294 MICROBIOLOGY OF SOIL 



' existing between soil ventilation and the mechanical composition of the 

 soil material is bound to react on the microbial factors involved. It is 

 well known that the rate of flow of air through soils is inversely propor- 

 tional to the fineness of the material; in other words, the fine-grained 

 soils, notwithstanding their greater pore space, will not allow air to 

 pass through them as rapidly as coarse-grained soils. King shows, for 

 instance, that 5,000 c.c. of air passed through a column of fine gravel 

 in thirty-seven seconds, whereas in similar columns of medium sand, 

 fine sand, loam and fine claly soil the same amount of air required for its 

 passage 1,178, 44,310, 282,200, and 2,057,000 seconds respectively; 



Aerobic and Anaerobic Activities. — ^The more rapid diffusion 

 of gases from open soils naturally leads to a more frequent renewal of 

 their oxygen supply. In its turn, the latter affects the ratio of aerobes 

 to anerobes; it follows, therefore, that in clay soils and clay loam soils 

 the activities of aerobic species are retarded to a greater extent than 

 they are in sandy loams or sandy soils. It follows, also, that in fine- 

 grained soils the activities of the aerobes are confined to a shallower 

 soil layer than in coarser grained soils. The reverse is true of anaerobic 

 species. Methods of soil treatment tending to improve soil ventilation 

 react both on the amount of chemical change produced by definite 

 species, as well as the numerical ratio of different species to one another. 

 Among such methods may be included drainage, liming, manuring and 

 tillage. 



Rate or Oxidation of Carbon, Hydrogen and Nitrogen. — : 

 Experiments carried out by Wollny proved conclusively that the pro- 

 duction of carbon dioxide in soils is directly affected by the amount of 

 oxygen supplied; that is, by the more or less thorough aeration of the 

 soil. In one of these experiments air containing varying proportions 

 qf oxygen and nitrogen was passed through columns of soil. When 

 this air contained 21 per cent of oxygen there were produced for every 

 1,000 volumes of air 12.51 volumes of carbon dioxide; while with 2 per 

 cent of oxygen in the entering air there were produced only 3.62 

 volumes of carbon dioxide. Similar observations were made by 

 Schloesing in connection with the formation of carbon dioxide and of 

 nitric acid. Deherain and many others have recorded the favorable 

 influence of aeration on the rate of nitrate formation, while Lipman 

 and Koch have observed an increased fixation of nitrogen by Azotobacter, 

 consequent upon a better supply of oxygen. 



