96 



ELEMENTS OE AGRICULTURE 



nitrogen. During the process of decay some of the 



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nitrogen escapes into the air in the form of ammonia; 

 the smell of ammonia may often be noticed rising from 

 decaying organic matter. Then some of the nitrates 

 are washed from the soil by rain water. In some way 

 this loss must be made good, else the store of nitro- 

 gen in time becomes exhausted. The nitrogen com- 

 pounds washed from the air are not in sufficient 

 quantity to make good this loss, the minerals in the 

 soil can supply no nitrogen, and the only remaining 

 source is the free nitrogen of the air. 



It is only within the last few years that we have 

 learned that some plants make use of the nitrogen of 

 the air for food. Beans, peas, clover and other plants 

 of the botanical family which is known as the 

 leguminos^: or pulse family, all contain notably 

 large quantities of protein. Yet these crops often 

 grow and flourish on soils which will not produce 

 crops of wheat, corn, or oats until some nitrogeneous 

 fertilizer is added. That the plants of the leguminous 

 family could draw on some supply of nitrogen which 

 was not available to other crops has been a recognized 

 fact for many years. But how or where they got their 

 nitrogen was not known until recently. Now, we 

 know that leguminous plants obtain parts of their 

 nitrogen from the air, and that it is supplied to them 

 by bacteria. On the roots of clover, beans, peas, and 

 other plants of this family are found, ordinarily, small 

 enlargements or knots which are called tubercles. 

 These tubercles are the homes of the bacteria which 

 make the nitrogen compounds from the nitrogen of 



