116 AGRICULTURE 



Their decaying roots, stems and leaves form an important 

 part of the soil, called humus. This vegetable matter not 

 only enriches the soil, but makes it lighter and more por- 

 ous, so that air can better get to the roots of growing 

 plants.' It also favors proper drainage in damp soils. 



Legumes and the nitrogen of the soil. Legumes, 

 however, enrich the soil in a special way, and are widely 

 cultivated for this purpose. In order to understand how 

 legumes do their work of improving the soil, it must be 

 known that plants demand certain foods from the soil. One 

 of the most necessary plant foods is nitrogen. If this is 

 lacking in the soil, the crop does not grow well, and the 

 yield is reduced. For example, wheat takes much nitrogen 

 from the soil ; this is the main reason why wheat can not be 

 grown on the same land year after year. The nitrogen be- 

 comes exhausted. The soil is "worn out." It is estimated 

 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. 



