30 FERTILITY OF THE SOIL 



3. How do plants obtain nitrogen? 



4. How may nitrogen be increased in the soil ? 



5. By what methods do farmers keep up the fertility of the 

 soil? 



6. What is meant by the term "texture of the soil" ? 



7. What condition of soil requires an application of lime ? 



8. How does humus improve the soil ? 



PRACTICAL EXERCISES 



1. Dig up the roots of clover or alfalfa and find the nodules in 

 which are the colonies of bacteria that take nitrogen from the air 

 and build it into nitrogenous substances that the plants can use. 



2. Make a ball of puddled clay and leave it to dry. Note the 

 hard clod that forms. Make a similar ball of clay mixed with 

 a little sand and one of clay mixed with a little lime. Do the 

 dried balls form hard clods? How does lime or sand affect 

 the texture of a clay soil ? 



REFERENCES 



Farmers' Bulletins: 44, Commercial fertilizers, composition 

 and use ; 192, Barnyard manure. 



