Significance of Soil pH
It is defined as the -ve log of the hydrogen ion activity of a soil.
pH = – log (H+)
Where (H+) represents the hydrogen ion activity in mol L-1. The pH scale is the logarithm to the base 10 of the reciprocal of the hydrogen ion activity. As the pH of a solution goes from 7 to 6, the hydrogen concentration increases 10 times and OH ions decrease by 10 times. The pH scale extends from 1 to 14, with pH 7 as being the neutral point. Soils with pH less than 7 are acidic and those with a pH above 7 are alkaline or basic. This means that at pH 7, hydrogen and hydroxyl ion concentrations are equal at 10-7 moles per liter (e.g. water).
Importance of soil pH:
- It is major factor in determining which trees, shrubs or grasses will dominate the land under natural conditions.
- pH influences the processes involved in the formation and development of soils.
- Most minerals are soluble in acid soils than in alkaline soils thus releasing ions toxic to plants e.g. Al.
- It affects the availability of nutrients to the plants. Alkaline pH reduces the solubility of all the micronutrients (particularly Fe, Zn, Cu & Mn) except Mo and Cl. Within a pH range of 6.5 – 7.5, most of the essential nutrients (especially phosphate) are available to plants.
- The soil pH also affects plant growth by influencing the activity of beneficial soil microbes. Most N-fixing bacteria are not very active in strongly acidic soils. Bacteria that decompose soil organic matter and thus release nitrogen and other nutrients for plant use are depressed by strong acidity. Fungi usually tolerate acidity better than do other microbes.
- Plant growth is also affected at high pH due to an excess of sodium ions both in soil exchange complex and solution, which actually deteriorate soil’s physical conditions for plant growth. Moreover, nutrient imbalance and sodium toxicity may also decrease plant growth.
Syed Shabbar Hussain Shah
Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan
M +92 304 6020238 E [email protected]