INOCULATION OF SOIL WITH MICROORGANISMS 229 



treatments so modify the soil constituents that the supply of avail- 

 able foods is increased and the soil organisms show a sudden and 

 pronounced response. 



Inoculation of Soil with Microorganisms. — Although 

 many of the soil organisms appear to be widely distributed and are 

 quite generally present in most soils, conditions appear where cer- 

 tain desirable types are absent. This suggests the possibility of 

 actual modification of the soil population by inoculation of the 

 organisms which are lacking. An investigation into the causes 

 for the absence of these organisms may suggest whether inoculation 

 may be expected to be of any value. 



The organisms may be absent because conditions are unfavor- 

 able. This may be due to the fact that the soil reaction or aeration 

 is unfavorable. It may also be due to the absence of available 

 food materials. Certain organisms are limited in their nutrition 

 to few sources of energy; some sulfur bacteria are able to utilize 

 only inorganic incompletely oxidized compounds of sulfur, or ele- 

 mentary sulfur itself. The occurrence of the organisms which 

 develop in association with leguminous plants within the nodules 

 on their roots is frequently limited to soils which have recently 

 grown this particular legume, or representatives of the cross-inocu- 

 lation group to which this legume belongs. In the presence of the 

 legume host the organisms find food to satisfy their requirements, 

 but in the absence of the plant the organisms may disappear in 

 course of time, especially in acid soils. 



On the other hand, certain bacteria may never have become 

 established in soil even though conditions are favorable to their 

 development. This may be due to the fact that conditions have 

 been altered recently by agricultural practices; the new conditions 

 may be favorable to organisms which could not develop previous 

 to the alteration. 



The possibility also exists that certain strains of organisms may 

 be developed which are capable of performing important soil 

 processes more efficiently than those already resident in the soil. 

 Their addition to soils may improve conditions for plant devel- 

 opment. 



It is known that forest soils, peat soils, and ordinary field and 

 garden soils possess characteristic floras. In the forest soils, espe- 

 cially the raw-humus soils, fungi predominate both in the amount 

 of active cell substance present and in the transformation of 



