66 Mineral Nutrition of Plants 



in terms of shifts in the bonding energy curves, comparing for instance, 

 potassium-calcium with potassium-hydrogen systems and calcium-po- 

 tassium with calcium-hydrogen systems. 



Evidence obtained with single monovalent cations 



The complexity of clay titration curves may be illustrated by the 

 case of 2.8 per cent hydrogen bentonite and sodium hydroxide (Figure 



ph 



900 



800 



700 



AF 600 



(calories 500 

 400 

 300 

 200 

 100 

 



per 

 equivalent ) 



« 10 



20 40 60 80 100 120 



Miiliequivolents Na OH per 100 q cloy 



140 



Figure 2. Clay titration curves for 2.8 per cent Wyoming 

 bentonite with sodium hydroxide. A: pH titration curve. 

 B: Sodium ion activity plotted against base added. C: Mean 

 free energy of sodium ions plotted against base added. 



2). Three curves are presented. A is the ordinary pH titration curve, 

 characteristically smooth and with a single reasonably well-defined in- 

 flexion point. Curve B relates the sodium ion activities to the amount 

 of base employed. Its complex characteristics are well established for 



