Hans Jenny ny 



rival theories, a choice may be made on the basis of the intrinsic value 

 of the theories themselves. There is one important mechanistic aspect 

 which renders the contact theory particularly attractive. 



Between negative surfaces (negative inner layers), transfer of cations 

 may be accomplished without the aid of anions. Conversely, between 



Distance 



Figure 4. Model of contact exchange between inter- 

 mingling ion swarms. Two parallel negative clay plates 

 with undisturbed positive ion swarms on the outer sur- 

 faces and interpenetrating mixed swarms between the 

 plates. 



positive surfaces (positive inner layers), anions may be transferred 

 without cations. Generally speaking, an ion is transferred without an 

 accompanying partner. Although water is usually present, it is not 

 essential. Theoretically, contact exchange may occur in any medium, 

 provided an electric double layer exists. 



Sengupta (42) measured ammonium-sodium exchange between 2.02 

 m.e. ammonium colloid (Ion-X, < 0.1 mm. particle size) and 1.50 

 m.e. sodium colloid (Amberlite, > 0.5 mm. particle size) in 100 cc. of 

 various media. The transfer of ammonium from Ion-X to Amberlite 

 was 43.1 per cent in water, 45.4 per cent in methyl alcohol, and 21.7 

 per cent in benzene. In the latter system the rate of transfer was slow 

 and equilibrium probably had not been attained. Ion exchange must 

 have been primarily by contact. 



In applying the idea of contact exchange to the mineral nutrition of 

 plants in soils, it is postulated that ion swarms of the root and of the 

 soil particles intermingle. Transfer of ions is accomplished without 

 the aid of the soil solution. The uptake of adsorbed ions by contact is 



