Modification of Soil Reaction 



331 



tain soils, however, it seems to have been estabhshed beyond donbt 

 that the presence of fungi is essential for normal tree development. 

 In addition to these two groups of microorganisms— the legume 

 bacteria and mycorrhiza fungi— it has also been found that occa- 

 sionally inoculation of soils with other organisms may result in 

 increased plant growth. Among these organisms, it is sufficient to 

 mention the nitrifjing bacteria, sulfur-oxidizing bacteria, bacteria 

 pathogenic to Japanese beetles or other insects, and nematodes para- 



FiG. 130. Ectotroi^hic mycorrhiza developing on roots of Piniis sylvcstris (from 



Melin). 



sitic upon insects or destructive to other injurious nematodes. In 

 some cases the advisability of microbial inoculation of soil is ques- 

 tionable, unless accompanied by certain soil treatments. This is true 

 of the use of certain saprophytic fungi which are believed to act as 

 a check upon the development of pathogenic fungi, of fungi for im- 

 proving soil structure, and of "all-soil inoculants" ( Azotogen ) . 



Enrichment of the soil with organisms not present there originally 

 may lead to development of antagonists, which bring about the de- 

 struction of the introduced bacteria. 



Modification of Soil Reaction and Microbiological Activities 



There is no one particular reaction which is favorable alike to all 

 groups of soil microorganisms. When the soil is acid, especially at 

 a reaction less than /;H 6.0, it may become injurious to the growth 

 of many bacteria, notably the nitrifying and the nitrogen-fixing types, 

 and favorable to the development of fungi. This may be because 



