314 MICROBIOLOGY OF SOIL 



weighed quantities of soil may serve as a very accurate measure of 

 decay from the nitrogen standpoint. Corresponding determination of 

 nitrates may similarly be employed in tracing protein cleavage and 

 transformation as influenced by the various 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-baderia 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 su'gar solu- 

 tions 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 stu,dy of 

 ammonification, nitrification and azotofication under controlled con- 

 ditions 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 In some of his still unpublished experiments with Azoto- 

 bacter Lipman employed solutions of mannit in distilled water, provided 

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

 with the essential mine^ral constituents. In this manner interesting 

 data were secured on the availability of phosphorus compounds in 

 different soils; similarly, Christensen has suggested the use of Azofo- 

 bacier for determining ■ the lime requirements of soils, and Butkevich 

 has experimented with cultures of Aspergillus niger in determining the 

 availability of the mineral constituents. 



