The Relation to Nitrogen 231 



tionably the addition of ammonium salts to the soil gave 

 increased yields, and the inference was that they were 

 directly beneficial; that is, that they were absorbed as 

 salts of ammonium. 



Subsequently, when it was determined that, as a result 

 of nitrification, ammonium compounds may be oxidized, 

 ultimately to nitrates, the dominant view was to regard 

 nitrates as practically the only source of nitrogen for crops. 

 This is still held by many, but the relatively recent exper- 

 iments of Maze, 1 Hutchinson and Miller, 2 and others 

 seem to indicate that salts of ammonium are directly 

 absorbed. In some cases nitrogen in the latter form 

 afforded growth equal to that where nitrates were em- 

 ployed, and the nitrogen content of peas is reported greater 

 when ammonia is the source of nitrogen. Nevertheless, 

 the salts of ammonium are more toxic than nitrates, this 

 toxicity exhibiting itself at the lower concentrations merely 

 in depressing the growth of roots. 



125. The sources of soil nitrates and ammonia. 

 Briefly stated, the supply of nitrates and ammonium com- 

 pounds in the soil annually removed by crops, by leach- 

 ing, and through denitrification (section 130) are or may 

 be renewed by the following means, most of which are 

 subsequently discussed : 



(1) By the decomposition or decay of organic matter, 

 accomplished by microorganisms. 



(2) By means of the bacteria producing and inhabiting 

 the root-tubercles of leguminous plants, which bacteria 

 possess the power to " fix " atmospheric nitrogen. 



1 Maze, P., Ann. de 1'inst. Pasteur, 14 : 23-45, 1898. 

 * Hutchinson, H. B., and Miller, N. H. J., Journ. Agl. Sci., 3 : 179- 

 194, 1909. 



