274 AZOFICATION 



methods differ greatly, the relative efficiency of the organisms is 

 about the same in both cases. In testing soils similar results are 

 obtained, as may be seen from the following results, which are the 

 average for several hundred determinations made on different soils 

 by the two methods. 



Nitrogen fixed in: 



~~100 c.c. ofAshby's 

 100 gm. of soil containing 1.5 gm. 



Depth of Sample. 1.5 gm. of mannite, of mannite 



mgm. mgm. 



Firstfoot . 5.28 2.11 



Second foot . . . 2.42 0.77 



Third foot . . 1.55 0.58 



Although the greater aeration in the sand and soil culture probably 

 play a great part, there is little doubt that the colloids also assist. 



Relation of Azotobacter to other Organisms. In the early study of 

 nitrogen-fixation, the view was held that algae growing on or near 

 the surface of soil are able to fix nitrogen. Frank in 1888 had 

 observed such a growth on sand exposed to light and found that the 

 soil showed a considerable increase in nitrogen. In 1892 Schlosing 

 and Laurent proved, both by determining the nitrogen fixed by a 

 soil in a closed vessel and by observing the diminution of the nitrogen 

 gas in the enclosed air, that a soil exposed to light gains in nitrogen 

 if algae are allowed to grow on the surface, and that the gain is 

 confined to the upper few millimeters. They did not, however, 

 employ a pure soil or pure cultures of algae. Kossowitsch, working 

 with pure cultures of two green algae, found no fixation, but observed 

 a considerable increase of soil nitrogen when they were grown with 

 soil bacteria. Later, Kruger and Schneidewind, employing pure 

 cultures of many other chlorophyceae, obtained no nitrogen-fixation, 

 Hellriegel and Deherain had found a large increase in the nitrogen 

 content of sand in pots when exposed to the light, which was always 

 accompanied by a development of algae. In the light of such results, 

 the conclusion has been reached that algae alone cannot assimilate 

 free nitrogen, but only in concurrence with soil bacteria, the former 

 producing carbohydrates which are used by the latter as a source 

 of energy for the nitrogen-fixation. Heinze actually observed rapid 

 fixation of nitrogen when cultures of alga? were inoculated with Azoto- 

 bacter or other nitrogen-fixing organisms. Stoklasa found that Azoto- 

 bacter are especially abundant in soils having a vigorous growth of 

 blue-green algae. Azotobacter are often absent from virgin soils, 

 but are always found in such soils when there is a vigorous growth 

 of algae. Bottomley claims that both Azotobacter and Pseudomonas 

 live in true symbiosis with cycas. It, therefore, appears certain 

 that the nitrogen-fixing powers of Azotobacter are greatly enhanced 

 when growing with algae, but the exact role played by each is yet to 

 be explained. This offers a rich and inviting field for research. 



Nor is it alone in combination with algae that these organisms 



