THE SOIL AND ITS IMPORTANCE TO PLANTS 



33 



two types of nitrifying bacteria; the nitrite bacteria, which change 

 ammonia to nitrites, and the nitrate bacteria, which in turn 

 convert nitrites into nitrates. In this form nitrogen is readily 

 absorbed and assimilated by plants, and is ultimately returned 

 to the soil again in the bodies of plants or animals, thus complet- 

 ing the cycle. Through the activity of another group of these 



Dcnitrificafion 

 The Soil Ammonia 



iNJtrate 



Salts 



Ni+nfmnc 

 Bacterio 



Fig. 19.— The Nitrogen Cycle. 



minute organisms, certain of the seed plants are also able to take 

 advantage of the enormous supply of nitrogen in the atmosphere, 

 which is ordinarily quite unavailable. These are the nitrogen- 

 fixing bacteria. They are present in most soils and cause the 

 development of the tubercles or nodules usually found on the 

 roots of plants belonging to the Legume family (Fig, 20), which 

 includes beans, peas, clover, alfalfa, and similar plants. These 

 bacteria are able to absorb the free gaseous nitrogen of the air 

 and to build it into nitrogenous compounds in their bodies, 

 whence it ultimately becomes available to the particular plant 

 on the roots of which the bacteria grew. Without drawing at all 

 upon the nitrogen compounds in the soil, a leguminous plant 

 is consequently able to acquire an abundant supply of this impor- 

 tant element. 



