250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



Table 3. — Specific acidity and alkalinity equivalents of Pb values. 



The substances which may yield hydrogen-ion to the soil solution 

 are listed in the following table: 



Table 4. — Soil constituents yielding hydrogen-ion. 



1. Directly {when treated with water alone): 



A. Inorganic. 



(a) Strong, highly ionized acids, like hydrochloric, sulphuric, etc. 



(6) 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 aluminium chloride, 



ammonium sulphate, etc., which are slightly hydrolyzed, and 

 therefore yield a small amount of hydrogen-ion. 



B. Organic. 



(a) Strong, highly ionized acids, like oxalic. 

 (&) Weak, slightly ionized acids, like acetic. 



(c) Acid salts, like potassium acid oxalate, which may be moderately 



or slightly ionized (as acids). 



(d) Salts of weak bases with strong acids, like aluminium citrate, am- 



monium oxalate, etc., which are hydrolyzed, as in A (d). 



(e) Amino acids, like aspartic (aminosuccinic) acid, which are inter- 



nal salts in the sense that the acidity is neutralized by the 

 amino group, and which may be moderately or slightly ionized. 



(f) Humic acids, which, if they exist at all, are slightly ionized. 



2. Indirectly (when treated with solutions of salts): 



A. Inorganic, especially colloidal clay. 



B. Organic, especially colloidal humus. 



From the above tabulation it is evident that soil acidity is likely 

 to be a rather complex phenomenon, and it is certainly misleading 

 for an investigator to look to any single substance or type of sub- 

 stances as the source of the hydrogen-ion producing it in all cases. 

 It seems probable, however, that comparatively few of these possible 

 sources of hydrogen-ion, and accordingly of acidity, coexist in appre- 

 ciable amounts in any one soil. 



It is desirable to tabulate next the methods which have been sug- 

 gested for measuring soil acidity, in more or less chronological 

 sequence, bringing together related ones. 



