ABSORPTION OF NITROGEN FROM SOIL AND AIR 55 



connection with the purely chemical idea of acid and basic 

 salts. 



The harmful effect of such physiological acidity in the nutri- 

 tive solution may be avoided by changing the solution frequently. 

 In soil cultures the effect is avoided by introducing calcium car- 

 bonate (chalk or marl) into the soil, which neutralizes the free acid 

 formed in the process of plant nutrition. Experiments carried 

 out by Prianishnikov have shown that when ammonium sulphate is 

 added to a soil simultaneously with chalk its fertilizing effect is 

 no less than that of nitrates. 



Ammonium nitrate, NH4NO3, is one of the best sources of 

 nitrogen for plants, as both of its ions are utilized at an almost equal 

 rate. The introduction of this salt into a nutritive solution does 

 not cause its reaction to shift in either direction. 



18. The Products of Nitrogen Assimilation and Its Connection 

 with Photosynthesis. — As has been mentioned, the plant requires 

 nitrogen for the purpose of forming protein substances, which 

 are an indispensable part of the protoplasm. The proteins are the 

 products of nitrogen assimilation. Their molecules, though, are 

 too complex to be considered as primary products of nitrogen 

 assimilation. It is accepted that protein formation is preceded by 

 the formation of some simpler compounds of the type of amino- 

 acids. Unfortunately, however, the chemistry of nitrogen assimila- 

 tion is not quite so clear to us as is that of carbon assimilation. 



It is easy to trace the utilization of nitrates in plants by the use 

 of a series of color reactions. These reactions enable one to know 

 that the nitric salts are absorbed by the root hairs, whence they 

 penetrate unchanged into the vessels of the root system and then 

 are transferred with water in the same unaltered state through the 

 vascular bundles, up the stem and into the leaves. When the 

 plant is placed into a dark room, an accumulation of nitrates 

 occurs in the leaves, primarily in the chlorenchyma cells. This 

 concentration is very small, however, because the ascent of water 

 is also considerably delayed in darkness. When the plant is 

 exposed to light, nitrates disappear from the cells of the leaf and a 

 simultaneous increase in protein content may be observed. 



The process of nitrate assimilation thus is closely connected 

 with that of photosynthesis. It is of importance to note that, in 

 nitric acid, nitrogen is combined with oxygen, whereas in the pro- 

 tein molecule nitrogen is in the form of the amino, — NH2 group, 



