130 BACTERIOLOGY OF THE SOIL 



Incubate at 22 C, and when the different colonies are well 

 developed examine the organisms of each. Make sketches and 

 describe the colonies. 



Try and determine the species by cultivation on gelatine, agar, 

 and potato, and in Bouillon, milk, and other liquid media. 

 (Compare, if necessary, with type cultures, obtainable from the 

 Author.) 



4. Action of Antiseptics and Heat upon the Soil. Soil 

 to which carbon disulphide has first been added and then 

 allowed to evaporate is found to be capable of growing 

 a larger crop than a similar untreated soil. The same 

 increased fertility is observed when soil is treated with 

 chloroform, ether, toluene, xylol, and other poisonous 

 volatile compounds. Nitrification and the organisms 

 which produce it are destroyed, but partial sterilization 

 by means of antiseptic agents give rise after a short time 

 to a large increase of ammonium compounds, to which 

 latter the greater fertility of the soil is chiefly due. The 

 ammonium salts may be directly absorbed and assimi- 

 lated by plants, but partially sterilized soils rapidly 

 become reinfected with living nitrifying bacteria through 

 access of dust, unless special precautions are taken to 

 prevent this from happening, and the crop then receives 

 its nitrogen in the usual manner as nitrate. 



Many investigations have been made to determine the 

 nature of the beneficial effect produced by the action of 

 antiseptics upon soil. 



It is probable that part of the improvement observed 

 is caused by direct chemical action of those substances 

 upon the insoluble soil constituents, some of the latter 

 becoming available for plant food in the process. 



It has been suggested also that these poisonous com- 

 pounds stimulate absorption by the roots and other physio- 



