ABSORPTION OF MINERAL ELEMENTS BY PLANTS 273 



in which there are potential reserves of anions and cations able 

 to ionize and neutrahze the H+ and 0H~ ions. The soil always 

 contains, to a greater or less degree, the above-mentioned com- 

 binations. For this reason, a soil solution is much more stable 

 in regard to pH than aqueous solutions. The soils showing the 

 greatest buffer action are those rich in organic substances, e.g., 

 peaty or muck soils. Soils showing the least buffer action are 

 sandy soils, gray leached soils, and podsols. 



Besides the actual acidity, soils also possess potential acidities 

 apparent only when the soil is fertilized by the application of 

 mineral fertilizers. This form of acidity is explained by the 

 adsorption of the added salts with the liberation of free acids or 

 by the replacement of the base of the salt by aluminum. The 

 newly formed salt is hydrolyzed in turn into the slightly dis- 

 sociated aluminum hydroxide and the strongly dissociated free 

 acid, which determines the concentration of hydrogen ions. The 

 significance of these types of acidity for plants has not been fully 

 studied. 



63. Physiological Basis of Fertilization. — It is customary to 

 regard fertilizers as nutritive substances, the addition of which 

 to the soil directly contributes to an increase of the yield. As a 

 matter of fact, a much more complicated phenomenon is met 

 with here. It has already been emphasized that only a part 

 of the substances absorbed by the roots, chiefly nitrogenous 

 substances as well as anions of phosphoric and sulphuric acid, 

 should be regarded as substances participating in the composition 

 of the most important constituents of the protoplasm, the protein 

 substances. Only these elements should be compared in impor- 

 tance with the source of carbon, the carbon dioxide of the air. 

 The main role of their action is the regulation of the vital proc- 

 esses, and their influence on the ac<^umulation of organic sub- 

 stance by the plant is mostly indirect. 



This is obvious from the fact that the introduction of fertilizers 

 into the soil leads not only to an increase in the plant of the total 

 amount of ash and nitrogen, but far more to an increase of the 

 total quantity of organic substance, most of which consists of 

 carbohydrates, which of themselves do not contain either mineral 

 elements or nitrogen. That is why it is necessary to search for 

 an explanation of the action of ash elements, especially of cations, 

 upon the yield through their influence upon the vital processes 



