Soil Reaction: Methods, Factors and Influence

After reading this article you will learn about:- 1. Methods of Expressing Soil Reaction 2. Ranges in Soil Reaction 3. Factors 4. Influence on Nutrient Availability and Plant Growth 5. Influence on the Activity of Microorganisms.

Methods of Expressing Soil Reaction:

Since soil reaction is the relationship between the concentration of H⁺ and OH ions in the soil suspension which will determine whether the soil is acidic or alkaline or neutral reaction. Soil reaction can be measured by determining the pH. So pH may be defined as the logarithm of the reciprocal of the H⁺ ion activity and it can be expressed in moles per litre.

pH = log 1/A_H+ or – log A_H+

where A_H+ is the hydrogen ion activity in moles per litre.

A solution with an H⁺ ion activity of 0.001 M will have a pH of 3.0, one with an H⁺ ion activity of 0.0001 M, 4.0 and so on.

In any solution in which water is the solvent, the product of the concentration of these H⁺ and OH^– ions is approximately 10^-14 at 25°C.

H₂O = H⁺ + OH^–

[H⁺] × [OH^–] = 10^-14

If pH is 6.0, the H⁺ ion concentration of OH^– ions will be 10^-14/ 10^-6= 10^-8M per litre. At neutrality, the H⁺ion concentration is 0.0000001 M or 10^-7 M per litre of solution.

pH = log 1/10^-7

= 1/0.0000001

= log 10,000,000

= 7

From pH 7 to 0 the soil is increasingly more acidic, from pH 7 to 14 the soil is increasingly more alkaline (basic). The H⁺ ion concentration has a ten fold change between each whole pH number. Thus a soil of pH 5 has 100 times (10²) more H⁺ ions in solution than a soil solution with a pH of 7.0.

The relationship between H⁺ ion concentration and pH, pH and pOH are presented in Table 13.1 and 13.2

Ranges in Soil Reaction:

For mineral soils the extreme range in soil reaction or pH extends from 3.5 to 10.5. Sometimes in peat soils, soil reaction or pH may be low to 3.0 or less and some alkali soils the reaction or pH may be high as 11.0. The ranges in soil pH for most mineral soils are shown in Fig. 13.1.

Factors Affecting Soil Reaction:

There are various factors which influence the soil reaction viz. nature of soil colloids, percentage base saturation and kind of adsorbed bases, soil solution, climate, soil management and oxidation-reduction state of soil.

1. Nature of Soil Colloids:

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

2. Percentage Base Saturation and Kind of Adsorbed Base:

Soils having low percentage base saturation show acidic reaction. Sodium saturated soils have much higher pH values than those dominated by calcium and magnesium.

3. Soil Solution:

The soil solution comes a number of salts dissolved in water. The cations of the salts inter-mingle or mix together with those of the diffuse double layer of the clay colloidal particles and increase the concentration.

In other words, the concentration of cations in the bulk of the solution is more or less the same as that near the particle surfaces.

For a base unsaturated soil, a large number of hydrogen ions (H⁺) dissociating 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 solution, the higher the pH value. Hence the pH tends to decrease as the soil gets progressively dry.

Soil reaction is also influenced by the presence of CO₂ in soil air liberated from plant roots, micro-organisms and decomposition of organic matter. As the concentration of CO₂ increases, the soil pH falls or decreases.

4. Climate:

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

5. 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 cations are lost from the soil through leaching and crop removal. Besides, the continual use of fertilizers is also responsible for the changes in soil reaction. Acidic fertilizers like ammonium sulphate make the soil acidic, while basic fertilizers like chelian nitrate (NaNO₃) make the soil more alkaline in reaction.

6. Oxidation-Reduction State of Soil:

The ions of certain substance present in the soil change their valency depending upon the oxidising or reducing conditions prevailing in the soil. Iron (Fe) and manganese (Mn) ions change their valency with the change in aeration.

Under aerobic conditions these ions (Fe and Mn) are present in their oxidised state as their higher valence (Fe³⁺ and Mn⁴⁺), whereas under anaerobic or reduced conditions their valencies are reduced to ferrous (Fe²⁺) and manganous (Mn²⁺) state.

This brings about a change in soil reaction and the pH is increased. Under similar reduced conditions, sulphur is usually present as sulphide (S^2-). Under similar reduced conditions, sulphur is usually present as sulphide (S^2-). Upon aeration sulphur is converted to its oxidised sulphate (SO₄^2-) and is available to plant. This also increases the soil acidity and lowers the soil pH.

Influence of Soil Reaction on Nutrient Availability and Plant Growth:

Plants can grow on soils with a wide range of soil reaction of pH as well as diversified types of soil conditions. The diversity in plants growing on soils with a varying range of pH is the best evidence. However, plants do have preferences for a specific range of soil reaction or soil pH. Soil reaction preferences for some crops grown in India are presented in Table 13.3.

Soil reaction and plant growth are inter-related as a result of the significant effect of soil reaction on soil environmental influence arising from increase or decrease in availability of nutrient element or from increasing concentration of certain plant nutrients to the toxic level. e.g., of Al³⁺, Fe²⁺ or Mn²⁺, in acid soils.

The other environmental factor is soil physical condition which is rendered unfavorable under low or high pH.

Soil reaction (pH) is the most important factor which governs availability of various essential as well as functional elements in soil by influencing the various soil properties like, physical, chemical and biological etc. A broad generalization of soil, reaction and availability of nutrient elements in organic soils as well as mineral soils are shown in Figs. 13.2. (a) and 13.2 (b).

From the figure it is found that the primary and secondary nutrients-nitrogen, phosphorus, calcium and magnesium are as available as or more available at a pH of 5.5 and 6.5 for organic and mineral soils than at any other pH. Molybdenum availability is significantly dependent on pH.

In strongly acid soils it is quite unavailable. However, molybdenum, boron and copper availabilities are also relatively high at a range of pH 5.5. to 6.5. The micronutrients like iron, manganese and zinc are less available at pH of 5.5 and 6.5 than at more acidic reactions.

Influence of Soil Reaction on the Activity of Microorganisms:

It is evident that the activity of microorganisms is influenced by the variation in soil reaction or soil pH.

In general, bacteria prefer near neutral to slightly alkaline reaction between pH 6.5 and 8.0, the activity being sharply curtailed when the pH drops below 5.5. Fungi grow in acidic reaction between pH 4.5 and 6.5 and actinomycetes prefer slightly alkaline conditions.

Nitrification, nitrogen fixation, de-nitrification and other beneficial processes, carried out by different microorganisms, are markedly influenced by the soil reaction or fluctuations of soil pH. Therefore, the soil pH regulates the qualitative composition of soil flora. All in all, a soil in the intermediate pH range presents the most satisfactory biological regime.