Exchangeable Acidity Defined

Colloids

• Exchangeable acidity depends on the presence of colloids in the soil. Colloids are particles too big to dissolve in a water solution but so small that they will not settle out of the solution. They often have an electrical charge. Clay particles in soils are often colloids.

Exchangeable Acidity

• Positively charged particles called cations stick to colloidal clay particles that have an opposite electrical charge. Some ions (electrically charged atoms) are acidic, especially hydrogen ions and aluminum ions with a +3 charge. Cations sticking to clay colloids are exchangeable; the hydrogen and aluminum cations sticking to colloidal clay constitute the exchangeable acidity of the soil. In contrast, the pH value does not measure any ions that stick to colloids, but only hydrogen ions that are actually dissolved in a soil solution.

Effect on Plants

• Hydrogen and aluminum ions can exchange places with ions in soil solution, thereby raising the pH. Some aluminum ions will come into contact with plant roots and may have a toxic effect, according to the Food and Agriculture Organization of the United Nations.