60 Mineral Nutrition of Plants 



whole soil solution as here defined. Where appreciable colloid is 

 present, it takes a high salt concentration to achieve this. Where the 

 colloid content is very low, a much lower salt concentration will suffice. 

 This explains why those who have worked with plant growth problems 

 in highly saline soils and those who have had similar experience only 

 in extremely light sands have been satisfied with a concept involving 

 simple solutions of ordinary electrolytes. However, the great preponder- 

 ance of productive agricultural soils are neither highly saline nor de- 

 void of ionizing soil colloids. 



Turning to the case in which a ( . is much greater than a B , we arrive 

 at the formula for membrane hydrolysis, in which the compound under 

 consideration is, in the above example, potassium hydroxide. Experi- 

 mentally this is very simple, since (a c -f- a B ) is given by the cationic 

 activity of the whole soil system, a B by the hydroxyl ion activity of the 

 whole soil system, and a E corresponds to the potassium hydroxide in 

 the expressed liquid. Thus, if we know the pH of the soil system and 

 its cation activity, we can immediately determine the composition of 

 the expressed liquid. The strength of the soil colloid as an electrolyte 

 comes into this picture only through a e , the cation activity of the colloid 

 under the given moisture conditions. The expressed liquid will be alka- 

 line, hence this equilibrium will be very sensitive to the presence of 

 weak acids, such as carbonic, in the system. It is easy to see that such an 

 acid will greatly increase the potassium activity in the expressed liquid. 

 It does not do this, however, primarily by competition with the soil col- 

 loids, which are much stronger acids than carbonic, but indirectly 

 through its effect on the Donnan membrane hydrolysis. 



Similar considerations may be applied to other cationic constituents 

 of soil colloids; as for instance, calcium-saturated soils in equilibrium 

 with calcium chloride. For this case the equation corresponding to (2) 

 is a c /a B = q z - 1, graphically represented also in Figure 1. Here again 

 we reach the limiting condition for membrane hydrolysis when a c is 

 very large in comparison with « B . The hydrolysis can be calculated from 

 the equilibrium with respect to calcium hydroxide, using the appropri- 

 ate Donnan equation. It will be a sensitive function of the pH of the 

 soil system, since the hydroxyl ion concentration comes in as the second 

 power. 



