C. Edmund Marshall 69 



Evidence obtained with single divalent cations 



In comparing results obtained for divalent cations with those for 

 monovalent, certain requirements as regards energy must be kept in 

 mind. Suppose that two identical, neighboring, exchange spots release 

 the same amount of energy when combining with two monovalent 

 cations as they do with one divalent cation. The bonding energy per 

 equivalent is therefore the same in the two cases. However, in terms 

 of ions, the bonding energy per divalent cation is twice that per mono- 

 valent cation. From the logarithmic formula relating bonding energy 

 per mole ion to the fraction active, one can readily see that the fraction 

 active of the divalent cation would be the square of the fraction active 

 of the monovalent cation. It is therefore to be expected that divalent 

 cations will show very low ionization from silicate surfaces. Thus, if the 

 fraction active for a certain monovalent cation is 0.1, then that for a 

 divalent cation releasing the same energy per equivalent should be 0.01. 



This is often found experimentally to be the order of magnitude of 

 the relationship, allowing, of course, for variations due to cationic in- 

 dividuality as it may show itself in hydration or in geometrical relation 

 to the silicate surface. 



Figure 3 gives the curves, analogous to those of Figure 2, for hydrogen 

 montmorillonite titrated with calcium hydroxide. The activity of cal- 

 cium is seen to be almost constant over a considerable range, extending 

 from about 30-70 per cent saturation with base. Then it rises very 

 rapidly through the point of equivalence. This general behavior is found 

 for all four clays (montmorillonite, beidellite, illite, and kaolinite) and 

 for the three cations (magnesium, calcium, and barium) with some 

 variation in the extent of the flat position and the subsequent rise. The 

 actual values of the fraction active are summarized in Table IV. 



Since calcium is the major exchange cation in most fertile soils, a 

 great deal of practical significance emerges from a study of these 

 figures. Acid soils, if less than 70 per cent saturated with calcium, may 

 be expected to give a very low calcium activity. The lime required to 

 bring the saturation up to 70 per cent will make relatively little differ- 

 ence as regards the chemical environment of the plant root. Between 

 70 and 100 per cent saturation the addition of lime greatly increases the 



