10 T. Lyttleton Lyon and James A. Bizzell 



to the use of most higher plants, there is undoubtedly a difference between 

 plants of different species in their relation to the nitrate nitrogen content 

 of the soil. The disappearance of nitrate nitrogen from the soil during 

 the growth of different kinds of plants is not always proportional to the 

 total quantity of nitrogen removed by the crop. Thus, it is greater for 

 oats than for maize, although the latter crop may contain a much larger 

 quantity of nitrogen. Both the question of a possible influence of certain 

 plants on the process of nitrate formation in the soil, and that of the degree 

 to which some plants may use forms of nitrogen other than nitrates, are 

 raised by these phenomena. 



Boussingault first demonstrated the importance of nitrates for higher 

 plants. Previous to that time ammonia had been considered the chief 

 source of nitrogen, and still earlier humus had been considered the source. 

 Liebig gave the weight of his influence in favor of ammonia as the supply. 

 He was unaware, of course, of the transformation of ammonia nitrogen 

 into nitrates in the soil. Since the publication of the experiments by 

 Boussingault and the later work on nitrification, there has been a tendency 

 to consider nitrate nitrogen as the only available supply of nitrogen for 

 autotrophic plants. 



It is questionable whether nitrates are so universally and decidedly 

 important to plant growth as has been commonly supposed since the time 

 of Boussingault. The preference that swamp rice plants exhibit for ammo- 

 nium salts, as proved by Kellner (1884) and also by Kelley (1911), indi- 

 cates an adaptation of this variety to the prevailing condition of its 

 environment, which must be reversed on its transfer to upland soils. 

 Other plants also may adapt themselves to the use of the more abun- 

 dant form of nitrogen. 



Ammonium salts are toxic to most higher plants^ except in very dilute 

 solutions. Maze (1900) obtained as great a growth of maize with a very 

 dilute solution of ammonium sulfate as with a correspondingly strong 

 solution of nitrate. It is possible that the toxic action has masked the 

 nutritive value of ammonia in many experiments. 



In a similar way results of experiments with ammonium salts applied ^ 

 to soils may have been influenced by the acid residue remaining when the 

 nitrogen was removed. 



Hutchinson and Miller (1911) found that peas obtained nitrogen from 

 ammonium salts as readily as they did from sodium nitrate, but that 



