256 



AZOFICATION 



the presence of sodium salts is unnecessary and depressing at least 

 to the growth of Azotobacter. The beneficial effect ascribed to sodium 

 chlorid solution in inoculating agar plates is due to the fact that 

 this liquid is isotonic with the cell content solution, but the sodium 

 hydroxid is a far less advantageous neutralizing agent than is cal- 

 cium or magnesium carbonate. Furthermore, Lipman failed to 

 stimulate the azofiers with any of the sodium salts. 



Food Requirements of the Azofiers.' These organisms probably 

 require for their nutrition the same elements as do the higher plants, 

 namely carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, 

 sulphur, calcium, magnesium, and iron, and possibly aluminum and 

 manganese. 



They obtain their carbon and hydrogen from organic compounds, 

 preferably from carbohydrates, which are considered in detail under 

 sources of energy. Oxygen is obtained either from the atmosphere 

 or from combined sources depending on the species and the condi- 

 tions under which they are grown. 



A marked difference between these and the higher plants is that 

 they possess the power of obtaining their nitrogen from the air, but 

 in the presence of combined nitrogen they obtain but little from the 

 a'lr. ' Lipman, Stranak, Heinze, and Stoklasa found that small 

 quantities of nitrates stimulated Azotobacter, whereas large quanti- 

 ties discouraged nitrogen-fixation since the organisms live on the 

 nitrates. This is the case whether the nitrates are added to the soil 

 or to the solution in which nitrogen-fixation is taking place. Cole- 

 man considers this action as due to several different factors : namely, 

 (a) a direct toxic action of the salt, (6) antagonism of other organ- 

 isms which it favors, (c) the using up of the energy supply by 

 these organisms, and (d) the discouragement of fixation by the use 

 of sodium nitrate.AThe last would seem to be the most important 

 factor when viewed in connection with the following results reported 

 by Hills: 



