62 



Mineral Nutrition of Plants 



CATIONIC ACTIVITIES IN HOMIONIC SOILS 



To bring these aspects into sharper focus some concrete results of 

 cationic activity determinations in various soil-water mixtures may be 

 considered. Table II, taken from the data of Marshall and McLean 

 (/6), gives potassium and calcium activities on two homionic soils con- 

 taining no added salts. The potassium and calcium ion activities were 

 determined by clay membrane electrodes using techniques fully de- 

 scribed elsewhere (g, w, 11 , 14). The fraction active is the ratio of the 



TABLE II 

 Potassium and Calcium Activities Measured at Different Soil-Water Ratios 



on Homionic Soils 



measured activity to the total concentration of the ion concerned. In 

 true solutions it is known as the activity coefficient. The following con- 

 clusions then emerge from an examination of Tables I and II together. 



1. In spite of the fact that the clay colloids are weakly ionized; the 

 cationic activities of salt-free soils in the moisture range where plants 

 actually grow are significant. Even in the case of calcium we have activ- 

 ity values of the order of 3 X io~ 3 , which is considerably greater than 

 would be given by calcium carbonate in equilibrium with ten times the 

 carbon dioxide pressure of ordinary air. 



2. The actual activities obtained depend on the nature of the soil col- 

 loid present as well as on the amount. 



3. The cationic activities are a sensitive function of the soil-water 



