THE USEFUL MICROBES 
61 
to produce anything. Instead of allowing this, 
the farmer plants wheat one year and the next 
year changes to some plant that will bring 
nitrogen to the soil. In this way he keeps the 
soil fertile, and does not need to fear that his 
land is “wearing out.” 
On farms where land has been completely 
“run down” by the continuous planting of such 
crops as wheat, tobacco, and cotton, it has been 
found that the growing of alfalfa will soon 
“build up” the soil. Alfalfa is a plant similar 
to clover and of the same family. It is a hardy 
perennial; that is, it does not “kill out” during 
the winter months. This enables alfalfa to send 
its roots far into the earth, where it may draw 
food from soil untouched by the average plant. 
As alfalfa is a nitrogen gatherer, its roots are 
soon covered with numerous nitrogen-factories 
—those small nodules which collect the free 
nitrogen from the air and deposit it later in the 
soil around it. In this way the soil is gradually 
freshened. Alfalfa grows naturally in most re¬ 
gions, but where it does not, the science of agri- 
How can soil 
be freshened? 
