SOIL AND SUBSOIL, 14$ 



the presence of free oxygen (" aerobic "), while others flour- 

 ish best, or only, in the absence of air (" anaerobic "), or are 

 able to avail themselves of the presence of combined oxygen, 

 by reduction of oxicls present. Their number is found, in 

 general, to be greatest in cultivated lands, and bacteria are 

 there by far predominant over the moulds. On the other hand, 

 the moulds gain precedence in woodlands and meadows, at least 

 so far as air can gain access; while in the deeper layers of the 

 same, as well as in peaty lands, bacterial life is always scanty. 

 This holds particularly in respect to the nitrifying organisms, 

 and others whose life-functions are dependent upon abundant 

 access of oxygen (aerobic). 



Food Material Required. All bacteria, like the fungi, are 

 dependent for their development upon the presence of adequate 

 amounts of some organic food-material, best apparently in 

 water-soluble form. In the soil it seems to be chiefly com- 

 pounds of the carbohydrate group, especially various gums 

 derived from the decaying plant substance, or from stable ma- 

 nure ; in artificial cultures, glucose is mostly found to be a 

 highly available food. When the decaying substance reaches 

 the state of humus, the latter seems to be available as food 

 only to comparatively few bacteria. The very abundant de- 

 velopment of bacterial life seems to be among the most im- 

 portant effects produced by stable manure upon the surface 

 soil, in establishing good tilth ("Bodengare" in German). 



Functions of the Bacteria. While there is still much uncer- 

 tainty as to the exact functions performed by most of these 

 bacteria in respect to soil-formation and plant growth, there 

 are several kinds whose activity has been proved to be of the 

 utmost importance in one or lx>th directions; it having been 

 shown that when the soil is sterilized cither by heat or anti- 

 septic agents, certain essential processes are completely sup- 

 pressed until the soil is re-infected and the conditions of bac- 

 terial life restored. 



Probably the chief in importance are those connected with 

 the processes of nitrification and dcnitrification, bearing as they 

 do upon the supply to plants of the most costly of the three 

 substances furnished by fertilizers. These organisms have 

 been first extensively studied by Winogradsky, while the con- 

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