94 SOIL CONDITIONS AND PLANT GROWTH 



Mg. of Nitrogen 

 Fixed. 



7*5 to 10 

 5 to 7-5 



2-5 to 5 



i to 2-5 



Nil 



Mannitol, xylose, lactose, laevulose, inulin, galactose, maltose, dextrin, 



sucrose + calcium carbonate. 

 Sucrose alone, dextrose, sodium tartrate + calcium carbonate, glycerol 



4- calcium carbonate. 



Starch, sodium tartrate, sodium succinate, calcium lactate. 

 Sodium propionate, sodium citrate, glycerol alone. 

 Calcium butyrate, potassium oxalate. 



Little is known of the chemistry of the change, even the fate of the 

 sugar has not been definitely ascertained. The only obvious product 

 is carbon dioxide, fatty acids being formed only in small quantities, in 

 sharp contrast with clostridium. 



Starting with 15-9 grams of dextrose Stoklasa (276) recovered 7*9 

 as carbon dioxide, 0*3 as ethyl alcohol, O'2 as formic acid, 07 as acetic 

 acid, O'2 as lactic acid, but could not trace the remaining 6*5 grams. 



Beijerinck's solution works satisfactorily for crude cultures but not 

 for pure cultures. Various hypotheses have been put forward in ex- 

 planation ; it was supposed that azotobacter required the presence of 

 some other organism, or that it lost its efficiency on cultivation- 

 Krzemieniewski (1570) found that neither of these views is correct, 

 and in an important investigation he showed that the determining factor 

 is the presence of a little soil ; so long as this is added pure cultures 

 retain their effectiveness. The active agent is the humus, but its effect 

 is not to furnish carbon or nitrogen to the organism; further, the 

 humus loses its power after treatment with hydrochloric acid. Remy and 

 Rosing (237^) frankly call it a stimulating action and attribute it to 

 the iron invariably present. 



The nitrogen is found partly in compounds dissolved in the liquid, 

 but mostly in the bacterial mass. The organism is remarkably active, 

 one gram weight evolving no less than I '3 grams of CO 2 in twenty-four 

 hours (273). An adequate supply of phosphate, calcium carbonate and 

 other mineral nutrients is essential, any deficiency limiting the amount 

 of fixation. Traces of nitrogen compounds are helpful in the early 

 stages, but larger quantities reduce the amount of fixation, and may 

 themselves suffer some change : thus sodium nitrate is partially re- 

 duced to nitrite and ammonia. Several forms of azotobacter are 

 now known : A. agilis^ A. vinelandii, etc., and also various less 

 efficient nitrogen fixers that more resemble clostridium, such as amylo- 

 bacter (53) and granulobacter, some of which are aerobic and others 

 anaerobic, but all form spores. 1 The great distinction between the two 

 groups is that the azotobacter give carbon dioxide as the chief product 



1 A list is given by C. B. Lipman in Journ. Biol. Chern., 191 1, x., 169-82. 



