272 PLANT PHYSIOLOGY 



the solubility and absorption of phosphates. The solubility of 

 the iron and aluminum salts equally depends on the pH of the 

 soil. The investigations of Uspensky with algae have shown 

 that the more iron the organism requires, the more the pH 

 optmium for its development shifts to the acid side, since the 

 solubility of iron increases with acidity. Conversely, algae that 

 are susceptible to an excess of iron, e.g., Cladophora, are injured 

 in consequence of a high iron content in water the reaction of 

 which is lower than pH = 7. Similar conditions may hold for 

 higher plants. The chlorosis of some plants on alkaline soils 

 rich in calcium carbonate may be explained not so much by the 

 injurious influence of the reaction as such, as by the lack of 

 iron that is transformed in an alkaline medium in the presence 

 of calcium and phosphates into an insoluble form. Chlorosis 

 is observed in the soil less frequently than in water cultures, 

 since the soil is usually rich in organic substances that produce 

 complex compounds with iron. These compounds are retained 

 in the solution even with an alkaline reaction. 



On the other hand, the pH of the soil may influence the plant 

 by acting on the solubility of the poisonous A1+++ ion. Gener- 

 ally, in neutral soils aluminum is quite insoluble, but in acid 

 and in alkaline soils, the solubility of aluminum sharply increases. 

 Plants may suffer greatly in acid and in strongly alkaline soils, 

 not only from the H+ or 0H~ ions, but also from the toxicity of 

 aluminum. At the present time, these questions are being 

 investigated in detail by physiologists as well as by soil scientists, 

 but they are still far from being solved. 



Besides the actual acidity determined either electrometrically 

 or by indicators as described in physical chemistry, the potential 

 or titratable acidity, determined by the usual methods of titra- 

 tion, may have an influence on plants. Solutions with the same 

 actual acidity may show different amounts of titratable acidity, 

 depending on the degree of dissociation of the acids in the solution 

 and their buffer action. '^ Buffer action" is the property of the 

 solution to minimize changes of the H+ concentration on dilution 

 or on the addition of H+ or 0H~ ions. Buffer action depends 

 on the presence of mixtures of weak acids in the solution, or of 

 weak bases, with their salts. Well-buffered solutions may be 

 mixtures of carbonates of alkalies, or alkaline earth metals with 

 carbon dioxide, or mixtures of phosphates of different basicity 



