370 PRINCIPLES OF SOIL MICROBIOLOGY 



necessary for the continuation of life and characteristic of the living 

 cell. Most of these reactions are carried on in the cell by the agency 

 of organic catalysts or enzymes, which may also be secreted outside 

 of the cell; this allows certain chemical reactions to take place outside 

 of the cell. 



The chemical reactions depend on the presence of specific substances 

 or substrates, the chemical and physical condition in which these 

 substances are present, temperature, reaction, etc. The activities 

 of the microorganisms will result in a change both in nature of the 

 substrate as well as in the condition of the medium in which they work. 

 Celluloses and proteins, substances of high molecular weight and low 

 osmotic pressure, will be changed by processes of hydrolysis, to sugars 

 and organic acids or to peptides and amino acids, substances of low 

 molecular weight and high osmotic pressure. On the other hand, 

 the absorption of soluble nitrogen salts and minerals and their syn- 

 thesis into microbial protoplasm will bring about a reverse condition. 

 The ionic exchange in the living cell, as the absorption of the base in 

 the case of ammonium salts or absorption of the acid in the case of 

 nitrates, will tend to leave the medium more acid or more alkaline 

 respectively. The formation of organic acids, such as gluconic, citric, 

 oxalic and fumaric by fungi, lactic, formic, propionic and acetic 

 by bacteria, and inorganic acids, such as carbonic, nitrous, nitric 

 and sulfuric, will also lead to a change in the hydrogen-ion concentra- 

 tion of the medium. These acids will combine with the insoluble and 

 soluble bases forming new salts. Some of these, such as the salts of 

 organic acids, may be used as sources of carbon, liberating the bases 

 which will combine with the carbon dioxide of the atmosphere to form 

 carbonates, and again change the reaction of the medium from acid 

 to alkaline. Others, like the nitrates and sulfates, may be again 

 assimilated by microorganisms and higher plants. These may be washed 

 from the soil in the drainage waters, they may be absorbed by the soil 

 colloids, or they may form simple or complex salts with the various 

 inorganic or organic soil constituents. 



All of these reactions bring about constant changes in the osmotic 

 concentration and the reaction of the medium. This is further ac- 

 centuated by the formation of electrolytes from non-electrolytes (am- 

 monia and nitrates from proteins and amino acids, phosphates and 

 sulfates from complex protoplasm) and vice versa. It is important, 

 therefore, to gain knowledge of the osmotic concentration of the soil 



