324 Microorganisms and Soil Fertility 



soil found to be so lacking in specific organisms that their introduc- 

 tion is required to bring about a particular soil process, occasionally 

 the growth of certain crops and the need for specific reactions in the 

 soil make such inoculation desirable. This is true, for example, of 

 the growth of various forest trees, for which certain mycorrhiza 

 fungi are required. It is also true for certain types of orchids and 

 other plants. In addition, the use of sulfur bacteria in very specific 

 cases and of nitrifying organisms in others nearly exhausts the occa- 

 sional needs for artificial introduction of organisms into the soil. All 

 other claims for the favorable effects obtained by inoculation of soil 

 with various bacteria or fungi, ranging from Azotobacter and spore- 

 forming bacteria, namely, the "all-crop inoculants," to certain fungi 

 or earthworms, are exaggerated claims, based more on hope than on 

 fact, and always with an eye on the immediate benefit to the seller 

 of those cultures. 



Among the other important processes in which considerable im- 

 provement has resulted from knowledge of the microbiological popu- 

 lation, the preparation of composts, discussed in the preceding chap- 

 ter, the preservation of manures, and the conservation of soil deserve 

 particular attention. 



Preservation of Manures 



When stable manures or plant residues supplemented with in- 

 organic fertilizer are placed in composts and conditions are made 

 favorable to the activities of microorganisms, through proper aera- 

 tion and sufficient moisture, numerous microbiological reactions im- 

 mediately set in. These are accompanied by a rapid rise in tem- 

 perature. Among the major chemical changes that take place during 

 the process of composting, the reduction of the cellulose and hemi- 

 celluloses and the relative increase in ash, lignin, and protein are 

 most significant. The latter occurs at the expense of the water- 

 soluble forms of nitrogen, which are utilized by the microorganisms 

 for their synthetic needs and are thereby converted into complex 

 organic forms. There is hardly any need for specific inoculation. 

 Plant residues and soils carry enough organisms which will imme- 

 diately become active when favorable conditions are established. 

 Addition of a few more organisms will scarcely modify the many 

 changes set in motion by the microorganisms already present. 



One of the major economic problems involved in the preservation 

 of stable manures is the loss of nitrogen, which may amount to as 



