350 MICROBIOLOGY OF SOIL 



The addition of ammonium and potassium salts, for example, re- 

 sults in the displacement of the corresponding calcium salts, which can 

 be washed out, and the formation of insoluble nitrogen or potassium 

 compounds which remain in the soil. On adding sodium and magnes- 

 ium salts to the soil, displacement of some of the insoluble potassium 

 salts may take place and these may become available for plant growth. 

 The interchanges taking place between the salts existing in the soil 

 and those added in the form of fertilizers have an important effect upon 

 soil biological phenomena and plant nutrition. On heating or drying 

 soils, an increase in the amount of soluble food is produced which is 

 probably a result of the change produced in the colloids. It is in this 

 colloidal complex of organic and inorganic compounds, saturated with 

 water and surrounded by the mineral particles that most of the soil 

 biological phenomena take place. 



AERATION 



MECHANICAL COMPOSITION OF SOILS. Soil ventilation is an impor- 

 tant factor in crop production. It provides for the proper supply of 

 elementary oxygen so essential to decomposition processes in normal 

 soils; for the supply of elementary nitrogen required by nitrogen-fixing 

 species; for the removal of excessive amounts of carbon dioxide; and 

 for the destruction of various toxic substances. The intimate relation 

 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 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 anaerobes; 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- 



