ii4 Mineral Nutrition of Plants 



roots having low-salt status. High-salt roots may release copious 

 amounts of ions to solutions. 



In view of the fact that a semipermeable membrane inserted between 

 root and clay prevents contact depletion, it appears impossible to resolve 

 this type of root behavior in clay systems in terms of conventional soil 

 solution theories. With Ratner (39, 40) we believe that desorption of 

 root-ions by clays constitutes an important plant physiological process 

 occurring in soils. 



Uptake of radioactive coliimbium (Cb 95 ) 



Columbium added to clay behaves as an insoluble compound. At 

 a pH value of 1.0 and at pH values commensurable with root activities, 

 no columbium can be detected in the intermicellar liquid. Yet, as 

 Jacobson and Overstreet (14) have shown, dwarf pea plants success- 

 fully compete with the clay for columbium and accumulate it in the 

 root, and, to a small extent, in the top. These findings have been con- 

 firmed with carrots by Vlamis and Pearson.* We cannot at present 

 comprehend the transfer of columbium on the basis of the carbon 

 dioxide-solution theory. 



Utilization of nonexchangeable potassium 



Ramona loam contains 76 p.p.m. of exchangeable potassium as 

 determined by leaching with neutral, normal ammonium acetate. Rye 

 seedlings (Neubauer test) will assimilate, within 18 days, 264 p.p.m. 

 of potassium from the same soil. Evidently these plants extract 188 

 p.p.m. of nonexchangeable potassium. 



According to Peech (38), many investigators have postulated the 

 following equilibria between the different forms of soil potassium: 



Nonexchangeable K ^ Exchangeable K „ K in solution 



The removal of solution-potassium by the plant releases exchange 

 able potassium into the solution which, in consequence, causes conver- 

 sion of nonexchangeable potassium to the exchangeable form. The 



*J. Vlamis, and G. A. Pearson, "Absorption of radioactive zirconium and 

 niobium by plant roots from soils and its theoretical significance," Science, 111: 

 112-113 (1950). 



