160 AGRICULTURE 



that a twenty-bushel crop of wheat removes about thirty- 

 five pounds of nitrogen from each acre. 



Nitrogen may be returned to the soil in several ways. 

 One of the most common ways is by means of barnyard 

 manure, which is rich in nitrogen. Another way is through 

 commercial fertilizers, such as sodium nitrate, ammonium 

 sulphate, or dried blood saved from slaughter-houses. But 

 the amount of manure is limited, and the commercial fer- 

 tilizers are expensive. And this is where the legumes come 

 in to help. Legumes are able to gather nitrogen from the 

 air and deposit it in the soil. 



There are millions of pounds of nitrogen in the atmos- 

 phere resting on every acre of ground. But the plants 

 can not make use of this nitrogen in the form in which it 

 exists in the air. It has to be made over for them. This 

 is accomplished by bacteria which have their homes in the 

 tubercles or nodules found on the roots of leguminous 

 plants. 



In the small tubercles to be seen on the roots of clover, 

 alfalfa, soy-beans or cow-peas are millions of bacteria, each 

 able in the process of its own growth to take nitrogen from 

 the air, change it into the form needed by growing 

 plants, and leave it in the soil for the next crop. In this 

 way the legume is able not only to secure its own growth, 

 but to leave the soil richer through the action of its bac- 

 teria friends. The raising of legumes is therefore one of 

 the best and most economical ways of enriching the soil. So 

 important is the group of legumes that a separate chapter 

 will be given to the most important ones. 



TOPICS FOR INVESTIGATION 



1. What are the chief grasses used for pasturage in 

 your neighborhood? For hay? For lawns? 



2. What are the principal legumes cultivated in your 

 vicinity? Which are annuals? Which are perennials? 



