 7.
Thus the pH can be defined as the logarithm of reciprocal of H+ ion concentration or the negative logarithm of H ion concentration (hydrogen ion activity). It is also called as soil reaction.
If the concentration of H ion in a solution is 0.000001 g/l (10-6) the pH is 6.0. Then at pH 6 the concentration of H ion is 10 times more as compared to that at pH 7. Since the product of concentration of H+ and OH- ions is always equal to 10-14 we can know the concentration of OH- ions if we know the pH value. A pH of 6.0 would indicate the concentration of OH- ions as 10-8. Since the soil acidity is mainly due to H+ ions a pH 7.0 indicates neutrality as the concentration of H+ and OH- are equal. Above pH 7.0 the concentration of OH- is more and it indicates alkalinity.