ACIDITY

pH is a measure of the acidity or alkalinity of soil. You can test the pH of substances using indicators which change colour with differing pH.

Colour chart for Universal Indicator an indicator for determining pH in the range 10 to 2

In soil, the pH of the soil determines what nutrients or toxins are available to be taken up by plants, and what are bound up on soil surfaces. Heavy metals such as cadmium and zinc become more available to plants at low pH. Some salts and elements such as aluminium and manganese become much more available at low pHs (acid soils) so that they become toxic to plants.

2 Al(OH)₃ + 3H₂SO_{4 <}—> 2 Al3+ + 3SO₄2- + 6H₂O

MnO₂ + 4H+ + 2 e– <—> Mn2+ + 2H₂O

At high pHs, essential nutrients become bound up chemically in the soil and are unavailable to plants, which is a problem in the calcereous soils of southern Australia. Even though they are still in the soil, plants cannot get at them because they are bound up.

Soils become more acid through the use of nitrogen fertilisers and legumes to improve crop nutrition of crops and pastures. Nitrogen is oxidised to form acids.

(NH₄)₂SO₄ + 4O₂ — 2HNO₃ + H₂SO₄ + 2H₂O

In agricultural systems, the removal of plant and animal material removes certain salts of calcium, magnesium and potassium which are alkaline in nature. This results in a steady acidification of the soil because these salts do not re-enter the soil.