258 Science of Plant Life 



trition of the crop, for among the multitude of substances 

 it contains are simple carbon and nitrogen compounds which 

 the higher plants may absorb and use in small amounts. 



Soil bacteria and nitrogen. In order to manufacture pro- 

 teins, seed plants must have a supply of nitrogen. This they 

 secure principally in the form of nitrates. There may be 

 other nitrogen compounds in the soil, but they are unavail- 

 able until certain nitrifying bacteria change them to nitrates. 

 Ammonia is one of the nitrogen compounds produced in the 

 process of humus formation. The ammonia may be acted 

 upon by certain bacteria and changed to nitrites, which in 

 turn are changed by other bacteria into nitrates. These 

 nitrates are used by the green plant. The nitrifying bacteria 

 are of great importance to the seed plants, because they break 

 down the nitrogen compounds in humus and other soils, and 

 convert the nitrogen into an available form. 



Still other soil bacteria bring about a process known as 

 nitrogen fixation, by which nitrogen is actually taken from the 

 air and built into compounds which are added to the soil. 

 The nitrogen-fixing bacteria are, with a few exceptions, the 

 only plants that can take nitrogen from the air and combine 

 it to form nitrogen compounds. They require rich, well- 

 drained soil in order to flourish. They are of great importance 

 in agriculture because nitrogen is one of the elements that is 

 most commonly lacking in soils, and because it is the most 

 expensive of all the elements that are purchased for fertilizers. 



Bacteria and legumes. Clover, alfalfa, beans, soy beans, 

 and peas belong to a family of plants called legumes. They 

 increase the nitrogen in soils on which they are grown, and 

 for many years they have been used in crop rotations, fol- 

 lowing wheat or corn. The practice of using legumes in 



