NON-SYMBIOTIC NITROGEN FIXATION 143 



at all depths to four feet, but this must be considered an ex- 

 ception for the examination of numerous soils in Utah has shown 

 a gradual decrease in nitrogen-iixing powers with depth. The 

 average of several hundred determinations in both solution and 

 soil media are given below: 



Depth of 

 Sample 



Nitrogen Fixed in loo gm. of 

 Soil with 7.5 gm. Mannite 



Nitrogen Fixed in 100 cc, of 

 Ashh-fs Solution with 1,5 Mannite 



First Foot 

 Second Foot 

 Third Foot 



mgm, 



5.28 

 2.42 



1-55 



mgm. 

 2.11 

 0.77 

 0.58 



These samples were collected with such great care that there 

 was no possibility of the mixing of one foot-section with another. 

 There is about one-half as much nitrogen fixed in the second as 

 in the first foot and one-fourth as much in the third as in the 

 first 



Condition for Growth. — ^The distribution and the physiologi- 

 cal efficiency of the nitrogen-fixing organisms, especially of the 

 A%otohacter species, are governed by the physical and chemical 

 properties of the soil, foremost among which is its calcium or 

 magnesium carbonate content. Some workers make use of this 

 in obtaining pure cultures of Axotohacter, for it is found that by 

 picking out the crystals of the carbonate from the soil and seed- 

 ing them into nitrogen-free media the likelihood of obtaining 

 the organism is greatly increased. The addition of calcium car- 

 bonate to a soil often increases its nitrogen-fixing power, the 

 extent depending on the lime requirements of the soil and on 

 the fineness of the added limestone. 



Christensen has suggested that the Azotobacter be used as an 

 index of the lime requirements of a soil. The test should in- 

 clude both a search for the organisms in the soil and a test of their 

 ability to grow when innoculated into the soil. He and Larson 

 examined more than a hundred soils of known lime requirement. 



