jo Mineral Nutrition of Plants 



calcium activity. This picture, of course, may need modification for 

 soils in which organic matter plays a predominant role, but it accords 

 very well with practical experience on soils high in clay. The compari- 

 son of kaolinitic soils with those dominated by members of the mont- 

 morillonite group is also extremely interesting. 



1300 

 1200 



AF 1100 



(calories, „ 

 per 1000 



equivalent ) 



900 



800 



700 



600 



500 



20 40 60 80 100 120 



Milliequivalents Co(OH) 2 per 100 q.clay 



140 



0*10 



Figure 3. Clay titration curves for 1.07 per cent Wyoming 

 bentonite with calcium hydroxide. A: pH titration curve. 

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

 free energy of calcium ions (per equivalent) plotted against 

 base added. 



Under any given set of circumstances the actual value of the fraction 

 active determines the mean bonding energy of the cations present and, 

 hence, controls the ease with which they can be exchanged for other 

 cations. It is entirely possible to have two soils, dominated by different 

 clay minerals, which would give the same calcium activity but different 



