270 PLANT PHYSIOLOGY 



one that can only be attained by the accumulation of sodium 

 salts. Calcium salts can increase the actual acidity of the soil 

 only to pH 8.3. Chernozem^ soils, rich in carbonates, have also 

 an alkaline reaction; podsoP soils, from which all soluble bases 

 are washed out, on the contrary, are acid. Soils of bogs are 

 usually very acid.^ 



Recent investigations have shown that the hydrogen-ion 

 concentration is one of the essential factors in determining plant 

 distribution. Some plants grow best at one pH concentration, 

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

 less exactly of the active acidity of the soil from the specific 

 association of a meadow vegetation. For instance, the Danish 

 investigator Olsen (1923) has proved that the predominance of 

 Deschampsia flexuosa among the meadow grasses indicates a pH 

 of 3.5 to 3.9; Carex goudenougii, a pH of 4.5 to 4.9; Deschampsia 

 caespitosa, a pH of 5.5 to 6.4; and Tussilago farfara, a pH of 

 7.5 to 7.9. These figures have been confirmed by artificial cul- 

 tures of these plants in nutrient solutions. It has been shown 

 that each plant has its optimum, maximum, and minimum 

 concentration of pH for growth (Fig. 80). 



The numerous investigations of the last few years, especially 

 the work of Arrhenius, have helped to elucidate the response of 

 plants to the reaction of the soil. It has been ascertained that 

 different species show different requirements with regard to the 

 optimum concentration of H+ ion and are moreover distinguished 



1 Chernozem (black earth) : A very productive soil developed under sub- 

 humid or semiarid cUmatic conditions on open grassland. The soil is 

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

 4 ft. and that 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 (ashhke 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. 



