142 



BACTERIA AND SOIL FERTILITY 



and molds, if conditions are ideal. But the A%otobacter appear 

 to be king of all non-symbiotic nitrogen gatherers. 



Where Do Nitrogen Fixers Occur?— The nitrogen-fixing 

 organisms are widely distributed, occurring in most soils. Lipman 

 examined forty-six soils from various parts of the world and 

 found the Azotobacter in about one-third of them. 



Several hundred Utah soils have been examined and all were 

 found to fix nitrogen — many of them without the addition of 

 carbohydrates. Aerobic Azotohacter are present in nearly all 

 Utah soils. Hutchinson found the Azotohacter 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. 



Azotohacter were found in only two out of sixty- four locali- 

 ties 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 Azotohacter occur 

 commonly in soils which contain sufficient calcium carbonate to 

 effervesce when acid is added and that they scarcely ever occur 

 in acid soils. 



Soil 



Depth {cm.) 



Average Nitrogen Fixed (mgrn.) 



Little Hoos 

 Little Hoos 

 Little Hoos 



10 

 20 



JO 



9-^3 

 7.29 



4,60 



They 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 loess soils of 

 Nebraska. They are most active in the upper few inches of soils, 

 as is indicated by the above results obtained by Ashby. 



Reports on some Hawaiian soils show them to be equally active 



