Soil pH

The soil pH is a measure of the acidity or alkalinity in soils. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+
or, more precisely, H
3O+
aq) in a solution. In water, it normally ranges from -1 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is alkaline. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH range for most plants is between 5.5 and 7.0, however many plants have adapted to thrive at pH values outside this range.

Classification of soil pH ranges

The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows:

Sources of soil pH

Sources of Acidity

Acidity in soils comes from H⁺ and Al³⁺ ions in the soil solution and sorbed to soil surfaces. While pH is the measure of H⁺ in solution, Al³⁺ is important in acid soils because between pH 4 and 6, Al³⁺ reacts with water (H₂O) forming AlOH²⁺, and Al(OH)₂⁺, releasing extra H⁺ ions. Every Al³⁺ ion can create 3 H⁺ ions. Many other processes contribute to the formation of acid soils including rainfall, fertilizer use, plant root activity and the weathering of primary and secondary soil minerals. Acid soils can also be caused by pollutants such as acid rain and mine spoilings.