124 PLANT PHYSIOLOGY 



ions, e. g., cations, may then leave the remaining anions free to 

 replace anions in other salts, thus ultimately releasing in the 

 soil solution anions that are different from those originally added 

 to the soil. 



If cations are taken up by the soil colloids and other mineral 

 cations or bases are freed into the solution, this exchange of basic 

 ions is commonly spoken of as base exchange. The fertility of the 

 soil depends to a large extent, for reasons that should be already 

 clear, upon the chief, available bases in the soil. If sodium or 

 magnesium ions become too available, the soil becomes too al- 

 kaline and toxic to many plants. If, as often happens, the hydrogen 

 ion becomes predominant or in excess, the soil will be acid. 



Hydrogen Ions.— This brings us to one of the most interesting 

 phases of plant nutrition. As we have seen, hydrogen is not more 

 important in plant nutrition than carbon or oxygen; but hydrogen 

 is one of the ions formed by the dissociation of water, the " uni- 

 versal" solvent, around the properties of which life itself is organ- 

 ized. Furthermore, hydrogen is one of the ions formed when acids 

 dissociate; and the previous sections have shown us the impor- 

 tance of this active little ion in the problems of antagonism. 



All acids, alkalis, and salts when in solution, as well as water 

 itself, dissociate into cations (with + electrical charges) and anions 

 (with — charges). A "neutral" solution is not a solution with 

 no acid (H) ions or alkali (OH) ions, but rather one in which the 

 number of acid ions is balanced by the number of alkaline ones. 

 When water dissociates, it does so according to the "law of mass 

 action" which "governs" such reactions: 



[H+]X[OH-] _ 



[H 2 0] 



This means that the product of the numbers of H + and OH - ions 

 divided by the number of undissociated molecules is a constant. 

 Pure water dissociates to an extremely small degree, which means 

 that the numerator of this fraction will be very small and the 

 denominator very large; the result is that K will be very small. 

 In fact it has been found by experimental methods that in the 

 case of pure water K is 10 -14 g. molecules per liter. Since water 

 is neutral, the number of H and OH ions must be equal, and 

 therefore the H ions in pure water (or a neutral solution) is 

 10~ 7 g. mol. per liter. This is another way of saying that pure 



