THE ABSORPTION OF MINERAL ELEMENTS 97 



hypothesis that at optimum pH for a given plant, the permeability 

 of the protoplasm to salts is the least, and that with the shifting of 

 pH to the one or to the other side of this optimum the permeability 

 may increase too much, resulting in injury to the plant. 



The soil, which is the natural medium for the development of 

 plants, represents a much more complex substratum than the solu- 

 tions used in water and sand cultures. According to many authors, 

 ihe influence of pH of the soil may be directly upon the plant or 

 may act indirectly by affecting the solubility of the nutrients and 

 poisonous soil substances. Thus, it is known that acid soils, or 

 physiologically acid fertilizers, applied to the soils promote the 

 solubility and absorption of phosphates. The solubility of the 

 Fe and Al salts equally depends on the pH of the soil. The investi- 

 gations of Uspensky with algae have shown that the more iron the 

 organism requires, the more the pH optimum for its development 

 shifts to the acid side, since the solubility of iron increases with 

 acidity. On the contrary, algae which are susceptible to an excess 

 of iron, for instance, Cladophora, are injured in consequence of a 

 high Fe content, in water, the reaction of which is lower than 

 pH = 7. 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 Fe insoluble 

 in an alkaline medium in the presence of calcium and phosphates. 

 But chlorosis is observed in the soil less frequently than in water 

 cultures, since the soil is usually rich in organic substances pro- 

 ducing complex compounds with Fe. 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 Al +++ ion. Aluminum is 

 quite insoluble at acidities from pH of 4.5 to 8.5. In a more alkaline 

 or more acid medium, the solubility of aluminum sharply increases. 

 Plants may suffer greatly in acid and in strongly alkaline soils not 

 only from the H and OH ions, but also from the toxicity of alum- 

 inum. At the present time, these questions are being investigated 

 in detail by botanists as well as by soil scientists, but they are still 

 far from being solved. 



Besides the actual acidity determined either electrometrically 

 or by indicators described in physical chemistry, the titrable 

 acidity determined by the usual methods of titration, likewise 

 may have an influence on plants. Solutions with the same actual 



