PO 36: Define the following terms

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.