Study Notes on Soil pH

Soil reaction is measured by pH (Puissance de Hydrogen) of a suspension of soil in water. The concept of soil pH may be explained with reference to pure water, which is amphoteric, and in which hydrogen and hydroxyl ions are in equilibrium with un-dissociated water molecules. The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration (or activity) of hydrogen (H⁺) or hydroxyl (OH ) ions present in it.

Acidity is due to the excess of H ions over OH ions, and alkalinity is due to the excess of OH ions over H ions. A neutral reaction is produced by an equal activity of H and OH ion (Fig. 7.1). According to the theory of dissociation, the activity is due to the dissociation or ionisation of compounds into ions. The greater the degree of ionisation, the greater is the activity of the ions.

Even pure water which is neutral in reaction dissociates into H and OH ions:

as it shows a very slight but definite conductivity.

The most convenient method of expressing the relationship between H⁺ and OH- is pH.

pH is defined as the logarithm of the reciprocal (or negative logarithm) of the hydrogen-ion concentration in gram per litre; represented in equation form as follows:

At neutrality, the hydrogen-ion concentration is: 0.0000001 or 1×10^-7 gram of hydrogen per litre of solution. Substituting this concentration into the formula,

At a soil pH of 6, there is 0.0000001 gm of active hydrogen, or 10 times more the concentration of H⁺ than at a pH of 7. At each smaller pH units, the H⁺ increases by 10 in concentration. It therefore follows that a pH of 6 is 10 times more acidic than a pH of 7; a pH of 5 is 10 times more acid than a pH of 6, and so on.

The soil pH value, therefore, represents the amount of free or active acidity and not the total quantity of potential (or combined) acidity. In other words, it represents the intensity of acidity of a solution. In this scale, the pH value ranges from 0-14, where pH 0 represents the highest limit of active acidity, and pH 14 the highest degree of basicity (or alkalinity). Neutrality represents pH 7 (Fig. 7.2). Therefore, pH 7 shows a neutral reaction. When the pH is less than 7, the solution is acidic; when it is above 7, it is alkaline.