DISTRIBUTION 253 



Aerobic Azotobacter are present in nearly all Utah soils. Hutchinson 

 found the Azotobacter in all the Indian soils examined. They occur 

 in cultivated more frequently and in greater numbers than in virgin 

 soils. This probably accounts for the much higher nitrogen-fixing 

 power of cultivated soils. 



Azotobacter were found in only two out of 64 localities in the soils 

 of Danish forests. Both of the soils which gave positive tests were 

 from beechwood forests and contained calcium carbonate. Although 

 the soils of these forests rarely contain enough carbonate to effervesce 

 they are usually neutral or slightly alkaline. They contain calcium, 

 but in forms other than the carbonate. It is generally understood 

 that Azotobacter occur commonly in soils which contain sufficient 

 calcium carbonate to effervesce when acid is added and that they 

 scarcely ever occur in acid soils. Their disappearance from soil is 

 usually due to the absence of basic substances, especially of calcium 

 and magnesium carbonate, and not to the presence of toxic sub- 

 stances. However, they are frequently not present in peaty soils, 

 where their absence cannot be attributed to a lack of lime. 



The aerobic nitrogen-fixers are probably more widely distributed 

 in soils than are the anaerobic, for, although both groups are gener- 

 ally found in the Russian soils, the aerobic are found in the sands 

 of Kirghese steppes and in the peat soils of the Province of Arch- 

 angel in which the anaerobic forms are absent. Anaerobic nitrogen- 

 fixers are, however, quite widely distributed in soils and are at times 

 found on the leaves of forests trees. 



The nitrogen-fixing organisms are confined almost entirely to the 

 first three feet of soil, although they have been found in soil at all 

 depths down to the tenth foot in the very favorably constituted 

 loose soils of Nebraska. 



They are most active in the upper few inches of soil, as is indicated 

 by results obtained by Ashby. 



Average 



Depth nitrogen fixed. 



Soil. inches. mgm. 



Little Hoos 10 9.23 



Little Hoos 20 7.29 



Little Hoos ... .30 4.60 



Reports on some Hawaiian soils show them to be equally active 

 at all depths to 4 feet, but this must be considered an exception, 

 for the examination of numerous soils in Utah has shown a gradual 

 decrease in nitrogen-fixing powers with depth. The average of 

 several hundred determinations, in both solution and soil media, 

 are given below : 



Nitrogen fixed in 



Nitrogen fixed in 100 cc. of Ashby's 



100 gm. of soil + solution with 1.5 



1.5 gm. of mannite. gm. of mannite. 



Depth of sample. mgm. mgm. 



Firstfoot 5.28 2.11 



Second foot . .2.42 0.77 



Third foot 1.55 0.58 



