MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 231 



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

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

 stance, that 5000 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 clay 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 an- 

 aerobes; 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 OF OXIDATION or CARBON, HYDROGEN AND NITROGEN. Ex- 

 periments carried out by Wollny proved conclusively that the production 

 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 

 21 per cent of oxygen there were produced for every 1000 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 or- 



