200 SOIL CONDITIONS AND PLANT GRO WTH 



shown both in culture solutions and in soil.^ Several forms 

 of azotobacter are now known : A. agilis, A. vinelandii, etc., 

 and also various less efficient nitrogen fixers that more re- 

 semble Clostridium, such as amylobacter (52) and granulo- 

 bacter, some of which are aerobic and others anaerobic.^ The 

 great distinction between the two groups is that the azotobacter 

 give carbon dioxide as the chief product from the sugar, while 

 the others, even the aerobic organisms, form butyric acid in 

 considerable amount and fix smaller quantities of nitrogen. 

 Amylobacter also makes and stores glycogen, a property pos- 

 sessed by few other micro-organisms. 



Kossowitch ( 1 54(3:) has shown that a mixture of azotobacter 

 and algae, especially nostoc, can work together very well, the 

 algae furnishing the necessary carbon compounds, while the 

 azotobacter fixes nitrogen, an observation that has been con- 

 firmed by Bouillac (43 and 44). Sand has been found to gain 

 nitrogen where the growth of algae was possible and the 

 proper bacteria were present.^ Evidence is now being ad- 

 duced that some of the algae can fix nitrogen by themselves, 

 and without the co-operation of azotobacter* (p. 255). 



How far azotobacter is active in the soil in natural condi- 

 tions has not been definitely ascertained, partly because of the 

 analytical difficulties of measuring small gains of nitrogen, 

 and partly because of the losses of nitrogen that, as we have 

 seen, go on in presence of organic matter. The mere occur- 

 rence of azotobacter in the soil is no proof that it is actually 

 fixing nitrogen, the only satisfactory evidence would be a 

 demonstrated gain in nitrogen effected by azotobacter, all 

 other possibilities being ruled out by the experimental condi- 

 tions. The usual method of investigation has been to add 

 sugar, or other carbohydrates, to the soil and measure the 

 change in nitrogen content after various intervals of time. 



^ D. A. Coleman obtained less fixation in presence of nitrate (Soil Sci., 



1917. 4. 345). 



^ A list is given by C. B. Lipman in yourn. Biol. Chem., 1911, x., 169-182. 

 ^A. Koch has collected instances in Lafar, Tech. Mykologie, Bd. iii., p. 15. 

 •* B. Moore and T. A. Webster, Proc. Roy. Soc, 1920, B 91, 201. 



