MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 245 



formation as influenced by the numerous factors of season, soil and 

 cultivation. 



ADDITION OF NITROGEN. At least one other bacteriological factor 

 in soils should be mentioned here as deserving attention in a systematic 

 study of soil fertility from the nitrogen standpoint. It is known that 

 Azo-bacteria are widely distributed in arable soils, and that they are 

 more prominent in some regions than they are in others. The student 

 of soil fertility finds it desirable, therefore, to study azotofication in 

 different soils, and employs (for this purpose) mannit solutions like 

 those proposed by Beyerinck, sand cultures supplied with sugar solutions 

 like those proposed by Fischer, or weighed quantities of soil mixed with 

 sugar as suggested by Koch. 



The methods referred to above make possible thus the study of 

 ammonincation, nitrification and azotofication under controlled conditions 

 and permit, thereby, the measure of bacteriological factors in soil fertility 

 from the nitrogen standpoint. 



REACTIONS CONCERNING CALCIUM, MAGNESIUM, SULPHUR AND 

 PHOSPHORUS. In addition to the purely chemical methods available for 

 the study of these constituents, microbiological methods have also been 

 suggested. In some of his still unpublished experiments with A zotobacter 

 Lipman employed solutions of mannit in distilled water, provided with 

 small quantities of sterile soil which were to supply the organisms with 

 the essential mineral constituents. In this manner interesting data were 

 secured on the availability of phosphorus compounds in different soils; 

 similarly, Christensen has suggested the use of Azotobacter for determining 

 the lime requirements of soils, and Butkevich has experimented with 

 cultures of Aspcrgillus niger in determining the availability of the mineral 

 constituents. 



