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Soils - Part 4: Soil pH

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Soil pH is defined and its implications for crop production are described in this lesson. How are soil pH and buffer pH determined? How are these assessments used in lime recommendations? The factors that influence pH variations in soils, the chemistry involved in changing the pH of a soil, and the benefits associated with liming acid soils will be discussed.

[This lesson, as well as the other nine lessons in the Soils series, is taken from the "Soils Home Study Course," published in 1999 by the University of Nebraska Cooperative Extension.]

What is pH?

Acidity and alkalinity of an aqueous system (solution) are described (measured) by the term pH. pH is the negative logarithm of the hydrogen ion [H+] concentration. Hydrogen is a component of all acids. When hydrogen ionizes to form ions [H+], its concentration determines the acid properties of soil solutions.

Pure water, free of all minerals such as calcium, magnesium, etc., and free of carbon dioxide [CO2], has a pH of 7.0, which is called neutral. Water [HOH] ionizes to form hydrogen ions [H+] and hydroxyl [OH-] ions according to the following equation:

  HOH <--> H+ + OH          (1)

If carbon dioxide is dissolved in water in the absence of other compounds or minerals, the water becomes slightly to moderately acid. Equation 2 shows the effect of distilled water open to CO2 in the atmosphere:

   HOH + CO2 <--> H+ + HCO3(Carbonic acid)          (2)

There is now more H+ than OH-; thus, the pH of distilled water that has absorbed CO2 from the atmosphere may range from 4.0 to 7.0, depending on temperature and the amount of CO2 in the atmosphere. The reaction in Equation 2 is probably responsible for seasonal fluctuation of 0.2 to 0.5 units in soil pH.

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