MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 351 



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 

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

 this air contained 2 1 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. 



THE MINERALIZATION OF ORGANIC MATTER. Conditions that favor 

 the intense activities of decay bacteria lead to a relatively rapid restora- 

 tion of the phosphorus, sulphur, calcium, magnesium and potassium 

 that had been made fast in plant tissues, to the stock of available plant 

 food in the soil; indeed, in extremely well-aerated soils the decomposition 

 of organic matter and its ultimate mineralization proceed too fast. It 

 often happens that the farmer is unable to maintain a proper supply 

 of humus in these soils because of their openness and is forced to adopt 

 measures that will retard soil aeration. He resorts therefore, to rolling, 

 marling, manuring and green manuring. 



On the other hand, heavy, fine-grained soils are not sufficiently well 

 aerated to allow a rapid mineralization of the organic matter. Under 

 extreme conditions the decomposition processes do not keep pace with 

 the process making toward the accumulation of organic matter, and a 

 more or less considerable increase in the amount of the latter takes 

 place. This occurs in low lying meadows, and, more particularly, in 



