THE MICROBES OF THE SOIL 281 



gallons of any fluid over an acre of land, and to a 

 width of 23 feet. Therefore, it would be advan- 

 tageous to use this machine for watering streets, 

 cattle markets, etc., with a weak solution of ' sanitas,' 

 carbolic acid, or any other cheap disinfectant. 



Concerning cultivated soils, nitrogen is a most 

 important element in the growth of crops. Berthe- 

 lot 1 has shown that a fixation of atmospheric 

 nitrogen takes place in certain vegetable soils by 

 the action of microbes and other fungi. 



Hellriegel and Wilfrath have proved that legu- 

 minous plants obtain their great supplies of nitrogen 

 from the air. This power of absorbing free nitrogen 

 is due to the roots of leguminous plants becoming 

 inoculated with the microbes present in soil. The 

 microbes, which give rise to tubercles on the roots 

 and rootlets, enter into a partnership or symbiotic 

 relationship with the leguminous plant for mutual 

 advantage. These microbes have the power of 

 bringing the free nitrogen into organic combination. 



Perhaps the chief soil-microbe which enters into 

 symbiosis with leguminous plants is Dr Beyerinck's 

 Bacillus radicicola. This microbe has been isolated 

 from cultivated soils as well as from the tubercles 

 on the roots of Vicia faba (the field bean) ; and 

 Beyerinck has inoculated the roots of seedling- 

 beans with this microbe, and in each case it 

 multiplied within the roots, ultimately giving rise 

 to tubercles. 



The process of nitrification or the conversion of 

 organic and ammoniacal nitrogen into nitrates was 



1 Comptes Rendut, vol. cviii. 



