1912] Lipman: Bacteria in Soils of Arid Regions 13 



therefore, in general, that where soils in the arid region are 

 supplied with a moderate amount of humus, with the proper tex- 

 ture and chemical constitution, as well as freedom from alkali, 

 all of which is true of the large majority of our soils, nitrification 

 as well as ammonification is found to obtain vigorously in the 

 lower layers of the soil for four feet at least, and in some cases 

 to six and to nine foot depths. 



NITROGEN FIXATION IN SOIL COLUMNS 



The next point of interest to determine in these soil-column 

 investigations from the bacteriological standpoint was to show 

 whether or not the supply of nitrogen, at the disposal of the 

 ammonia-forming and nitrate-forming organisms, which we have 

 found developed to such great depths, and enabling roots to 

 have a soluble nitrogen supply there, was provided merely by 

 the humus content of the soil at those depths and produced from 

 decaying roots, or carried down from the upper layers; or, was 

 that nitrogen supply in part a new one obtained directly from 

 the atmosphere by nitrogen-fixing bacteria. If such were the 

 ease, we should, of course, have enormous quantities of nitrogen 

 fixed per acre, since the fixation would not be limited to the 

 upper foot of soil. Accordingly, experiments were inaugurated 

 to obtain the facts which exist with reference to this matter. 



Here the necessary mannite solution was inoculated with five 

 grams of soil in each case, and a culture prepared from every 

 foot in depth in the case of every soil. Table III shows in 

 tabular form the results obtained, which are set forth qualita- 

 tively. The numbers at the heads of the columns refer again 

 to the numbers used in the description of soils, and one plus 

 sign is intended to show the presence of Azotobacter, two of 

 a fairly vigorous development of these organisms, and three of 

 a very vigorous development. In this qualitative way, there- 

 fore, nitrogen fixation has been judged by the development of 

 Azotobacter as a criterion. It may justly be argued against this 

 that other organisms are capable of fixing nitrogen and that the 

 quantitative figures would be preferable to the qualitative one 

 showing merely the presence of Azotobacter. While this argu- 

 ment may in part be true, it appears from my results, which 



