THE BALANCING FUNCTION OF SALTS 127 



the reaction of a culture solution in 12 hrs. from pH 3.9 to 6.3, 

 due possibly to a larger absorption of N0 3 . Plants, in general, 

 tend to bring the reaction of the solution to pH 5.0-7.0 when in 

 solutions more acid or alkaline than this. Rice, however, formed 

 an exception (Jacobson) in that it changed the reaction from pH 

 5.0 to pH 3.0 in three days. This was attributed to (a) less ab- 

 sorption of sulphates than of cations and (b) the excretion of 

 carbonic acid by the roots. 



On the other hand, the original pH of the soil solution determines 

 to some degree what ions are absorbed. Thus Hoagland found 

 that absorption of N0 3 , Ca, and P0 4 was greater at pH 5.0-5.5 

 than in neutral soils by crops that thrive best at these pH values. 

 There is, consequently, a mutual interaction between the plant 

 and the soil, the details of which are not yet sufficiently explored 

 and correlated. 



Plants and animals contain also an internal mechanism which 

 permits them to control to some extent their internal pH value. 

 This consists of the action of the ampholyte and " buffer" systems 

 of the organism. By an ampholyte is meant organic compounds 

 (proteins, lecithins, amino acids), which can split off either alkali 

 or acid groups to suit the occasion and thus neutralize the harmful 

 effect of incoming H and OH ions. The " buffer" systems are 

 inorganic salts which serve to regulate the dissociation of their 

 acids and thus serve as potential reserves of anions and cations. 

 Active, Titrable, and Potential Acidity. — In addition to the 

 active acidity, which is measured in terms of pH value, mention 

 should be made of the total (or titrable) acidity, which refers to 

 the acids present that are not dissociated. It must be borne in 

 mind that " active" acidity refers only to the H ions that are 

 active in the solution. If these are removed, then more may ap- 

 pear until the acid present is completely dissociated. These un- 

 dissociated acids also may influence crop production, although 

 they are of more interest when they are dissociated or active. 

 Potential acidity refers to the aluminum, iron, and other com- 

 pounds of the soil, which under certain conditions might be re- 

 placed or exchanged and thus free H ions into the soil. The rela- 

 tion of this type of acidity to plant growth has been only slightly 

 studied. 



Conclusion. — Plants originated in the sea and were developed 

 under an environment containing a certain quantity of salts, 



