pH of Soil and Factors Affecting it

After reading this article you will learn about the pH of soil and factors affecting it.

pH of Soil:

When soils contain more hydrogen ions than hydroxyl ions, they are acidic in reaction. If they contain more hydroxyl ions than hydrogen ions, they are alkaline in reaction. If they contain equal number of hydrogen and hydroxyl ions, they are neutral in reaction. The word “pH” has originated from the French word “pouvoir hydrogen” which means powers of hydrogen. pH is the logarithm to the base ten of the reciprocal of hydrogen ion concentration.

Hence pH of a soil is also defined as the negative logarithm to the base ten of its hydrogen ion concentration. The product of hydrogen ion concentration and hydroxyl ion concentration of a solution is called the ionic product of water which has a constant value of 10^-14gms per litre for all aqueous solution.

Therefore when the hydrogen ion concentration of an aqueous solution increases, its hydroxyl ion concentration decreases in order to maintain the value of the ionic product of water constant i.e. 10^-14gms per litre e.g. if hydrogen ion concentration is 10^-6gms per litre, then hydroxyl ion constriction will be 10⁵gms of water when the ionic product of water will be 10^-6 x 10^-8 = 10¹⁴ gms/litre.

Factors Affecting Soil Reaction, pH:

Amount of hydrogen and aluminum ions present in the soil. When the soil contains more hydrogen ions than hydroxyl ions then it becomes acidic in reaction. When aluminum ions are present in the soil, they react with water to liberate hydrogen ions, which increase the soil acidity

Percentage Base Saturation:

A low percentage base saturation of soils means soil acidity. Basic elements have been leached down from the soils of a humid region, the percentage base saturation decreases much below 80 and they become acidic in reaction. If it is above 80 then it is neutral in reaction. If it is 90, then it is alkaline reaction.

Nature of the Micelle:

More hydrogen ions are absorbed and released by the high cation exchange capacity micelle than by the low cation exchange capacity micelle. For example, if the cation exchange capacities of Montmorillonite and kaolinite are 80meq/100gms and 3meq/100gms respectively, then at 50 per cent base saturation Montmorillonite and kaolinite holds 40meq/100 and 1.5meq/100 gm. of hydrogen respectively.

The Montmorillonite soil solution will contain more hydrogen ions than the kaolinitic soil solution at the same percentage base saturation, since hydrogen ions absorbed by the micelle are in equilibrium with the hydrogen ions in the soil solution. Hence the pH of the Montmorillonite soil will be less than that of the kaolinitic soil.

Kind of Exchangeable Ions:

Sodium ions are held less strongly by the micelle calcium ions and are therefore more easily hydrolyzed than calcium ions as shown below:

Hence more hydroxyl ions are released from the hydrolysis of sodium ions than from the hydrolysis of calcium ions as shown in the Equations (1) and (2) respectively. Hence the pH of sodium saturated soils is higher than that of calcium saturated soils.

Formation of Carbon dioxide under Natural Condition:

Soil microorganisms and plant roots respire to produce carbon dioxide which is also liberated from the decomposition of organic matter in the soil. Carbon dioxide reacts with water to form carbonic acid, which, in turn react with the clay and humic micelle (suppose they contain Ca⁺⁺ Mg⁺⁺, K⁺ Na⁺ and H⁺ ions in the ratios of (54:8:3:5:30) and insoluble primary minerals present in the soil as shown in the following equations:

The soluble bicarbonates thus formed are washed down by rain in the high rainfall areas. So the above reversible reactions proceed in the forward direction. Hence more basic elements are gradually replaced from the clay and humic micelle to form soluble bicarbonates.

Insoluble primary minerals are gradually converted to soluble bicarbonates. These soluble bicarbonates are washed down by high rainfall and the pH of soils gradually decreases. Ultimately soils become acidic in reaction in humid region. Drying of soil slightly increases soil acidity.

On account of the decrease in the thickness of the diffuse layer of cations around the clay micelle caused by drying as explained below.

The clay micelle possesses a diffuse layer of cation around its negatively charged surface. The concentration of cations including hydrogen ions is greatest just near its negatively charged surface and gradually decreases towards the outer side of the diffuse cationic layer.

So more hydrogen ion occur in the inner portion of the diffuse layer of cation than in its outer portion. The soil pH which is commonly measured indicates the quantity of hydrogen ions occurring at the outer portion of the diffuse layer of cation (including hydrogen ions).

The increase in the soil moisture content increases the thickness of the diffuse layer when less hydrogen ion would be present in the outer portion of the diffuse layer than in its inner portion. So little higher pH would be indicated by the pH measuring device.

When the soil dries up, the thickness of the diffuse layer decreases. The concentration of cations including hydrogen would increase at its outer portion. So the pH measuring device would indicate a little less pH than when the soil was moist. Ammonium ions are oxidized to form nitric acid in the soil.

Nitric acid reacts with calcium carbonate and clay and humic micelle as shown below:

The soluble calcium nitrates and calcium bicarbonates thus formed are leached down by high rainfall resulting in the increase in soil acidity. Hence continuous application of ammoniacal fertilizer increases soil acidity. Continu­ous application of urea also increases soil acidity because urea undergoes transformation in the soil to form nitric acid in the soil.

As shown below:

Soil aeration status also affects the soil reaction. In poorly aerated soil, electrons are available from the microbial oxidation of organic compounds. These electrons are utilized in the process of reduction of ferric iron to ferrous iron.

When H⁺ ions are consumed as shown below:

Summing up:

Hydrogen H⁺ ions are also consumed when sulphate id reduced to sulphide is poorly aerated soil and also when carbon dioxide is reduced to glucose in poorly aerated soil as shown below:

So poorly aerated soil are lower pH then well aerated soil.