NITROGEN ASSIMILATION; THE NITROGEN CYCLE 99 



7. Are there any dangers from the direct use of sewage as fertilizer? 



8. What is a catalyst? 



9. What is the Haber process of nitrogen fixation? 



10. Do soils ever lose any nitrogen aerobically? 



11. Why were the farmers of the southern United States so interested in 

 the Muscle Shoals power project? 



12. Is cremation or burial more wasteful from the point of view of the nitro- 

 gen cycle? 



13. Does the abundant use of nitrogen on crops ever result in any dis- 

 advantages? 



14. Can Rhizobium radicicola free in the soil fix nitrogen or must it be in the 

 nodules of some host plant? See Soil Science, 29 :37, 1930. 



REFERENCES 



Bristol, M., and Page, H.— A critical inquiry into the alleged fixation of 

 nitrogen by green algae. Ann. Appl. Biol, 10:378, 1923. 



Cottrell, F. G .— Fertilizer from the air. Sci. Mo., 21 :245, 1925. 



Gowda, R— Nitrification and nitrifying organisms. J. Bad., 9:251, 1924. 



Gray, P. H— Soil bacteria and fertility. Sci. Prog., 23:444, 1929. 



Rayner, M. — Nitrogen fixation in the Ericaceae. Bot. Gaz., 73:226, 1922. 



Russell, E. J.— Soil Conditions and Plant Growth, Longmans. 1921. 



Senn, G— The assimilation of the molecular nitrogen of the air by lower 

 plants, especially by fungi. Biol. Rev. & Biol. Proc. Cambridge Phil. Soc, 



3:77, 1928. 

 Spratt, Ethel. — A comparative account of the root nodules of legumes. 



Ann. Bot., 33:189, 1919. 

 Wann, F. — Fixation of nitrogen by green plants. Am. J. Bot., 8:1, 1921. 

 Winters, N. — Soil conditions which promote nitrogen fixation. /. Am. Soc. 



Agron., 16:701, 1924. 



