ver street an d Dean i o I 



where m\ is the molality of the ion and Yi is its activity coefficient. 

 Yi becomes unity in an infinitely dilute solution. Many soil filtrates are 

 dilute to the extent that the activity coefficients of their constituent ions 

 can be taken as unity and the activities of the ions can be considered 

 equal to their molalities. In such cases, where the total concentrations 

 in each phase are known, it is possible to calculate relative activity co- 

 efficients for the ions in the soil suspension or gel. 



When this calculation is made, it is found very often that the activity 

 coefficient of a soil cation is very much less than unity, often of the 

 order of o.ooi. On the other hand, we find that with soil suspensions 

 that display such Donnan effects as negative adsorption (41), the activ- 

 ity coefficient of a soil anion may be considerably greater than unity. 

 This, however, is not an uncommon situation even with ordinary elec- 

 trolytes (cf. the mean activity coefficient of 2 M HCl). 



Ion absorption rates in soil suspensions and their filtrates 



In a series of studies dealing with the "contact" and "soil solution" 

 theories, Jenny and Overstreet (24, 25, jj) were able to show that with 

 certain clay or soil suspensions containing adsorbed Na+, K+, or Rb+, 

 the rates of absorption of the ions by barley roots was greater than from 

 the corresponding filtrates. In terms of the present argument, these ex- 

 periments are rather strong evidence that the absorption rates of some 

 soil ions are functions of their concentrations in the culture medium 

 rather than their activities. That is, although the ion activities were the 

 same in suspension and filtrate, the rate was greatest in the phase where 

 the concentration was greatest. 



A similar situation has been encountered in other branches of chem- 

 istry. Although most theories on reaction kinetics predict that rates will 

 be functions of the activities of the reactants, a rather impressive num- 

 ber of reactions have been discovered in which the rates are determined 

 by concentrations {34). 



In our present state of knowledge, predications beforehand of the 

 rate-determining factors of chemical reactions are uncertain. Also, we 

 are probably not justified in making any generalizations concerning 

 the absorption of soil ions. To date, the effect has been established for 

 Na+, K+, and Rb+ ions onlv. 



