314 MAN'S USE AND CONSERVATION OF SOILS 



have been put there either by the decay of plant and animal 

 matter organic matter or by the direct efforts of cer- 

 tain kinds of bacteria. 



Nitrogen is a gas that constitutes about four fifths of the 

 atmosphere. Yet the higher forms of plant and animal 

 life can no more use the free nitrogen of the atmosphere 



SOIL IN GOOD TILTH 



than a human being can digest carbon. The nitrogen must 

 be chemically united with other elements into compounds 

 that are soluble in water before the plant can make use 

 of it for food. Directly or indirectly, plants furnish the 

 entire nitrogen supply of animals. Partially decayed organic 

 matter in the soil is called humus. 



We have learned that decay is caused by minute living 

 things, germs, the most important of which are the numerous 

 kinds of bacteria. The soil teems with this germ life. 

 It has been estimated that there are fifty thousand 

 germs of various kinds in a gram of fertile soil. Certain 

 kinds of bacteria work the humus over and over, each 



