.MICROORGANISMS AND SOIL FERTILITY 75 



bacteria. Il is, therefore, essential that the soils in which 

 tin- acid reaction is pronounced be neutralized by the addi- 

 tion of lime, in order that the decomposition of the organic 

 matter be favored, and hence conditions established for a 

 luxuriant growth of the seeded crop. 



The decomposition of organic matter under anaerobic con- 

 ditions is incomplete; acids persist thus marsh lands are 

 often acid in reaction. By drainage of such lands, air is 

 admitted and the growth of molds is made possible. These 

 organisms will gradually destroy the acids, and the reaction 

 of the soil will be changed in time so as to be more favor- 

 able for bacterial action and for plant growth. Drainage 

 and cultivation may soon make a marsh soil a fitting place 

 for plants that are very susceptible to acid. 



Number of organisms in soil. The number of bacteria 

 in a soil, as has been shown, is dependent on many factors, 

 chief among which is the amount of organic matter. The 

 bacterial content of a sandy soil may be but a few hundred 

 thousand per gram, that of a rich garden soil several mil- 

 lion per gram. The bacterial content of soil is determine! 1 

 by the plate-culture method, using some particular kind of 

 culture medium. It should be understood that only a por- 

 tion, probably a very small portion, of the total bacterial 

 flora of the soil is thus revealed ; for only the bacteria that 

 can grow on the medium used, and at the temperature and 

 in the oxygen tension at which the cultures are kept, can 

 develop. It seems probable that, in any soil that will per- 

 mit the growth of higher plants, the bacteria are to be meas- 

 ured by the hundreds of millions per gram. The higher the 

 fertility of the soil, the higher will be its bacterial content ; 

 for the bacteria are the direct cause of the former, through 

 their elaboration of plant food. 



The soil also contains many molds and yeasts. Little is 



