1912] Lipman: Bacteria in Soils of Arid Regions 15 



aware, constitutes the only published case of even this extent of 

 penetration of Azotobacter organisms. It has been reported to 

 me, however, that Azobacter organisms have been found in the 

 twelfth foot of soil in some of the very favorably constituted loess 

 soils of Nebraska. The question put in the introduction to this 

 subject of nitrogen fixation is therefore answered in the negative. 

 For the greater depths, at any rate, in which ammonification 

 manifestly is vigorous in our soils, Azotobacter organisms do 

 not penetrate and are not the source of the supply of nitrogen 

 which can be transformed at those depths by the ammonifying 

 organisms or by the nitrifying organisms. The nitrogen supply 

 of these, therefore, in the lower layers of the soil must be the 

 humus produced from the decaying roots at those depths, or 

 the humus brought down in solution from the upper layers of 

 the soil. 



As regards soil No. 6 we have here again a total absence 

 of Azotobacter organisms, possibly due in part, at least, if not 

 wholly, to the absence of any but very small amounts of humus, 

 by which I have already tried to explain the feeble nitrification 

 only in the first foot of this same soil. The same remarks 

 also which were made above, with reference to soil Nos. 8, 9, and 

 10, as regards their ammonifying and particularly their nitrify- 

 ing power, apply again in the case of their nitrogen-fixing power. 

 No Azotobacter organisms and no fixation of nitrogen were ever 

 observed in any of these soils, no matter from what depth of soil 

 the cultures were prepared. 



In justice to this subject it must further be stated here that 

 the comparatively slight penetration of Azotobacter organisms 

 in our soils may be due to factors other than merely a lack of 

 a plentiful supply of oxygen. There is evidently some other 

 circumstance which controls the presence or absence in many 

 of our soils of Azotobacter organisms and that may also limit 

 the depth to which th&se organisms may penetrate. Just what 

 this factor may be is not at present clear to the writer, but the 

 fact remains that frequently soils with a good chemical and 

 physical constitution and producing good crops, will yet show 

 no Azotobacter organisms. 



