CHAPTER 



O The Activities of Cations 

 Held by Soil Colloids 



and the Chemical Envi- 

 ronment of Plant Roots 



C. EDMUND MARSHALL 



T 



he concept of the soil solution encounters difficulties as 

 soon as ionizing colloidal constituents contribute appreciably. In the 

 author's opinion, these difficulties become insurmountable if the soil 

 solution is defined either according to Burd's recent dictum (2) in which 

 the contributions of the colloidal constituents or ionizing surfaces are 

 excluded, or according to earlier views identifying the removable soil 

 solution (obtained by displacement or by pressure) with the soil solu- 

 tion in situ. On the other hand, a frank recognition of the complexities 

 involved, together with the utilization of modern methods for de- 

 termining cationic activities in colloidal systems, opens up the possi- 

 bility of their resolution. 



THEORY OF THE SOIL SOLUTION 



In what follows, the soil solution will be defined as the complete 

 external aqueous chemical environment of the plant root. This is a 

 macro concept, that is, it involves the mean chemical environment for 

 each ionic and molecular species present. In actual soils variations un- 

 doubtedly occur from place to place on a single root hair, but we are 

 dealing here with averages taken over large numbers of points of con- 

 tact and immense multitudes of ions. It is also a static concept, avoid- 

 ing reference to changes with time except in so far as these may be 



