254 AZOFICATION 



These samples were collected with such great care that there was 

 no possibility of the mixing of one foot section with another. It is 

 interesting to note that while the actual gain in nitrogen per gram 

 of mannite is over twice as great in the soil as in the solution, vet 

 the relative gain per foot section is the same in both. 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. 



The nitrogen-fixing organisms are not confined to the soil alone, 

 for Beijerinck and van Delden first isolated Azotobacter agilis from 

 canal water in Holland. Azotobacter chroococcum and B. Clostridium 

 pasteurianum are both found in many fresh and salt waters, living 

 on algse and plankton organism. 



Reaction of the Media. The distribution and the physiological 

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

 bacter species, are governed by the physical and chemical properties 

 of the soil, foremost among which is the basicity of the soil, namely, 

 its calcium or magnesium carbonate content.^ Ashby bases his 

 method for obtaining pure cultures of Azotobacter upon this property, 

 for he finds that by picking out the crystals of the carbonate from 

 the soil and seeding them into nitrogen-free media the likelihood of 

 obtaining the organism is greatly increased. The addition of calcium 

 carbonate to a soil often increases its azofying power, the extent of 

 which increase depends 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 

 to the lime requirements of a soil. The test should include both a 

 search for the organism in the soil and a test of their ability to grow 

 when inoculated into the soil. He and Larson examined more than 

 one hundred soils of known lime requirement. They determined 

 the carbon dioxid set free by acids, the amount of calcium dissolved 

 by an ammonium chlorid solution, the behavior of the soil toward 

 litmus, and the biological test. The result of this test was that the 

 biological test agreed with the known condition in 90 per cent, of the 

 cases, the ammonium chlorid in 50 per cent., the litmus in 40 per 

 cent., and the carbon dioxid failed more often than not to indicate 

 the correct condition of the soil. 



Fischer failed to find Azotobacter in a heavy loam soil containing 

 only 0.145 per cent, of lime, while adjoining limed plots had an 

 Azotobacter flora. The quantity of calcium carbonate which must 

 be added to obtain maximum fixation varies with the soil. 



A West Virginia Dekalb silt loam, which required 0.175 per cent. 

 of calcium carbonate to render it neutral by the Veitch method, 

 gave greatest nitrogen fixation when 0.375 per cent, of calcium 

 carbonate was added. Above this concentration azofication 

 decreased, but when phosphorus was applied with the lime it was not 

 toxic even when present in quantities as great as 0.5 per cent. It is 



