6 Main Factors that Control Soil Reactions

This article throws light upon the six main factors that control soil reactions. The factors are: 1. Nature of Soil Colloids 2. Soil Solution 3. Climate 4. Soil Management 5. Microbial Action 6. Formation of Sulphuric and Nitric Acids in the Atmosphere.

Factor # 1. Nature of Soil Colloids:

The colloidal particles of the soil influence soil reaction to a very great extent. When hydrogen (H⁺) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.

Factor # 2. Soil Solution:

The soil solution carries a number of salts dissolved in capillary water. The cations of the salts intermingle with those of the diffuse double layer of the clay particle and increase the concentration. In other words, the concentration of cations in the bulk of the solution is more or less (or nearly) the same as that near the particle surfaces.

For a base unsaturated soil (clay), the number of hydrogen ions dissociate into the solution. This increases the acidity of the soil solution or lowers its pH.

Under field conditions, the concentration of salts varies with the moisture content of the soil. The more dilute the solution, the higher the pH value. Hence, the pH tends to drop as the soil gets progressively dry. Soil reaction is also influenced by the presence of CO₂ in soil air.

As the CO₂ concentration increases, the soil pH falls. This principle of increasing the concentration of CO₂ in soil air is also used in the reclamation of alkali soils. Under field conditions, plant roots and microorganism liberate enough CO₂.

Factor # 3. Climate:

Rainfall plays an important role in determining the reaction of its soil. In general, soil formed in region of high rainfall are acidic (low pH value), while those formed in regions of low rainfall are alkaline (high pH value).

Factor # 4. Soil Management:

Cultural operations in general tend to increase soil acidity. They make an acid soil more acidic, and an alkaline soil less alkaline. As a result of constant cultivation, basic elements are lost from the soil through leaching and crop removal.

Another factor that controls soil reaction is the state of oxidation or reduction. The ions of certain substance present in the soil change their valency depending upon the oxidising or reducing conditions prevailing in the soil. Iron and manganese ions change their valency with the change in aeration.

Under aerobic conditions these ions are present in the oxidised state having a higher valency as ferric (Fe⁺⁺⁺) and maganic (Mn ⁺⁺⁺) ions. These ferric and maganic ions forms are available to plant. Under conditions of reduced aeration as in water-logged soils, their valencies are reduced to ferrous (Fe⁺⁺) and maganous (Mn⁺⁺) states. This brings about a change in pH also be reducing the valency the pH is increased, and by increasing it the pH is lowered. Thus, the pH of a water-logged soil will be high and the soil will be more alkaline than when it is well-aerated.

Under similar circumstances, a change in the degree of oxidation of anions, such as sulphide and sulphite to sulphate. Sulphur is usually present as sulphide (S-) under reduced conditions in poorly-aerated soils. Upon aeration, it is converted to fully oxidised sulphate (SO₄-) which is available form to plant. This also increases the soil acidity and lowers its pH.

The continual use of fertilizers is responsible for a marked change in soil pH. Acid fertilizers like ammonium sulphate make the soil acidic, while basic fertilizers like sodium nitrate make it alkaline. Leaching also encourages acidity because it removes the base-forming cations that might compete with hydrogen and aluminium on the exchange complex.

Factor # 5. Microbial Action:

Decomposition of organic matter produces carbonic acid (H₂CO₃). The solvent action of carbonic acid on the mineral constituents of the soil is responsible for the removal of large quantities of base-forming cations (e.g.,Ca⁺⁺and Mg⁺⁺) by dissolution and leaching.

Sulphuric and nitric acids are formed by the organic decay processes and by microbial action on inorganic sulphur and nitrogen-containing materials (ammonium nitrate and ammonium sulphate). These acids encourage acidic conditions in the soil.

Factor # 6. Formation of Sulphuric and Nitric Acids in the Atmosphere:

Industrial complexes emit sulphuric and nitric acids from the combustion of coal, gasoline etc. The precipitation of these substances is called “acid rain”. The pH value of acid rain may range from 2.0 to 4.5. The hydrogen ions added to the soil are not sufficient to change the pH immediately but over a long period of time the continuous addition of hydrogen ions may have significant acidifying effects.