Glossary:Reserve acidity

The pH of a soil solution is a measure of the active acidity [concentration of dissolved free hydrogen ions (H+)] in the soil solution. A low pH indicates an excess of hydro gen ions compared to hydroxyl ions (OH-). The lower the pH the greater the acidity in the soil, and the scale is logarithmic. In very acid soils (pH < 5), trivalent aluminum (AI+3) which is a component of numerous clay minerals becomes increasingly soluble. When dissolved, each aluminum ion is strong enough to split three water molecules (by three stages of hydrolysis) in order to bond with the three hydroxyl ions thus released. This releases three hydrogen ions to the soil solution, and further increases the acidity. When chemical amendments are added to neutralize an acid soil, the amount of chemical amendment applied (lime) must be sufficient to neutralize both the free hydrogen ions (H+) and the three species of free aluminum ions [AI+3, AI(OH)+2, and AI(OH)2+1] which are still capable of causing further hydrolysis and acidification. Because of the complex reactions involved, the amount of lime required to balance the pH is determined by titrating a known mass of soil with a base representing lime. When the titration stabilizes at an appropriate endpoint (e.g., pH = 7.0), the amount of base used correlates with the amount of lime which must be added to neutralize the hydrogen ions and aluminum ions. Reserve acidity is the combined acid potential of H+, Al+3, AI (OH)+2, and AI (OH)2+1 ions adsorbed on clay colloids, whereas active acidity is free H+ in the soil solution (not adsorbed).