MICROORGANISMS AS A PACTOR, IN SOIL FERTILITY 3II 



teins, or they may be destroyed with the evolution of free nitrogen. 

 Moreover, there are groups of bacteria capable of joining non-nitro- 

 genous organic matter to elementary nitrogen and of producing thus 

 nitrogen compounds. Again, there are groups of bacteria bearing 

 distinct and important relations to the decomposition of cellulose, or 

 the transformation of its cleavage products, methane and hydrogen. 

 There are, likewise, definite groups of bacteria concerned in the 

 transformation of sulphur audits compounds, and of ferrous compounds. 



Methods or Study 



Quantitative Relations. — Since the early work of Koch in 1881 

 many investigators have determined the number of bacteria in soil 

 samples, by means of the plate method. It is well known, however, 

 that on ordinary gelatin or agar plates kept under aerobic conditions 

 but a fraction of the soil organisms produce visible colonies. The 

 anaerobic species do not appear, nor do aerobic Azotobacter, and nitro- 

 bacteria, while other common soil organisms form colonies sparingly 

 or not at all. By employing synthetic agar media instead of be'ef broth 

 gelatin or agar, Lipman and Brown have succeeded in securing the 

 growth of a much larger number of colonies from any given quantity 

 of soil, yet even these larger numbers were incomplete for reasons 

 mentioned above. 



H. Fischer recommends a simple medium of agar to which nothing 

 has been added but soil extract (prepared by extracting with a .1 

 per cent solution of NaaCOs) and potassium phosphate. Following 

 the path of Lipman and Brown in reducing the content of organic 

 matter, Temple employed i g. of peptone per 1. as a culture medium 

 and obtained satisfactory results. Brown has further modified the 

 formula of Lipman and Brown by replacing the .05 g. of peptone with 

 .1 g. of albumin, and obtained results which were somewhat superior. 

 In a comparison of culture media. Conn considers the former media 

 undesirable for quantitative purposes because they contain substances 

 of indefinite chemical composition, and offers an agar medium con- 

 taining no organic matter except agar, dextrose and sodium asparag- 

 inate, and also a soil-extract gelatin which is valuable for qualitative 

 purposes. Another medium that has been suggested, after a com- 

 parison of all of the above-mentioned media, is the urea-ammonium 



