64 



The free nitrogen present in the air is inert and can- 

 not be made use of by the plant directly for plant food. 

 Certain parasitic micro-organisms living on the roots of 

 plants have the power of converting the nitrogen of the 

 air into a form which becomes available as plant food, as 

 stated on page 38. There is another class of bacteria 

 that live on decaying organic matter, which has the power 



I-.B :*. .6-. 25 .26-.! 



DIAMETER OF THE GRAINS IN MILLIMETERS. 



FIG. 21. Texture of a typical bright tobacco land of Virginia and 

 North Carolina. (U. S. Dept. Agr.) 



of " fixing " atmospheric nitrogen 

 come available for plant growth, 

 on decaying organic matter are said 

 53. Soil Air. The composition 

 upon the amount of organic matter 

 with which it is decaying. The 

 differs considerably from that of 



in a form that may be- 



The bacteria that live 



to be saprophytic. 



of the soil air depends 



present and the rapidity 



composition of soil air 



the atmosphere. The 



