668 PLANT GROWTH AND J^LANT COMMUNITIES 



particles, and the reactions are then written, for example, as follows, 

 the rectangles denoting clay: 



Ca 



Ca Ca 



Na 



Clay 



Ca + 2NaCl , Ca 



Clay 



Ca Ca 



+ CaCls 



Na 



The extent of exchange depends on many factors, such as the na- 

 ture of the clay mineral, the salt concentration, the nature of the 

 anion, and particularly the sizes and the charges of the participating 

 cations ( lyotropic series, valency series ) . 



From the standpoint of plant nutrition and root-clay interactions, 

 the following exchanges deserve special consideration: 



hydrolysis: 



K-clay + HOH ( water ) :;=^ H-clay + KOH (in solution) 



carbonic acid exchange: 



K-clay + H HCO3 (CO2 + H2Q):; — ^ H-clay + KHCO3 (in solution) 



The carbon-dioxide reaction assumes importance in the vicinity of 

 the root surface, where a high CO2 gradient is assumed to exist. 



Excepting the early ( 1916 ) work of Devaux ( Mehlich and Drake, 

 1955), McGeorge was one of the first to demonstrate exchange proper- 

 ties of roots ( Mehlich and Drake, 1955 ) . Soon afterward workers in 

 California (Jenny and Overstreet, 1939; Williams and Coleman, 1950) 

 and in Sweden (Wiklander, 1957) became active in this field, and in 

 recent years, partly due to the stimulation by Drake (Drake et al., 

 1951.), the ion-exchange capacities of roots have been studied in many 

 parts of the world (Blanc, 1958; Helmy and Elgabaly, 1958). Keller 

 and Deuel ( 1957 ) demonstrated that the majority of exchange sites in 

 roots are attributable to carboxyl groups. 



Pertinent are the experiments of Williams and Coleman ( 1950 ) on 

 the exchange isotherms of the outer portions of the living root. The 

 reactions were confined to surface regions by restricting the exchange 

 process to a duration of only ten seconds. These investigators prepared 

 living H-roots by immersing roots of intact plants in distilled water 

 saturated with carbon dioxide. The roots were then rinsed repeatedly 

 in distilled water to remove occluded CO2. The washed roots were 

 dipped for ten seconds into dilute, neutral K2SO4 solution. Immediately 

 the salt solution turned acid. Titration curves revealed the presence of 

 H2SO4, not H2CO:>, or malic acid. The reaction may thus be depicted as 

 H-ion exchange: 



