THE CYCLE OF ELEMENTS IN NATURE 243 



ous important processes in which microorganisms take an active 

 part: rock disintegration and soil formation, decomposition of 

 organic matter in soil, liberation of nitrogen in an available form as 

 ammonia, liberation of carbon as carbon dioxide, formation of nitrate, 

 fixation of nitrogen, disappearance of available nitrogen from soil, 

 synthesis of microbial cell substance, oxidation and reduction of soil 

 constituents, transformations in composted stable manure, production 

 of artificial manure, disintegration of green manures, liberation of 

 mineral elements such as phosphorus from relatively insoluble sub- 

 stances, transformation of sulfur, the improvement of certain plant 

 growth following soil inoculation, changes following partial steriliza- 

 tion treatments, injury to plants and animals by bacterial and fungus 

 parasites. Some of these processes may be considered at some- 

 what greater length. 



Role of Microorganisms in the Cycle of Elements in 

 Nature. — The activities of microorganisms in the soil are respon- 

 sible for completing in nature the cycles of various elements, 

 especially those that enter into the composition of organic sub- 

 stances. Here may be included first of all the element carbon 

 which is the basis of all organic compounds. The element nitrogen 

 which, in combination with carbon, hydrogen, and oxygen, forms 

 the most important group of constituents of all hving protoplasm, 

 is of no less importance. The mineral elements sulfur, phosphorus, 

 and potassium are likewise absolutely essential for the growth of 

 green plants and are frequently the controUing factors in develop- 

 ment of plants in soil. A number of other elements, such as iron, 

 calcium, and magnesium, are directly or indirectly acted upon by 

 microorganisms in their cycle in nature. 



Chief among the activities of microorganisms is the minerali- 

 zation of organic matter in nature. The abundant life of green 

 plants in and on the surface of the earth, from the microscopic 

 algae to the thousand-year-old evergreen and deciduous trees, 

 tends to build up organic matter from inorganic elements or from 

 simple inorganic compounds. From the carbon dioxide in the 

 atmosphere as the only source of carbon, from the nitrogen present 

 in the soil in the form of simple compounds (ammonium salts and 

 nitrates), out of the phosphorus, potassium, sulfur, iron, calcium, 

 magnesium, and traces of other elements in the form of simple 

 inorganic compounds, in addition to water and gaseous oxygen, the 

 green plants are capable of manufacturing every year several tons 



