BOG XEROPHYTES AND ACID SOILS. 135 



the reaction of cane-sugar inversion is a measure of its acid, and is 

 probably the only method that can measure the acid bound up with 

 the solid soil phase. 



Walker (1920) has found that more acidity was developed in muck 

 soil under water-soaked anaerobic conditions than when the moist 

 soil was kept aerated by stirring. This agrees with Conner's re- 

 sults to the effect that a peat soil develops more acid the more water 

 it contains. In this case, the drier soils are better aerated by diffu- 

 sion than the wetter ones, and the oxidation thus made possible 

 decreases the acidity. 



Wherry (1920 : 164) regards soil acidity as probably a rather com- 

 plex phenomenon, and states that it is misleading to look to a single 

 substance or type of substances as the source of hydrogen-ion pro- 

 ducing it in all cases. It seems probable that comparatively few of 

 the possible sources of hydrogen-ion, and hence of acidity, coexist 

 in appreciable amounts in any one soil. A statement is given of the 

 various sources of hydrogen-ion, as follows : 



Soil constituents yielding hydrogen-ion. 

 1. Directly (when treated with water alone). 



A. Inorganic: 



a. Strong, highly ionized acids, Hke hydrochloric, sulfuric, etc. 



b. Weak, slightly ionized acids, especially carbonic. 



c. Acid salts, like potassium acid sulphate, which may be moderately or slightly 



ionized (as acids). 



d. Salts of weak bases with strong acids, like aluminum chloride, ammonium sulphate, 



etc., which are shghtly hydrolyzed and therefore yield a small amount of 

 hydrogen-ion. 



B. Organic: 



a. Strong, highly ionized acids, like oxalic. 



b. Weak, slightly ionized acids, like acetic. 



c. Acid salts, like potassium acid sulphate, which may be moderately or slightly 



ionized (as acids). 



d. Salts of weak bases with strong acids, hke aluminum citrate, ammonium oxalate, 



etc., which are hydrolyzed as in A d. 



e. Amino acids, like aspartic (aminosuccinic) acid, which are internal salts in the 



sense that the acidity is neutrahzed by the amino group, and which may be 

 moderately or slightly ionized. 

 /. Humic acids, which if they exist at all are slightly ionized. 



2. Indirectly (when treated with solutions op salts). 



A. Inorganic, especially colloidal clay. 



B. Organic, especially colloidal humus. 



Gillespie (1916) has investigated the hydrogen-ion concentration 

 of 22 soils in water suspensions and found the range of H-ions to be 

 from pH 4.4 to pH 8.6. 



Sharp and Hoagland (1916 : 123) have measured the acidity of 

 24 different soils by means of the hydrogen electrode, certain of them 

 giving an H-ion concentration as high as 0.2 X 10-' and hence pos- 

 sessing a considerable intensity of acidity. The hydrogen-ion 

 concentration of different soil suspensions varied within wide hmits, 



