216 Bureau ok Farmers' Institutes. 



in a soil. It is only under unusual conditions that a fanner 

 will be justified in having soil analyzed in order to learn its 

 plant food requirements. There are other and more certain 

 means to use for gaining this knowledge, which the fanner can 

 apply to best advantage, as will he indicated later. 



6. The Kelations of Micro-Organisms to Soil and to Plant 



Food. 



Nitrification and its Importance. — The absolute necessity of 

 nitrogen as food for plants has been mentioned and it has also 

 been stated that plants require this nitrogen to be supplied to 

 them in the form of nitric acid or nitrate compounds. A plant 

 cannot feed directly upon the nitrogen of the air, or upon the 

 nitrogen contained in ammonia or vegetable or animal matter. 

 All these forms of nitrogen must first be changed into nitrate 

 nitrogen, and the work of bringing about this change is accom- 

 plished by fermentation processes caused by several different 

 micro-organisms or germs, constituting a chain of workers in a 

 simple division of labor. The organic nitrogen of dried blood, 

 bone, cottonseed meal, stable manure, etc., is changed into am- 

 monia by one set of workers; the ammonia is taken by another 

 set and carried along to another stage called nitrous acid, and 

 this in turn is changed into nitric nitrogen by a third set of 

 works. The general process by which nitrogen in other forms 

 is changed into nitrate nitrogen is commonly spoken of as nitrifi- 

 cation. The importance of the work done by these different micro- 

 organisms working together can be appreciated, when we realize 

 that without their work no plants could be grown and all forms 

 of vegetable and animal life would perish. 



These micro-organisms are the smallest imaginable kinds of 

 plants and we cannot see them singly by the unaided eye. They 

 are commonly called germs. Another name for many of them is 

 bacteria. 



Number of Micro-Organisms in Soil. — The number of different 

 kinds of micro-organisms in the upper portion of ordinary cul- 

 tivated soils varies roughly from one to fifty millions to the 

 ounce. The number diminishes rapidly below a depth of 30 

 inches. At six feet below they nearly disappear. The number 

 is greater in cultivated than in uncultivated soils, in well-fertil- 

 ized soils than in unfertilized soils, in soils rich, in humus than 



