Skip to main content

Competency Area 5: Soil pH and Liming

PO 36. Define the following terms

  1. Soil pH
  2. Exchangeable acidity
  3. Buffer pH
  4. Alkalinity

Soil pH is the negative log of the H+ ion concentration: pH = -log H+ = log 1/H+.  The more H+ there is, the lower the pH and the greater the acidity.

  • pH 7.0 = -log 0.0000001 = log 1/10-7
  • pH 6.0 = -log 0.000001 = log 1/10-6
  • Properties of pH
    • The pH scale goes from 0 – 14.  pH < 7 is acidic, while pH = 7 is neutral (neither acid nor base), and pH > 7 is basic (alkaline).
    • 1 pH unit represents a tenfold change in acidity.  Thus, pH 5 is ten times more acidic than pH 6, and 100 times more acidic than pH 7.
  • Soils range between pH 3.5 and 9.  Hydrogen (H+) and aluminum ions (Al3+) and complexes are two primary sources of soil acidity.  Northeastern mineral soils generally have pH values between 4.5 – 8.2, while Northeastern muck soils tend to fall between 3.5 – 8.2.

Exchangeable acidity is a measure of the soil's ability to withstand a change in pH upon lime addition.  The higher the exchangeable acidity of a soil, the more lime is needed for a particular pH change.

Buffer pH is used to estimate a soil's exchangeable acidity; the degree of change in buffer pH is related to lime needs.  Buffering occurs when an acid (for instance, H+) and a base (for instance, OH-) react and form a neutral product (in this case, H+ + OH- → H2O).

Alkalinity is the term used to describe the amount of base in a soil when the pH is above 7.  At higher alkalinity, there are fewer hydrogen ions (H+), and more hydroxyl ions (OH-).

Basically, pH is a measure of active acidity, telling you whether you need lime or not.  Buffer pH and exchangeable acidity measure the buffer capacity, telling you how much lime you need to add.

There are several causes of soil acidity, including the leaching of basic cations (Ca2+, Mg2+, K+; leaving behind Al3+, which is acidic), crop uptake of basic cations and release of acids, decay of plant residues, acid rain, and the reaction of nitrogen fertilizer.

There are many benefits of liming: it prevents the toxic effects of aluminum, increases availability of essential nutrients, supplies plant needs for Ca and Mg, improves soil conditions for microorganisms, increases effectiveness of some key herbicides, and improves soil texture.