Soil reaction (pH) seems to be the single soil chemical condition that most strongly affects the environment of farm crops and soil micro-organisms. It is useful in diagnosing the fertility of the soils.

This fact must therefore be taken into consideration in the management of any farming enterprise, especially in tropical acid soils. Acknowledging the conditions which cause different soil reactions is of tremendous value to the student of soil science.

The large presence of aluminum and low activity clays in tropical soils enhance soil acidity, especially in areas that receive high rainfall.

Soil amelioration with lime improves the productivity of tropical acid soils by replenishing calcium and magnesium cations, improving plant root environment, improving soil conditions for microbial activities, and increasing the availability of the most essential nutrient elements.

Meaning of Soil Reaction

Soil reaction (pH) is an indication of the acidity or basicity of the soil and is measured in pH units.

The acid could be defined as any substance capable of yielding hydrogen ions (H⁺), which is a proton when dissolved in water.

Pure water dissociates to a small extent so that the quantity of water molecules (H₂O) present usually far outnumbers either the hydrogen (H⁺) or hydroxyl (OH^–) ion present.

In pure water, the concentrations of hydrogen and hydroxyl ions are always equal and the solution is said to be neutral. Soil reaction is also the degree of acidity or alkalinity in the soil.

Soil acidity is usually measured by determining the pH values which, according to Sorenson (1909), is defined as the negative logarithm of the hydrogen ion concentration.

In acid solution, the concentration of hydrogen ions is always greater than 10^-7 gm ions per liter and the pH is always lower than 7.

Conversely, in alkaline solutions, the pH values are always greater than 7. Thus the lower the pH the more acidic is the solution and the higher the pH the more alkaline the solution.

Active Acidity

An acid when mixed with water dissociates or ionizes to give hydrogen ions and accompanying anions.

The active acidity is the sum total of all the hydrogen ions in the acid solution or in the soil solution. In the above equation, the active acidity is represented by the H⁺ to the right; its strength dictates the strength of the acid solution.

The HA on the left side of the equation is the potential acidity otherwise referred to as reserved acidity. In soil solution reserved acidity is what is in the exchange site complex that is not yet in solution. It is the potential or reserve of H⁺ in soil and it is always in dynamic equilibrium with active acidity.

Although active acidity is only a small fraction of the total acidity, it is important because it is the one that actually affects the growing plant.

The total acidity of the soil is the sum of the concentrations of active acidity which is represented by the H⁺ ion concentration in the soil solution and the potential/reserved/exchange/latent acidity which is due to the hydrogen and aluminum ions adsorbed on the soil colloids.

The term actual acidity is also used for active acidity (Yagodin, 1984) and is defined as the acidity due to carbonic acid (H2 CO3); water-soluble organic acids, and hydrolytically acid salts which bear directly on the development of plants and micro-organisms.

Potential acidity is very important in determining the strength of active acidity since portions of adsorbed H⁺ and A1³⁺ can be released into the active pool as the acidity of the soil solution decreases.

That is, for as much as potential acidity exists there would always be active acidity. This stresses the practical significance of the relative amounts of both species of soil acidity in ameliorating by liming.

Yagodin (1984) suggested the concept of hydrolytic acidity which arises when soil is treated with a normal solution of a hydrolytically alkaline salt such as sodium acetate.

The hydrolytic acidity involves the less mobile portion of adsorbed hydrogen ions that are less readily exchangeable with the soil solution cations.

Buffer Systems

These are compounds that maintain the pH of a solution within a narrow range when small amounts of acid or base are added (Tisdale and Nelson, 1975). The term buffering is defined as the resistance to a change in pH. Acetic acid and sodium acetate form a common example of a buffer system.

The addition of an acid such as hydrochloric acid or a base such as sodium hydroxide to the system results in very little change in the active acidity of the solution.

The soil buffering ability is attributable to its humus and aluminum silicate clays which have the ability to retain hydrogen aluminum and other cations on their exchange sites with the net result that there is little change in the active acidity of the soil solution.

Acidity Ranges of Soils

When a soil sample is mixed with water the resultant solution is either acid (most common), alkaline (sometimes), or exactly neutral (Very rare). Application of lime in view of the difficulties of defining it on the basis of the relationship between pH, exchangeable hydrogen, and aluminum.

Soil with pH 5.0 in 0.OIM CaCl₂ (1:2.5) or about 5.5 in water (1:1) are classified as acid soils.

Evidence abounds in the literature (Agbede, 1984) to show that soil pH is the most important factor influencing crop performance in that it (pH) influences the rate of organic matter decomposition, microbial activities, forms, and extent of nutrient availability or even nutrient uptake by the crop.

Sources of Soil Acidity

Soil acidity originates from several sources among which are humus or organic matter, aluminum silicate clays, hydrous oxides of iron and aluminum, soluble salts, and carbon dioxide (Tisdale and Nelson, 1975).

Organic Matter

In organic matter or humus, the carboxylic, phenolic, amino, and sulphydryl functional groups can undergo ionization to release their hydrogen ions, H+, into the soil solution thus increasing soil acidity. The functional groups constitute a very significant source of reserved acidity in the soil.

In soils containing large amounts of organic matter or humus, e.g. peat, covalent bonded H which is pH dependent dissociates at a high pH value depending on the dissociation constant of the acid formed and leaves a net negative charge on the humus colloids as explained earlier.

Alumino Silicate Clay Minerals

The 1:1 or 2:1 clay minerals typified by Kaolinite and montmorillonite usually carry negative charges on them as a result of isomorphous substitution in their crystal lattice.

Additional charges on the clays also arise from the dissociation of hydrogen ions from hydroxyl groups or from bound water of constitution, both of which are structural components of the crystal lattice.

At the negatively charged sites of these clays, hydrogen ions, H+, come in to neutralize the charges so that we have conditions of neutrality; otherwise, it will be impossible to walk on the soil without experiencing an electric shock of some sort.

Any of the hydrogen ions, H+, on the edges of the layer silicate clay minerals that could be released into the soil solution under certain conditions.

Exchangeable Aluminum

Present in the inorganic component of the soil are bonded A1³⁺ which could be displaced from the clay minerals by other cations such as Fe³⁺ Mg²⁺, or Ca²⁺.

Another source of H⁺ is the hydrated oxides of aluminum and iron commonly called hydrous oxides which may be brought into solution as pH is lowered and release H⁺ ions by hydrolysis.