Soil pH

Soil pH is a measure of the acidity or basicity of a soil. It is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+ or, more precisely, H3O+ aq) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution, such as 0.01 M CaCl2), and normally falls between 3 and 10, with 7 being neutral.

Overview

Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra acidic soils (pH < 3.5) and very strongly alkaline soils (pH > 9) are rare. 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 and the reaction rates.

Soil pH and plants

Plants require certain pH levels to grow. Most plants prefer a pH between 6 and 7, although there are some notable acid-loving exceptions such as blueberries which can grow in soils with pH down to 4.5. Some plants, such as rice, are capable of growing in more alkaline soils.

Soil pH and microbes

Soil pH also affects the availability of nutrients for microbes. Many bacteria, for example, prefer slightly acidic conditions, and fungi tend to prefer slightly alkaline conditions.

Soil pH and nutrient availability

The availability of most plant nutrients is influenced by soil pH. Some nutrients, such as nitrogen, phosphorus, and potassium, are most available to plants in slightly acidic soils (pH 6 to 7), while others, such as iron, manganese, and zinc, are more available in more acidic soils (pH 4.5 to 6).

See also

• Soil science
• Soil type
• Soil fertility
• Soil degradation

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  World Soil pH map
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  Soil pH scale