After reading this article you will learn about the sources of hydrogen ions for acid soil.
Soil acidity is caused by ionisable hydrogen ions. Exchangeable hydrogen ions (which along with other such cations) are present in the soil to neutralize the negative charge arising from isomorphous substitution. They are displaceable by any cation which is added in fairly high concentration.
Moderately Acid Soils:
The exchangeable hydrogen ions are main sources of hydrogen ions in moderately acid soil (pH value between 5.0 and 6.5). Moderately acid soils have higher percentage of base saturation (Ca++, Mg++ etc.) than the strongly acid soils, the aluminium can no longer exist as ions but is converted to aluminium hydroxy ions (as shown in reaction).
In the soil solution, aluminium hydroxy ions produce hydrogen ions by the hydrolysis reaction.
Strongly acid soils:
Aluminium becomes soluble at very acid soil conditions (pH less than 5.0). Aluminium is either tightly bound by organic matter or a present in the form of aluminium or aluminium hydroxy cations. These exchangeable ions are adsorbed by the negatively charged soil colloids. The adsorbed aluminium ions are in equilibrium with aluminium ions in the soil solution.
In the soil solution, aluminium ions produce hydrogen ions by the following hydrolysis reaction:
The H+ ions lower the pH value of the soil solution.
Acid sulphate soils:
In case of acid sulphate soils (pH less than 3.5), the acidity is due to dissolved or free acidic substances, such as sulphuric acid, ferric and aluminium sulphate. The sulphuric acid is produced by oxidation of sulphur and sulphide. Acid sulphate soils occur in some of the low lying areas of Kerala.