Current Literature 57 



Soil Acidity. By J. E. Harris. Technical Bulletin 19, Michi- 

 gan Agricultural College Experiment Station. 1914. Pp. 1-13. 



The theory that soils become acid through the accumulation 

 of complex insoluble organic acids had its beginning with Spren- 

 gel in 1826, when he separated from a soil solution a brown pre- 

 cipitate to which he gave the name humic acid. During the next 

 few years, to humic acid were added ulmic acid, crenic acid and 

 apocrenic acid, as ingredients of acid soils. Although chemical 

 formulae were given these substances, it was demonstrated about 

 thirty years ago that their component parts held to no definite 

 ratios. At the present time most agricultural chemists, who have 

 done the most work upon them, do not regard the so-called humic 

 acids as definite compounds at all. Some of them hold that the 

 humus substances are colloidal in nature and that the so-called 

 humates are adsorption compounds. 



When acid soils are tested with blue litmus paper, it very 

 quickly turns red. Now the peculiar thing about this reaction is 

 that the litmus paper must be brought in direct contact with the 

 soil particles for the change of color to take place. When a clear 

 water solution of an acid soil is tested with litmus paper, it is 

 usually found to be neutral. If, however, we substitute for water 

 a solution of a neutral salt such as potassium nitrate, a consid- 

 erable quantity of free acid will be found in the resulting solu- 

 tion. These phenomena have been usually explained on the 

 ground that the soil acids are insoluble in water and that in the 

 case of the neutral salt, a double decomposition takes place be- 

 tween the insoluble acid and the salt, the base of the salt combin- 

 ing with the humic acid and the acid of the salt being set free. 



Numerous experiments showing that in the presence of colloids 

 certain bases are adsorbed with a setting free of their constituent 

 acids, have led chemists to doubt the explanation given above 

 for the behavior of soil acids. They point out that blue litmus will 

 turn red in contact for a short time with acid free cotton. This 

 phenomenon can be explained by ascribing to the cotton a selec- 

 tive adsorbing power for the base of the blue litmus salt, the 

 base being adsorbed while the red acid dye remains in the paper. 

 Now the action of acid soil towards litmus may be explained in 

 exactly the same way. Also in the action of the soil towards the 



