7o 



PHYSIOLOGY OF NUTRITION 



ganic nitrogen can be nitrified by nitrobacteria only after it has been changed 

 into ammonia or ammonium salts. The cooperation is thus necessary, of at 

 least one of the bacterial forms that give rise to ammonia from organic com- 

 pounds. Omelianskii was able to obtain nitrification of bouillon if he inocu- 

 lated the medium with three species of bacteria at the same time: Bacillus 

 ramosus, Nitrosomonas and Nitrobacter. If only B. ramosus and Nitrosomonas 

 are introduced the process is limited to the formation of nitrous acid (nitrites), 

 while B. ramosus and Nitrobacter produce only ammonia. Inoculation with 

 Nitrosomonas and Nitrobacter leaves the bouillon unchanged. All these rela- 

 tions may be shown by a diagram, reproduced below, in which the bacteria 

 that decompose organic compounds to form ammonia are represented by a, 

 those that form nitrites are represented by b, and those that oxidize nitrous 

 to nitric acid (nitrites to nitrates) are represented by c. 



Organic 

 Nitrogen 



Ammonia 

 Nitrogen 



Nitrite 

 Nitrogen 



Nitrate 

 Nitrogen 



a + b + 

 a + b 

 a + c 

 b + c 



No alteration of organic nitrogen. 



Fig. 42. — Comparison of the effect of nitrate and of ammonium salts on growth of plants 

 in bog-soil, which is poor in lime. O, no fertilizer; N0 3 , nitrate added; NH 3 , ammonium 

 salts added. (After P. Wagner.) 



Now that we have become acquainted with the process of nitrification, we 

 may consider the question whether higher plants are able to obtain their nitro- 

 gen only as nitrates or whether they can assimilate ammonium salts directly, 

 without previous nitrification of the latter. Recent discoveries favor the view 

 that nitrates act chiefly, if not exclusively, as the source of nitrogen for such 

 plants. The experiments of Wagner 1 have shown that nitrates and ammo- 

 nium salts have different effects according to the nature of the soil employed. 

 Turnips were grown in vessels of a bog-soil very poor in calcium. In one series 

 of experiments some of the vessels contained no nitrogen fertilizer, others each 

 contained 2 g. of nitrogen as nitrates, and still others each contained about 2 g. 



1 Wagner, Paul, Diingungsfragen unter Beriicksichtigung neuer Forschungsergebnisse. Heft. IV. Ber- 

 lin, 1898. 72 p. 



