REACTIONS OF AMMONIA WITH SOIL, III SORPTION OF AQUEOUS NH3 BY HOMOIONIC SOIL CLAYS

Homoionic clay suspensions were equilibrated at 25°C with aqueous NH, (0.01 to 0.30 M) by agitating for several days. Using clays extracted from three different soils, sorption of NH₃, measured by titration, followed the order Na > Ca > NH₄ -clay. The difference in NH₃ sorption between Na-clay and NH₄ -clay was matched by an increase in Na ⁺ ion concentration in the aqueous phase of suspensions of Na-clay treated with NH₃, but there was no such effect with Ca-clay. Sorption of NH₃ on NH₄ -clay in aqueous suspension is considered due to proton donation at sites of pH-dependent charge, and the additional sorption on Na-clay to displacement of Na ⁺ by NH₄⁺ ions. The additional sorption on Ca-clay seems largely due to co-ordination of Ca²⁺ ions with molecular NH. Exchangeable cations affect NH₃ sorption differently when an aqueous phase is present than when dried homoionic materials are treated with NH₃ gas.