SYNTHETIC ACTIVITIES OF MICROORGANISMS 245 



origin, are introduced into the soil, the numerous bacteria, fungi, 

 protozoa and other invertebrates immediately become active 

 and rapidly attack the various constituents, changing them back 

 to the simple compounds from which the plant started to manu- 

 facture its tissues. This process or group of processes may be 

 carried out by one or more than one group of organisms, through 

 one reaction or through a series of very compUcated reactions, 

 usually one organism following another or one competing with 

 another, the nature of the organisms and the mechanism of trans- 

 formation depending upon the nature of the organic matter and 

 environmental soil conditions. The sum total of the activities 

 of the various organisms in bringing about the transformation of 

 the complex plant and animal organic materials into simple inor- 

 ganic elements or compounds is known as the nmieralization of 

 organic matter, frequently spoken of in the older literature as decay 

 and putrefaction. 



In the process of the mineralization of organic matter, many of 

 the elements are liberated in forms which cannot be assimilated 

 directly by higher plants. The mineralized inorganic elements or 

 simple inorganic compounds may have to be transformed first 

 into compounds which the plants can utilize and assimilate more 

 readily. For example, in the decomposition of organic matter, 

 the nitrogen is liberated as ammonia, while most plants prefer and 

 many even require the nitrogen in the form of nitrate; the sulfur 

 may be hberated as hydrogen sulfide, while the plant needs its 

 sulfur as sulfate. A large part of the carbon may be left in the 

 form of various organic acids, aldehydes, and alcohols, while the 

 plant uses its carbon as carbon dioxide. The final transformation 

 of the materials into forms available for higher plants depends 

 again upon the activities of various groups of microbes, some oxi- 

 dizing the ammonia to nitrous acid then to nitric acid which com- 

 bines with the soil bases to give nitrates; others oxidize the sulfides 

 to sulfates; still others oxidize the various organic acids, alde- 

 hydes, and alcohols, hberating carbon dioxide. 



Synthetic Activities of Microorganisms. — As a result of 

 their various activities, the microorganisms in the soil build up con- 

 siderable organic matter of their own. Microorganisms are not 

 like catalysts, which do not increase or decrease in amount in 

 bringing about a certain reaction; on the contrary, the reactions 

 brought about by a microbe result in synthesis of microbial cell 



