22O PRINCIPLES OF AGRICULTURAL CHEMISTRY 



compounds of nitrogen placed in the soil tend to change to 

 nitrates. 



The absorption of ammonium salts by the roots of plants 

 can be shown in the same way, as for nitrates, namely, 

 by growing plants in solutions containing the salts. There 

 can be no doubt that plants absorb ammonia. To decide 

 whether ammonium salts can serve satisfactorily to produce 

 organic matter, is more difficult, since transformation of 

 the ammonia to nitrates must be excluded. Hampe grew 

 corn in solution containing ammonium salts and other 

 nutrients, which were repeatedly changed to exclude nitri- 

 fication. The corn was apparently not nourished well at 

 first, but later grew well. Kuhn and Wagner obtained 

 similar results. Muntz deprived a soil of nitrates by washing, 

 fertilized with ammonium salts, and took proper precautions, to 

 prevent nitrification. Corn, beans, barley, and hemp attained a 

 normal development, and their growth could only be attributed 

 to the influence of the ammonium salts used as a fertilizer. Other 

 experiments in the same direction could be cited. We must con- 

 clude that ammonium salts, as such, serve as nourishment for 

 plants. 1 



Fixation by Bacteria. Besides the bacteria which fix nitrogen 

 in connection with legumes, other nitrogen fixing bacteria occur 

 in the soil. 



These bacteria are not easily isolated, but may be separated 

 by the dilution method. They may also be separated by 

 plating. 2 The chief nitrogen-fixing bacteria are Chlostridum 

 Pasteurianum and species termed azotobacter. Under favorable 

 conditions, they decompose from 100 to 200 grams sugar for 

 each gram nitrogen fixed. 



Nitrogen fixing power is estimated by Lipman 3 by inoculating 

 loo cc. of the culture solution given below with 10 grams of soil. 

 After incubating 10 days at 28"" C. the total nitrogen is estimated, 

 the nitrogen in a portion of the original culture solution having 



1 Jour. Agr. Science 3, p. 179. 



2 Bulletin 66, Delaware Exp. Sta. 



3 Report New Jersey Exp. Sta., 1907, p. 181. 



