218* Bureau of Farmers' Institutes. 



(Hies. Hence, it is important to lime soils occasionally, in order 

 that the preparation of nitrate nitrogen can lake place. 



Conversion of Atmospheric Nitrogen into Plant "Food by Micro- 

 organisms. — Another most important work done by certain micro- 

 organisms is the production of nitrogenous plant food from the 

 nitrogen of the air. Certain plants, known as leguminous, such 

 as clover, peas, beans, etc., have been proved to utilize atmos- 

 pheric nitrogen as a source of food supply. They are able to 

 accomplish this through the action of certain germs. If the roots 

 of a healthy clover plant are examined, there will be found on 

 them little round bunches or tubercles, varying from the size of 

 a shot to that of a pea. These have been found to be colonies con- 

 taining enormous numbers of germs having the power to assimi- 

 late atmospheric nitrogen. They are accompanied by other germs 

 which change the nitrogen, thus taken from the air, into nitrate 

 nitrogen, which the plant uses as food. 



Loss of Nitrogen through Micro-Organ isms. — The soil con- 

 tains certain kinds of germs which have a power opposite to that 

 of nitrifying germs; that is, they change nitrates back through 

 different forms into some form that is likely to escape into the air 

 and be lost as plant food. This process is known as denitrifica- 

 tion, which is simply the destruction of nitrates as plant food. 

 These germs are abundant also in straw and in stable manure. It 

 is important to know the conditions under which this destructive 

 process takes place and thus learn to avoid such losses. 



(a) Absence of Air is favorable to the destruction of nitrates. 

 Too much water in the soil has the effect of shutting off the sup- 

 ply of air and thus the denitrifying organisms are enabled to do 

 their injurious work. Good drainage and thorough tillage will 

 overcome this condition. 



(b) An Excessive Amount of Humus in the soil favors the de- 

 struction of nitrate compounds. This is a condition not often 

 met in common farm practice. 



(c) Loss of Nitrogen from Stable Manure. — Those organisms 

 that feed upon nitrate are very abundant in stable manure and 

 straw. These losses can be prevented by using preservatives in 

 the manure as described later, and also bv using bedding material 

 free from decomposable organic matter. 



