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Soil pH

Soil pH is a term that is often used in soil and plant science.  It is a measure of the hydrogen ion activity (H+) in solution. The amount of H+ in solution can be relatively small and is expressed in exponential form.  For example, the H+ activity of pure water at 25o C is 1 x 10-7 moles per liter. Working with such numbers and expressions can be very cumbersome.  However, instead of saying that the activity of hydrogen ions in pure water is 1 x 10-7, it is customary to express the activity on a logarithmic basis and to say that the pH is 7.0.

The symbol, pH, is defined as the negative logarithm of the hydrogen-ion activity and allows for the use of arithmetic terms to describe the activity of hydrogen ions:

pH = -log(H+)

The pH scale ranges from 0 to 14. Values below 7 represent acidic conditions and values above 7.0 represent basic or alkaline conditions. The following figure illustrates the pH of some common products and soil conditions.

Soil pH is considered by many scientists as being one of the most important chemical reactions that occurs in soils. It is so important because it influences many chemical and biological reactions that occur in soils. Soil pH influences soil nutrient availability, potential for Al/Mn toxicities, microbial activity, fertilizer efficiency, and plant community composition. The following figure illustrates some of the potential affects that different pH levels may have on certain chemical and biological reactions occuring in soils. Consequently, in order to maintain a favorable soil environment for turfgrass it is essential to monitor soil pH every two to three years. For most turfgrass species in the southeast the most favorable pH range is from 5.5 to 6.5. In other regions of the country some species perform quite well at pH levels of 7.5.

Sometimes thatch pH differs from the underlying soil pH, especially in humid climates or if irrigation water tends to be neutral to acidic.  An occasional check on thatch pH may be useful because at pH levels below 6.0 bacteria populations decline.  Bacteria in the thatch are necessary for thatch decomposition.  If thatch pH is below 6.0, an application of finely ground (100-mesh) agriculture limestone at 2 to 4 lbs per 1000 ft2 may benefit thatch decomposition.



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