730 PLANT GROWTH AND PLANT COMMUNITIES 



the salt concentration in the external solution must be so low that for 

 all practical purposes the ionic atmospheres of the colloidal phase are 

 free from salt. 



This type of characterization gives such quantities as the "lime 

 potential" of Schofield, namely, pH — VzpCa; and the energy of ex- 



change of Woodruff, for example, RT In or 1,364 (pK — VapCa). 



V'^Ca 



These, of course, are intensity ratios, and their relation to the gross 

 cationic composition of the exchange complex has to be determined by 

 complete exchange. This last has always been regarded as an important 

 determination by soil scientists, since it expresses quantitatively the 

 absolute level of each of the readily available cationic reserves. From 

 the individual analytical results appropriate ratios are easily calculated. 



An important characteristic of the exchange complex can be de- 

 rived by dividing a particular activity ratio by the corresponding ratio 

 of total exchangeable quantities. Under the conditions specified (very 

 dilute external solution), the result is the ratio of the activity coeffi- 

 cient of the two cations of the colloid where these have the same 

 valency (Wiklander, 1946). This quantity is often described as the 

 equilibrium constant for the exchange reaction (kg), but for many ex- 

 changers it varies greatly with the proportions of the two cations on 

 the exchanger. In fact, the modern trend is to treat it as a variable, to 

 call it the "selectivity number" or "selectivity coefficient," and to char- 

 acterize a given exchanger in relation to any pair of cations by the 

 curve connecting kg with exchange composition. A soil in its natural 

 condition corresponds to one point on such a curve for each pair of 

 cations. These curves can be derived also from individual cation ac- 

 tivity measurements whenever they are available for bionic systems. No 

 doubt in the future we shall see increasing use of this method of char- 

 acterizing the exchange properties of soils and soil colloids. 



Can the same be done for plant roots under low rates of metabo- 

 lism? With roots it is extremely difficult to effect a small exchange even 

 at low temper atiu'es. There is, apparently, fairly rapid passage from 

 the interior of the root through the surface exchanger to the outer 

 solution. Higdon ( 1957 ) found that upon exchange against salt solu- 

 tions of 10"* molar, the amounts found in solution were often a quarter 

 of the exchange capacity of the root. Therefore, to make such determi- 

 nations without changing the composition of the exchanger, dilute salt 

 mixtures of the correct proportions should be used. If this type of de- 

 termination can be made in sufficient detail, evidence on the possible 

 variations in cationic bonding by different sites on plant roots should 

 emerge. This would throw light on the basic assumptions used by Ep- 

 stein and Hagan ( 1952 ) in their interpretation of the kinetics of uptake. 



