128 Mineral Nutrition of Plants 



TABLE III 

 Exchange Constants Between Adsorbents (K. Sengupta) 



TIME RATES OF ION TRANSFER BETWEEN ADSORBENTS 



The aforementioned adsorbent-adsorbent studies deal with equilib- 

 rium states. In so far as alkali metal ions are concerned, the transfer 

 may be visualized as taking place through solution or by contact. In 

 either case, the amount of net transfer is the same. It is, however, highly 

 probable that the rate of transfer is greatly accelerated by contact ex- 

 change. In fact, conditions exist under which solution transfer is so 

 slow that for practical purposes it is nonexistent. Good illustrations are 



provided by the behavior of divalent iron adsorbed on bentonite (20). 

 More recent experiments furnish information on columbium and non- 

 exchangeable potassium. 



Experiments with columbium (K. Sengupta) 



Radioactive columbium, Cb 95 , carrier-free and dissolved in 3N 

 sulfuric acid, was added to Amberlite and to clay suspensions. After 

 excess acid had been washed out the supernatant liquid was completely 

 free of columbium. A 0.17 per cent suspension of columbium-hydrogen 

 clay had a pH of 3.0. Adding hydrochloric acid to bring the suspension 

 to 0.025 AT hydrochloric acid did not release any columbium; neither 



