Soil pH

C. Owen Plank

Extension Agronomist

Soil pH or reaction 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. A pH of 7.0 is neutral which means the acid component, hydrogen ions (H+), and the basic or alkaline component, hydroxyl ions (OH-), are in equillibrium. 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:

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 7.0 to 7.5.

Correcting Low Soil pH

Soil pH should be monitored by soil tests ever 2 to 3 years especially areas that are heavily fertilized with ammonical-nitrogen fertilizers. Information can be obtained from your local County Extension Office or Garden Center on soil test services available in your area.  Low soil pH can be corrected using either dolomitic or calcitic limestone.  In instances where the soil tests low in magnesium, dolomitic limestone should be used.  If the pH is low and magnesium is adequate either calcitic or dolomitic limestone can be used. Correcting soil pH in established turf can be a slow process, especially if one is attempting to alter the surface 4 to 6 inches. When limestone is applied to the surface of soils it does not move downward appreciably.  Thus, several years may be required to change the pH of the surface 4 to 6 inches of soil.

The pH of the surface 2-3 inches can be adjusted with some liming materials in a relatively short period of time.  Generally, with conventional liming materials slight adjustments may occur within 2 to 3 months after application but normally 6 months reaction time is required for these materials to have much effect on soil pH. When more immediate results are desired some of the pellitized liming materials or finely ground (<100-mesh) limestone can be used.  If moisture conditions are good, these materials will usually effect a change in pH in 3 to 6 weeks.

Thatch pH

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.