C.Edmund Marshall 63 



ratio. This means that the chemical environment of the plant root 

 changes greatly with changing moisture conditions in the soil. 



4. Since different cations are ionized to different degrees, it becomes 

 exceedingly important to determine all the factors involved. In a nat- 

 ural soil system with a mixture of exchangeable cations present, the 

 relative activities may be quite different from the relative exchange 

 quantities. 



5. The expressed soil solution will be considerably different from the 

 soil solution in situ at the low salt contents of nonsaline soils containing 

 appreciable amounts of soil colloids. In many cases the cationic environ- 

 ment of plant roots will be provided chiefly by the soil colloids. 



In view of these conclusions the detailed study of the electrochemical 

 properties of soil colloids becomes of great importance to plant nutri- 

 tion. The main results thus far obtained are reviewed below. 



THE CATIONIC ACTIVITIES OF SOIL COLLOIDS 



The colloids of soils may be divided into two main groups, inorganic 

 and organic. In the inorganic group, the clay minerals are of predomi- 

 nant importance in contributing cations. The organic colloids are domi- 

 nated by humified materials of an electronegative character having, in 

 general, higher capacities for cation exchange than the clays. In a ma- 

 jority of fertile soils, the content of organic matter is less than that of 

 the clay minerals. 



The electrochemistry of these two groups of soil colloids had, until 

 the advent of clay membrane electrodes, been developed through pH 

 measurements and conductivity determinations. Potentiometric and 

 conductometric titrations of the free colloidal acids, generally purified 

 by electrodialysis, had led to the conclusion that both the clays and the 

 humic materials were genuine colloidal acids, owing their acidic char- 

 acter to inherent structural features and not acquired by adsorption of 

 soluble acids from solution. Many attempts were made to compare them 

 with weak soluble acids by ascribing to them approximate dissociation 

 constants. 



Evidence on the exchange properties of these soil colloids also con- 

 tributed to our knowledge. Exchange isotherms were obtained by meas- 

 uring analytically the exchange equilibria for different proportions of 



