242 MICROBIOLOGY OF SOIL. 



spherical organisms is markedly increased. This happens, particularly, 

 when large quantities of composted manure (rich in spherical organisms) 

 is added to the soil. 



Among the rod-shaped species B. mycoides, B. stibtilis, B. mesentericus 

 B. tumescens and other members of the subtilis group are quite prominent 

 Members of the amylobacter group are seldom absent. Members of the 

 proteus group and various fluorescents are always present, while Bact. 

 arogenes and allied species are common inhabitants of the soil. 



(Physiological Groups}. In the decomposition of organic matter in 

 the soil certain important changes in both nitrogenous and non-ni- 

 trogenous material are accomplished by definite groups of bacteria. The 

 breaking down of protein substances is accomplished by the formation of 

 ammonia, nitrites and nitrates. These in turn may be transformed back 

 into more complex amino- compounds, peptones, and proteins, or they may 

 be destroyed with the evolution of free nitrogen. Moreover, there are 

 groups of bacteria capable of joining non-nitrogenous organic matter to 

 elementary nitrogen and of fashioning 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 pro- 

 ducts, methane and hydrogen. There are, likewise, definite groups of 

 bacteria concerned in the transformation of sulphur and its compounds, 

 and of ferrous compounds. 



METHODS OF 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 beef 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. 

 It is evident, therefore, that the results secured in the counting of soil 

 bacteria have a relative value only. With the same media and methods 



