142 SOILS AND FERTILIZERS 



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 propagation 

 of many classes of organisms. Sewage-sick soils, clover- 

 sick soils, and flax-diseased lands are all the result 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. 



159. Products formed by Soil Organisms. In con- 

 sidering the part which micro-organisms take in plant 

 growth, as well as in all similar processes, there are two 

 important phases : (i) the action of the organism itself, 

 and (2) the chemical action of the product of the or- 

 ganism. In the case of nitrification, the action of the 

 organism brings about a change in the composition of 

 the organic matter of soils 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 soils rich in organic matter the 

 fermentation changes, which take place during humifi- 

 cation, result in the production of acid products. This 

 is simply the result of the action of the ferments. The 



