What wor\ 
do they do? 
Nodules of a 
clover plant 
60 WORLD OF INVISIBLE LIFE 
microbes are able to take the nitrogen from the 
decayed plant and animal matter, use it in their 
bodies, and then put it back into the soil, where 
it can be used by growing plants. 
The second, and by far the most important 
natural means of keeping the soil from “wear¬ 
ing out,” is the work of certain microbes in 
securing the “free” nitrogen from the air. 
These soil-microbes grow on the roots of such 
plants as clover and peas and beans, forming 
small round balls, or nodules, which are filled 
with nitrogen. As if to pay for living on these 
roots, the soil-microbes give this nitrogen to the 
plants they live upon. In turn, when the plants 
decay, they leave the nitrogen in the soil, where 
it may be taken up by other plants. 
Farmers make use of this knowledge through 
what is known as rotation of crops. As plants 
such as wheat and corn have no soil-microbes 
living upon them, they take nitrogen from the 
soil. If a farmer planted wheat on the same 
piece of ground every year, the soil would soon 
be too worn out, by the loss of all its nitrogen, 
Clover plants enrich the soil 
with nitrogen 
