132 



BACTERIOLOGY OF THE SOIL 



NITROGEN PRESENT AS AMMONIA. PARTS PER 

 MILLION OF DRY SOIL. 



The total nitrogen-content is not altered, the ammonia 

 being derived from humus, and the bodies of algae, 

 protozoa, mites, worms, and other living things which 

 have been killed by the antiseptic substance used. 



They conclude that in ordinary soils amcebae and 

 other protozoa devour and keep down the numbers of 

 bacteria ; after treatment with toluene or other anti- 

 septics these larger organisms are destroyed, the more 

 resistant bacteria and their spores being then free to 

 develop without competition. 



Soil which is heated to 100 C. or over becomes 

 sterile, and when the process is carried out thoroughly, 

 there is, of course, no subsequent development of 

 bacteria unless re-infection occurs. Moreover, heating 

 results in the formation of toxic substances which 

 prevent the germination of seeds and the growth of 

 plants for some time. After the disappearance of the 

 toxic compounds the soil is more fertile than unheated 

 soil of the same kind, the heating process having set free 

 some of its plant food. 



Soils which have been partially sterilized by heating 

 to 90 or 95 C. for two or three hours resemble those 



