SOURCES OF ENERGY FOR THE AZOTOBACTER 261 



colloid. The extent of the stimulation resulting varies with the form 

 in which the iron is applied and is most effective in the form of the 

 hydroxid and in the presence of cane sugar. In this case it is 

 probable that the saccharate is the active substance. Hence, the 

 contradictory results reported may be due to the different min- 

 eral constituents of the humus. 



These facts make it certain that colloids of the metals act as 

 stimulants to nitrogen-fixing bacteria, as does also crude humus. 

 Carefully purified humates do not possess this property, but it is 

 possessed by the aqueous extract, the alcoholic extract, and the 

 phosphotungstic fraction of the aqueous extract from "bacterized" 

 peat. Whether this influence is due to a catalytic effect, as suggested 

 by Sohngen, or whether the substance furnished a direct source of 

 nutritive material is not clear at the present time. 



Moreover, the colloid may act as a protection to the organism 

 against poison; for, when 10 parts per million of soluble arsenic is 

 maintained in a soil, it acts as a stimulant to Azotobacter. If, how- 

 ever, this proportion is added to the Ashby nutritive solution it stops 

 all nitrogen-fixation. This is due in part to the adsorption of the 

 arsenic by the soil. This adsorption would have to be attributed 

 largely to the silica compounds, for the nitrogen-fixing organisms are 

 stimulated by arsenic in quartz free from organic colloids. This 

 could readily be due to the arsenic becoming concentrated at the 

 surface layers of the silica, leaving the inner part of the water film 

 comparatively free from arsenic, in which part of the water film the 

 microorganisms multiply and carry on their metabolic processes. 

 This being the case, one should and probably could find a water 

 solution weak enough to stimulate bacteria. A great difference, 

 however, between the solution and the sand-culture method is the 

 greater aeration in the sand. That the aeration of a culture medium 

 does play an important part in determining the activity of the 

 nitrogen-fixing powers of a soil is strikingly brought out in Fig. 18, 

 page 124. 



Sources of Energy for the Azotobacter. The nitrogen-fixing organ- 

 isms differ widely from other plants in their energy requirements. 

 This is due to the fact that they are carrying on endothermic reac- 

 tions in which nitrogen is concerned. This necessitates a greater 

 supply of energy than is required by other bacteria. They are 

 similar to most other bacteria in that this energy must be supplied 

 by an organic compound, preferably one of the carbohydrates. 



Berthelot in his early work maintained that the gains in nitrogen 

 noted in some soils were due to the action of biological agents on 

 the humus of the soil. This was followed by the observation of 

 others that when forest leaves are allowed to decompose in soil there 

 is an increase in its nitrogen content. Koch in 1907 increased 

 nitrogen-fixation by the addition to soil of dextrose, cane sugar or 



