eChapter Name: Concept of Soil pH and Nutrient Availability
9788119103256
eBook Name: SOIL PROFILING AND MANAGEMENT
by B. Jena, R.K. Nayak, Shradha Mohanty, P. K. Samant, D. Sethy
Soil pH
Soil reaction is measured by pH (Puissance de Hydrogen) of a suspension of
soil in water. The word “pH” has originated from the French word “pouvoir
hydrogen” which means powers of hydrogen. The acidity, neutrality or
alkalinity of a soil is measured in terms of hydrogen ion activity (active
concentration) of the soil water system. Most of the agricultural soils have
a pH varying from 4.5 to 8.5. The active concentration of hydrogen ion is
termed as ‘active acidity’ which is 1/50,000 or 1/1,00,000 times lesser than the
exchange acidity or exchangeable hydrogen.
Soil pH is a measure of the activity of ionised H (H+) in the soil solution. The
pH of a solution can be defined as the negative logarithm to base 10 of the H+
ion activity or the logarithm of the reciprocal of the H+ ion activity.
pH = -log10
a[H
+] = log10
1/[H
+]
There is difference between activity and concentration. Activity and
concentration are related by the mathematical expression
a
c
F
= where a = activity of a species (moles L-1)
c = concentration of that species (moles L-1), f = activity coefficient
a = c for dilute solutions, f = 1
Activity coefficient of an ionic species is the degrees of freedom of that species
to participate in an equilibrium reaction or to maintain its charge under an
ionic environment.