124 SOILS AND FERTILIZERS 



fication process, and is the result of the workings of a 

 class of organisms which feed upon the nitrates form- 

 ing free nitrogen which is liberated as a gas. One of 

 the conditions for denitrification is absence of air, as 

 the organism belongs to the anaerobic class. Denitri- 

 fication readily takes place in soils saturated with 

 water, and where the soil is compacted so that air is 

 practically excluded. 4<5 



144. Number and Kinds of Organisms in Soils. — 



In addition to the micro-organisms which carry on the 

 work of nitrification, denitrification, and ammonifica- 

 tion, there are a great many others, some of which are 

 beneficial while others are injurious to crop growth. 

 It has been estimated that in a gram of an average 

 sample of soil there are from 60,000 to 500,000 bene- 

 ficial and injurious micro-organisms. 47 There are pro- 

 duced from many crop residues, by injurious ferments, 

 chemical products which may be destructive to crop 

 grow r th. Flax straw for example when it decays in 

 the soil forms products which are destructive to a suc- 

 ceeding flax crop. 



A moist soil, rich in organic matter, and containing 

 various salts, may form the medium for the propaga- 

 tion of all classes of organisms. Sewage-sick soils, 

 clover-sick soils, and flax-diseased lands are all the re- 

 sults of bacterial diseases. Many of the organisms 

 which are the cause of such diseases as typhoid fever, 

 cholera, and diphtheria, may propagate and develop 



