Notes on Buffering of Soil

This article provides notes on the buffering of soil.

Buffering refers to resistance to a change in pH. If 1 ml of 0.01 N HCl is added to 1 litre of pure distilled water of Neutral pH; the resulting solution would have a pH of about 5.0. Now, on the other hand if the small amount of acid is added to a soil suspension the resulting change in pH would be very small.

The soil suspension is buffered against a change in pH. The power to resist a change in pH is called “buffer action”. Buffer solution contains reserve acidity and alkalinity and does not change pH with small additions of acids or alkalis.

Soils also exhibit buffering action. In order to produce a certain change of pH, it is usually necessary to add much more acid or alkali than the amount of H⁺ and OH^– ions present in the soil suspension. This buffering action is due to the influence of weak acids and their salts. Soil behaves like a buffered weak acid and that will resist sharp changes in pH.

This buffering mechanism may be explained as follows:

Al³⁺ + H₂ODAIOH²⁺+H⁺

If these H⁺ ions are then neutralized by the addition of small amounts of a base and the aluminium ions in solution are precipitated as Al(OH)₃, the equilibrium of the system, hence the pH, will tend to be maintained by the movement of adsorbed aluminium ions to the soil solution. These aluminium ions in turn hydrolyze, producing H⁺ ions, and the pH tends to remain as it was before the addition of the base. As more base is added, then active H⁺ ions are neutralized.

In order to bring equilibrium, some of the H⁺ ions held by the clay (potential acidity) will dissociate and cause slight increase in the concentration of H⁺ ions in the active acidity and thereby resulting slight changes in pH.

As the process is continued, less and less of the hydrogen tends to ionize during neutralization process and thereby resulting a gradual decrease in the active acidity (acidity develops due to concentration of H⁺ ions in soil-solution) with a concomitant increase in soil pH.

On the other hand, if H⁺ ions are added to the system some of the additional H⁺ ions will be adsorbed by the clay with the release of other ions. As a result, there will be a slight increase in the active acidity and a corresponding decrease in soil pH.

So because of the tendency of the soil colloid either to adsorb H⁺ and Al³⁺ ions from soil solution or to give up those to the soil solution to maintain the equilibrium, the soil tends to resist sudden changes in pH and this action is said to be the buffering action of the soil.

Buffering Capacity is the ability of the soil to re-supply an ion to the soil solution.

Buffering capacity depends on:

i. Clay content and type

ii. CEC

iii. Organic matter

There are various factors which affect the buffering capacity of soils, the most important of which are amount and kind of clay, organic matter content, cation-exchange capacity, carbonate, bicarbonate, phosphate content and organic acid etc.

Soil containing large amounts of clay and organic matter are said to be highly buffered and require large amounts of lime for affecting a certain change in pH than an acid soil containing smaller amounts of clay and organic matter.

Keeping all other factors constant, the buffering capacity of a soil increased with increasing cation exchange capacity of a soil (CEC).

Soils containing large amounts of calcium carbonate, organic acids and phosphates show higher buffering capacity.

Soils containing large amounts of 1: 1 type clays are generally less strongly buffered than soils in which the predominant clay minerals are of the 2: 1 type.

The importance of buffering of soils is mainly two folds:

(i) The stabilization of soil pH, and

(ii) The amounts of amendments necessary to affect a certain change in soil reaction or soil pH.

An abrupt change in pH causes a radical modification in soil environment and if it fluctuates too widely, the plants and microorganisms would suffer seriously.

Changes in soil reaction not only have a direct influence on the plants but also exert an indirect influence on soil environment by creating sudden changes in the availability of nutrients. Deficiencies of certain plant nutrients and excess of others in toxic amounts would seriously upset the nutritional balance in the soil.

The amounts of amendments necessary to affect a given alteration in soil reaction also relate to the effectiveness of pH stabilization. The greater the buffering capacity of the soil, the larger must be the amounts of lime or sulphur to affect a given change in pH.

Problems:

(i) What is the pH of a solution having A_H+

= 4.2 × 10^-6 g ion litre^-1?

pH =-logA_H⁺ = – log (4.2 × 10^-6)

= -0.62-(-6)

= – 0.62 + 6

= 5.32

(ii) What is the Ah⁺ of a solution having

pH = 4.38?

A_H+= 10^-pH= 10^-4.38

= 10^0.62× 10^-5 = 4.2 × 10^-5 g ion litre^-1