Since in pure water, the number of H ions is equal to the number of OH ions, the concentration of each ion type must be 1 x 10-7 g ions per litre.
If the concentration of H ions is more than 10-7, the solution is acidic; if less solution is alkaline.
This method of expressing the H ions concentration is very inconvenient and therefore Sorenson (1969) suggested that H ions concentration be generally expressed as the numerical value of the negative power to which 10 must be raised in order to express the required concentration and this value be designated by the symbol pH.
Thus, technically pH is the negative logarithm of the H ion concentration or the logarithm to the base ten of the reciprocal of hydrogen ion concentration
i.e. H+ = 10-pH
log [H+] = -pH log10
- pH = log [H+]/ log10
pH = log [H+] / log 10
Since log 10 =1, therefore pH = -log [H+] (OR) pH = log 1/ [H+]
Soil reaction:
Soil reaction is one of the most important physiological characteristics of the soil solution.
The presence and development of micro - organisms and higher plants depend upon the chemical environment of soil. Therefore study of soil reaction is important in soil science.
The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration of H ions (acidity) or 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 ions. According to the theory of dissociation, the activity is due to the dissociation or ionization of compounds into ions.
Factors affecting Soil Reaction (pH)
Soil reaction varies due to following factors
1. Nature of soil colloids:
The colloidal particles of the soil influence soil reaction to a very greatest extent. When hydrogen (H+) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.
2. Soil solution:
The soil solution carries a number of salts dissolved in capillary water. 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 and increases the availability of the nutrients. Under field conditions, plant roots and micro-organism liberate enough CO2, which results in lowering the pH appreciably.
This principle of increasing the concentration of CO2 in soil air is also used in the reclamation of alkali soils.
3. Climate:
Rainfall plays important role in determining the reaction of soil. In general, soils formed in regions of high rainfall are acidic (low pH value), while those formed in regions of low rainfall are alkaline (high pH value).
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. This leads to change the soil reaction to the acid side.
5. Parent materials:
Soils developed from parent material of basic rocks generally have higher pH than those formed from acid rocks (e.g. granite).
The influence of parent material is not very important as it is completely masked by the climatic conditions under which the soil is developed.
6. Precipitation:
o As water from rainfall passes through the soil, basic nutrients such as calcium (Ca) and magnesium (Mg) are leached.
o They are replaced by acidic elements including Al, H and manganese (Mn). Therefore, soils formed under high rainfall conditions are more acid than those formed under arid conditions.
7. Decomposition of organic matter:
· Soil organic matter is continuously being decomposed by micro-organisms into organic acids, carbon dioxide (CO2) and water, forming carbonic acid.
· Carbonic acid, in turn, reacts with the Ca and Mg carbonates in the soil to form more soluble bicarbonates, which are leached away, leaving the soil more acid.
8. Native vegetation:
o Soils often become more acid when crops are harvested because of removal of bases.
o Type of crop determines the relative amounts of removal. For example, legumes generally contain higher levels of bases than do grasses.
o Calcium and Mg contents also vary according to the portion (s) of the plant harvested. Many legumes release H ions into their rhizosphere when actively fixing atmospheric N2.
o The acidity generated can vary from 0.2 to 0.7 pH units per mole of fixed N.
9. Soil depth:
· Except in low rainfall areas, acidity generally increases with depth, so the loss of topsoil by erosion can lead to a more acid pH in the plough layer.
· The reason is that more subsoil is included in the plow layer as topsoil is lost. There are areas, however, where subsoil pH is higher than that of the topsoil.
10. Nitrogen fertilization:
· Nitrogen from fertilizer, organic matter, manure and legume N fixation produces acidity.
· Nitrogen fertilization speeds up the rate at which acidity develops. At lower N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.
11. Flooding:
· The overall effect of submergence is an increase of pH in acid soils and a decrease in basic soils.
· Regardless of their original pH values, most soils reach pH of 6.5 to 7.2 within one month after flooding and remain at the level until dried.
· Consequently, liming is of little value in flooded rice production. Further, it can induce deficiencies of micronutrients such as zinc (Zn).
Buffering capacity
· Buffering refers to resistance to a change in pH.
· If 1 ml HCI (of 0.1 N) is added to one liter of pure distilled water of pH 7.0, the resulting solution would have a pH of about 5.0.
· If on the other hand, the same amount of acid is added to a liter of soil suspension the resulting change in pH would be very small.
· There is a distinct resistance to change in pH. This power to resist a change in pH is called buffer action.
· A buffer solution is one which contains reserve acidity and alkalinity and does not change pH with small additions of acids or alkalis. Buffer capacity:
· The colloidal complex acts as a powerful buffer in the soil and does not allow rapid and sudden changes in soil reaction.
· Buffering depends upon the amount of colloidal material present in soil. Clay soils rich in organic matter are more highly buffered than sandy soils.
· Buffer capacity of the soil varies with its cation exchange capacity (C.E.C.). The greater the C.E.C. the greater will be its buffer capacity.
· Thus, heavier the texture and the greater the organic matter content of a soil, the greater is the amount of acid or alkaline material required to change its pH.
Importance of buffering in agriculture
· Changes in soil reaction (pH) have a direct influence on the plants and it also affects the availabilities of plant nutrients.
· Deficiency of certain plant nutrients and excess availability of others in toxic amounts would seriously upset the nutritional balance in the soil.
· Buffering prevents sudden changes and fluctuation in soil pH. So it regulates the availability of nutrients and also checks direct toxic effect to plants.
Table 30. Interpretation of Soil pH
S. No pH Effect pH Range
1. Acid 0.0-6.0
2. Normal 6.1-8.5
3. Moderately alkaline / tending to alkaline 8.6-9.0
4. Alkaline >9.0
