Il8 SOILS AND FERTILIZERS 



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. 48 There are pro- 

 duced from many crop residues, by injurious ferments, 

 chemical products which may be destructive to crop 

 growth. Flax straw, for example, when it decays in 

 the soil forms chemical products which are destructive 

 to a succeeding 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, 

 and cholera, may propagate and develop in a moist 

 soil under certain conditions, and then find their 

 way through drain water into surface wells, and cause 

 these diseases to spread. 



148. Products Formed by Soil Organisms. In 

 considering the part which micro-organisms take in 

 plant growth, as well as in all similar processes, there 

 are two phases to be considered: (i) the action of the 

 organism itself, and (2) the chemical action of the pro- 

 duct of the organism. In the case of nitrification, the 

 action of the organism brings about a change in the 

 composition of the organic matter, producing nitric 

 acid which is merely a product formed as a result of 

 the action of the organism. The nitric acid then acts 

 upon the soil, producing nitrates. In the case of soils 

 rich in organic matter, the fermentation changes which 



