THE ABSORPTION OF MINERAL ELEMENTS 95 



the concentration of the H + ions. The active acidity of a solution 

 is usually indicated not in gram-equivalents per liter or normality, 

 but by the negative logarithm of the normal concentration denoted 

 by the sign pH, the hydrogen-ion exponent. Thus, the concentra- 

 tion of the H + ions at neutral reaction is denoted by pH = 7. 

 As pH is a negative logarithm, the lower the designation of pH, 

 the higher the concentration of the H + ions; and the more acid is 

 the reaction, and vice versa, the higher the pH is, the more alkaline 

 is the reaction. 



The development of plants is in general possible only within 

 comparatively narrow limits of concentration of the H + ions. 



The pH value of the soil may vary from 3 to 11. The first 

 figure indicates a very acid soil, the second a strongly alkaline one 

 that may only be attained by the accumulation of magnesium salts. 

 Calcium salts can increase the pH value of the soil only to 8.4. 

 Chernozem 1 soils, rich in carbonates, have also an alkaline reaction, 

 podsol 2 soils, on the contrary, are acid. Soils of bogs are very acid. 3 



Recent investigations have shown that the hydrogen-ion con- 

 centration is one of the essential factors in determining plant dis- 

 tribution. Some plants grow best at one pB. concentration, others 

 at another. Very often it is possible to judge more or less exactly 

 the active acidity of the soil from the specific complex of a meadow 

 vegetation. The Danish investigator, Olsen, has proven that the 

 predominance of Deschampsia flexuosa among the meadow grasses 



Chernozem (black earth). — A very productive soil developed under 

 subhumid or semiarid climatic conditions on open grassland. The soil is 

 characterized by a very dark surface horizon which varies in depth from 1 to 

 4 ft. and which possesses a well-developed granular structure. The soil does 

 not contain soluble toxic salts, but both the organic and mineral colloids are 

 saturated with calcium and magnesium. The normal reaction is neutral and 

 the typical topography smooth. The largest continuous area of chernozem 

 lies in the southern half of western Europe (Russian steppe) and extends 

 eastward into Siberia. 



2 Podsol (ash-like soil) . — A soil developed under forests of moderately 

 humid regions. It is characterized by an ashy-gray or nearly white horizon 

 just below the leaf litter. The horizon varies in thickness from several inches 

 to more than 2 ft. Below the ashy-gray or podsolized horizon lies a very 

 heavy, compact dark-brown or reddish-brown horizon of varying thickness, 

 usually 1 to 2 ft. The podsolized horizon is typically acid in reaction. The 

 topography varies from undulating to rough. Typical podsols are to be 

 found in the forested portion of northern United States. 



3 Bogs in northwestern United States are frequently alkaline. 



