When the concentration of hydroxyl ions (OH⁻) in the soil exceeds that of hydrogen ions (H⁺), resulting in a pH above 7.0, the soil becomes alkaline.
Soil with a pH above 7 is termed alkaline soil. Such soil typically contains more than 15% exchangeable sodium. Due to the high sodium content, alkaline soils often have fewer clay particles and less organic matter. As pH increases beyond 7, soil alkalinity also increases.
There are three main types of alkaline soil:
1. Saline soil,
2. Calcareous soil,
3. Alkaline soil.