Ionic Exchange.—The phenomena of exchange in many soils, especially significant in the study of fertilizers, are classed as “ionic (or basic) exchange.” The various soil colloids are the carriers of the interchange, where the negative particles show basic exchange while the electropositive particles show an exchange of anions. The exchange of bases in agricultural soils has long been known but is properly interpreted only through colloid chemistry. The gels of aluminum hydroxide and silicic acid, important components of clay, bind the ions of ammonium and potassium more strongly than those of sodium and even calcium at certain concentrations. The latter can for this reason be displaced by the former and substituted adsorptively. While ammonium and potassium remain insolubly combined in the soil, the dissolved sodium and calcium ions flow away, and the soil is decalcified by the addition of potassium. Similar phenomena of exchange also occur in natural soils and are of the greatest importance to the development of vegetation. Jenny (1926) explains the leaching of the rendzina soils of the Alps and their transformation into podsol and alpine humus soils, from the viewpoint of base exchange. The H  ions displace the cations on adsorbed humus and clay colloids of the rendzina soil according to the following graphic formula: (Braun-Blanquet 1932:163-4)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.