74 Mineral Nutrition of Plants 



and (c) calcium in presence of hydrogen or potassium can be worked 

 out. The data obtained under (c) are limited by the difficulty of measur- 

 ing very low activities of divalent ions in the presence of much higher 

 activities of monovalent. 



Percentoge Co 



Figure 5. Comparison of calcium ion activities and mean 

 free energy values (per equivalent) in calcium-hydrogen and 

 calcium-potassium clay systems (2.5 per cent Putnam clay). 



Figures 4 and 5 show how the mean bonding energy of potassium 

 is affected by the presence of calcium replacing hydrogen and how that 

 of calcium is affected by the presence of potassium replacing hydrogen. 

 The particular clay used was Putnam (beidellite). It is evident that 

 calcium greatly lowers the energy with which potassium is held. This 



