HOW NITROGEN ENTERS THE SOIL 



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73. Nitrogen is a gas, slightly lighter than air. It lacks 

 color, taste, and odor. It does not combine easily with 

 other substances, and its compounds break up very readily. 

 It makes up four fifths of the air, and it is probably the 

 most important food element for both plants and animals. 

 Neither plants nor animals, however, can make use of it 

 in its free state, as it is found in the air. They can take 

 it only from certain compounds, and the compounds of 



CURLY KALE. 

 The heavy crop shows abundance of plant food in the soil. 



nitrogen which are usable by plants are not common. 

 Much money and effort are expended to get these com- 

 pounds into the soil. 



74. Nitrogen enters the soil to become available for plants 

 in four ways: (1) through the use of barnyard manure; 



(2) through the decay of vegetable matter in the soil ; 



(3) through commercial fertilizers placed in the soil; 

 and (4) through the use of legumes, such as clover and 

 peas, with certain bacteria on their roots. 



a. Ammonia gas can often be smelled in fresh manure 



