BIOLOGY OF SOILS 141 



the sterile condition cannot be maintained unless the re- 

 entrance of bacteria is prevented by special precautions. 



Chemical Changes due to Bacteria. The chemical 

 changes due to bacteria are too numerous to recount, and 

 too complex to be explained here. In many cases the 

 nature of the change is either imperfectly understood 

 or totally unknown. The general tendency, however, is 

 to decompose compounds of complex molecular structure 

 and produce simpler substances by splitting off groups 

 or even single elements, and by hydrolysis, oxidation, 

 reduction, etc. By the researches of Pasteur and 

 others many diseases of animals, e.g., tuberculosis, 

 cholera, etc., have been traced to the action of bacteria ; the 

 souring of milk, fermentation of urine, and many other 

 familiar changes are due to the same cause. In some 

 instances the specific organisms which produce these various 

 effects have been isolated and identified ; the pathogenic 

 forms have received the most attention, and, as there are 

 more workers in that field, they are generally better known. 



Bacteria also take part in the formation of organic 

 matter in soils. They are found on bare rock surfaces, 

 at mountain tops and elsewhere, under conditions which 

 show that they must derive nourishment from the atmos- 

 phere. Notwithstanding the absence of chlorophyll, it is 

 believed that certain forms can obtain carbon from carbon 

 dioxide, and can assimilate free nitrogen or obtain that 

 element from ammonia. Small quantities of organic 

 matter are thus produced, and are generally found adhering 

 to the particles of disintegrated rocks in the early stages 

 of soil formation. The most important of the changes due 

 to bacteria in the soil, however, are the decomposition of 

 organic matter either nitrification or putrefactive decay 

 and the fixation of free nitrogen, directly like algse, or 

 indirectly by symbiotic association with higher plants, 

 chiefly, if not exclusively, belonging to the order leguminosae^ 



