THE ROTATION OF CROPS 51 



Practical Rotations. A good rotation alternates crops 

 from three classes. These classes are: (i) cultivated crops, 

 as corn and potatoes; (2) grain crops, such as wheat, oats, 

 and barley; (3) grass crops, including legumes. A three- 

 year rotation may be (i) corn, (2) oats, (3) clover. Where 

 corn is not grown a three-year rotation may be (i) potatoes, 

 (2) oats, (3) clover. A four-year rotation with corn may 

 be (i) corn, (2) grain, (3) hay, (4) pasture. 



QUESTIONS AND EXERCISES. 



1. What is the action of carbon dioxide in the soil upon 

 phosphoric acid and potash? 



2. Name some farm products that remove but little 

 plant food from the soil. 



3. Which plant food is removed most extensively from 

 the soil by grain crops? 



4. How much phosphoric acid would be removed from 

 the soil of an acre in three years if the rotation is clover, 

 corn, and oats, and if 3,000 pounds of clover, 2,000 pounds 

 of corn, and 1,000 pounds of oats are removed? 



5. How many tons of manure will it take to furnish this 

 phosphoric acid if one ton contains 5 pounds of this plant 

 food? If the crops are fed on the farm and the manure is 

 returned to the fields about one-half of the phosphoric acid 

 is returned. How may the rest be supplied? (See p. 37.) 



6. From what three classes of crops are rotations made? 



7. Make out a four-year rotation for a farm in your neigh- 

 borhood. 



8. One farmer sells grain and hay from his farm every 

 year. Compare the fertility of this farm with that of one 

 from which live stock and dairy products are sold. 



