THE SOIL 319 



the air in the form of carbon dioxide, and thus the plants 

 are provided with a means of taking carbon dioxide from 

 the air, breaking it up, and using the carbon to form 

 compounds for building material. The leaves of plants 

 are the organs which manufacture food of carbon dioxide 

 and water. A large part of the body of plants is carbon. 

 Of the elements which plants take from the soil, nitrogen 

 is one of the most important; it is also the most diffi- 

 cult to keep in the soil and the most expensive of all plant 

 foods when purchased in the form of commercial fertilizer. 



About 79 per cent of the air is nitrogen an inex- 

 haustible supply for plants. But plants growing in the 

 air would starve to death for nitrogen if they could not get 

 it from the soil in the form of soluble compounds. Plants 

 cannot take free nitrogen from the air. This being true, it 

 is necessary that farmers learn how to keep a supply of 

 nitrogen in the soil. The nitrogen compounds in the soil 

 decompose readily and the free nitrogen escapes into the 

 air. When humus decays soluble nitrates are formed, 

 and if there are no growing plants to use these nitrates 

 they will be broken up and the nitrogen will escape. 

 One method of keeping the nitrogen in the soil is to keep 

 a crop growing during the time when humus is decaying, 

 and to have on the ground in winter a cover crop of some 

 kind which will hold the nitrates and prevent them from 

 being leached out by the water. These cover crops can 

 be plowed under in the spring and will then form humus. 



There are four practical ways of getting nitrogen 

 compounds into the soil when they are deficient. First, 

 by growing weeds, rye, or other rapidly growing plants 

 and plowing them under while green but almost mature. 

 Second, by growing clover, cow peas, or some other 

 legume. If several crops of legumes are plowed under, 



