EDAPHIC OR SOIL FACTORS: CHEMICAL 167 



1. Acidophilous species and communities (calciphobes) ; soil reaction pH 6.7 to 

 3.8: 



a. Plants extremely acidophilous, soil strongly acid, pH .5.0 to 3.8. 



b. Plants moderately acidophilous, soil moderately acid, pH 6.2 to 5.0. 



c. Plants weakly acidophilous, soil slightly acid, pH 6.7 to 6.2. 



2. Neutrophilous species and communities, soil neutral, pH 7.0 to 6.7. 



3. Basopliilous species and communities, soil reaction, pH 8.5 to 7.0. 



n. Plants basophilous to neutrophilous, soil basic to neutral, pH 7.5 to 7.0. 

 b. Plants distinctly basophilous, soil basic, pH S.5 to 7.2. 



4. Indifferent species and communities on soils that range from basic to highly 

 acid. 



Active or Specific Acidity. — The logarithmic H number has the 

 disadvantage that it does not bring into adequate relief the considerable 

 differences in the active H ion concentrations. To remedy this, 

 Wherry (1922) has proposed a method which, starting from neutrality 

 (pH 7 = 1), gives the active acidity or alkaUnity, The active or 

 "specific acidity" indicates the quantity of H ions in one liter of solu- 

 tion, referred to the approximate H ion concentration of pure water 

 (0.0000001 g. per liter). The "specific alkalinity" means the corre- 

 sponding number of 0H~ ions. Wherry's method was promptly 

 accepted by North American plant ecologists but is little used in 

 Europe. In European literature it is mentioned by Christophersen 

 (1925), who, however, prefers the indication of acidity by Sorensen's 

 H numbers. In order to facilitate a direct comparison of Sorensen's 

 H numbers with active acidity, the corresponding acidity numbers are 

 given on page 168 with their pH equivalents. 



Measurement of pH. — Several methods are in use for the measure- 

 ment of the H ion concentration of the soil solution, and new and very 

 satisfactory apparatus for its determination are constantly appearing. 

 The colorimetric method is one of the simplest, and equipment for it is 

 least expensive and most portable. When used according to the direc- 

 tions accompanying the testing sets, the results are accurate to 0.1 to 

 0.2 pH unit, especially on the acid side. Olsen (1927) has expressed the 

 opinion that the colorimetric method is the most satisfactory and that 

 properly used it yields results that are quite sufficiently accurate for all 

 ordinary soil investigations. In Europe the "Folien colorimeter" after 

 Wulff and in America the "La Motte-Morgan soil-testing set" have 

 proved themselves satisfactory, simple, and easily manipulated. They 

 are well adapted for use in the field and, when it seems desirable, may 

 be checked against more elaborate apparatus in the laboratory. 



The most accurate method is probably the electrometric. The 

 equipment is here more elaborate and more expensive, but both in 

 Europe and in America portable sets arc available that are accurate to 



