THE USEFUL MICROBES 
59 
Enriching the soil, that is, making it fertile, 
is one of the most important uses of microbes. 
All plants, of course, depend on the fertility 
of the soil, for it is from the soil that they get 
most of their food. And as animal life feeds 
upon plants, it is therefore necessary always to 
keep the soil rich in food materials. A group 
of microbes which live in the soil in enormous 
numbers play a large part in keeping it 
supplied with one of the materials most need¬ 
ful for plant life. 
This is a chemical element called nitrogen . 
Nitrogen, mixed with other substances, helps 
to make up the bodies of all living things. 
But animals can obtain it only from plants. 
It is one of the gases of the air, but there it is 
wholly useless to all the ordinary plants, which 
can obtain it only from the soil in which they 
grow. As they grow, they use up the nitrogen 
in the soil, and it must be replaced in some way 
if the soil is to remain fertile. This is largely 
the work of the soil microbes. 
This work is done in two ways. Certain 
Are there 
microbes in 
soil? 
