276 FARM FRIENDS AND FARM FOES 



that is, combinations of nitrogen, oxygen, and some element 

 like lime or potash — which are readily absorbed through 

 the roots of plants. 



There are certain sorts of bacteria the special function 

 of which is to convert the organic nitrogen into nitrates. 

 These are called the Nitrifying Bacteria. During this 

 process the organic material is first converted into an am- 

 monia form, then into a nitrite form, and finally into the 

 nitrate form. In a general way this is a change from a 

 condition in which the nitrogen is insoluble into a condition 

 in which it is readily dissolved. 



During recent years the importance of this nitrifying 

 process has received a great deal of attention. It is now 

 recognized that good farming involves such treatment of 

 the soil that these friendly germs may do as much work as 

 possible, and that their abundance is an important test of 

 soil fertility. 



The Nitrogen-gathering Bacteria 



In addition to the store of nitrogen in the soil which is 

 unavailable until it is changed into the nitrate form by the 

 nitrifying bacteria, there is an even greater store of it in 

 the air. This supply is so great that it has been estimated 

 that there are seventy-five million pounds of nitrogen rest- 

 ing upon every acre of land. This nitrogen in the air, 

 however, cannot be used directly by plants, so that it 

 seems to have little value as a fertilizer. 



But here again the friendly germs come to the farmer's 

 help. If you dig up a vigorous red clover plant and wash 

 the soil carefully away from its roots, you will probably find 

 upon the latter many small, roundish tubercles. If you treat 

 a soy bean plant in the same way, you will find a condition 

 like that pictured on the opposite page. And you may easily 



