NITROGEN ASSIMILATION; THE NITROGEN CYCLE 97 



Page and others failed to confirm the work of Wann, and the 

 technique of Lipman and Taylor has also been found not above 

 reproach; for the present, therefore, these results are viewed as 

 decidedly heterodox! On the other hand, there is little doubt but 

 that blue-green algae, such as Anaboena, can fix free nitrogen; and 

 in the previous chapter mention was made of the symbiosis be- 

 tween this form and Azolla. Anaboena, however, is not at all de- 

 pendent upon the Azolla, and, according to Allison and Morris 

 (1930), who it may be added found no fixation with any green 

 algae, Anaboena can fix 5 mg. of nitrogen per 100 c. c. of culture 

 medium in 75 days. These workers believe also that in some agri- 

 cultural soils, especially rich in blue-green algae, these may be the 

 most important agents of nitrogen fixation. 



The possible relation of mycorrhiza to nitrogen fixation has also 

 been mentioned. Various mycorrhiza have been reported as nitro- 

 gen fixers, and Melin (1922) found that Pinus sylvestris could be 

 grown in a nutrient solution completely lacking in nitrogen if 

 inoculated with its mycorrhizal fungus. Thus the fixation of nitro- 

 gen by fungi (including mycorrhiza and bacteria) seems to be a 

 very common occurrence, and the sum total of the nitrogen fixed 

 by all these methods is of considerable economic importance. 

 It has been estimated (Arrhenius) that all the organisms working, 

 in the soil may fix per year about 25 pounds of nitrogen per acre 

 on the average; but a good crop of legumes may add eight times 

 that much! 



The Nitrogen Cycle.— The previous pages have showed the 

 importance of nitrogen in plant nutrition and the various ways in 

 which the nitrogen is maintained in the organic cycle, as well as 

 the methods by which the great reserves of free nitrogen in the 

 air may be drawn upon. Nitrogen is one of the most expensive of 

 the elements in our food supply. It is one of the elements in 

 which soils are commonly lacking and, for this reason, is generally 

 present in commercial fertilizers. The necessity for the conserva- 

 tion of our nitrogen resources cannot be overemphasized. The 

 nitrogen passes from the soil into plants, from there into animals, 

 and back again to the soil. Any retardation of the action of 

 natural forces in restoring the organic nitrogen to the inorganic 

 form, where it can be used again by plants, withdraws that much 

 nitrogen from circulation and is to be deprecated. Enormous 

 quantities of nitrogenous material are wasted in sewage in America, 



