Discussion 58 Determination of pH and lime requirement

Definition. Soil pH is defined as the negative logarithm (to the base of 10) of the H+ activity in the soil solution measured under the stated conditions.

The activity approximates to the concentration [H+] in mol l-1 for dilute solutions, thus:

Acids dissociate to yield hydrogen ions, which will reduce the pH value because 1/[H+] becomes smaller. Alkalis supply hydroxyl ions (OH-), which associate with hydrogen ions, thus [H+] decreases and 1/[H+] increases. Pure water partially dissociates into hydrogen and hydroxyl ions: H2O s H+ + OH- where [H+][OH-] = 10-14 mol l-1, and is termed the dissociation constant, Kw. Thus [H+] = [OH-] = 10-7 = pH 7 for a neutral solution. Soils with pH values < 7 are referred to as acid, and > 7 as alkaline, and if the pH is 7, as neutral.

Pure water in equilibrium with atmospheric CO2 has a pH of 5.6. If a soil pH is lower or higher than this, it is acting as an acid or base respectively. Several soil components act as buffers (hydroxy aluminium monomers or polymers, soil organic matter and undissolved carbonates), therefore lime requirement tests may also be required.

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