Explanation:

Complementary ion effect is defined as the influence of one adsorbed ion on the release of another from the surface of a colloid.

The complementary ion effect can be well explained where ammonium (NH4) in the soil solution is exchanging with calcium (Ca2+) on soil colloids. This exchange will take place more readily when the complementary cation on the exchange complex is aluminium rather than sodium. Ammonium will replace much more sodium that it willsaluminium. By ammonium becoming involved in exchange for sodium, there is less of it available for replacement of calcium. The more strongly held trivalent aluminium tends to satisfy a greater part of the cation exchange capacity and permits the exchange of ammonium for calcium to proceed more completely.