MAINTAINING SOIL FERTILITY 87 



95. Essentials of profitable crop rotation. A profitable crop 

 rotation for a general farm should include a " cleansing crop," 

 that is, a cultivated crop to rid the soil of weeds ; a so-called 

 " resting crop," such as a legume or grass crop, which tends to 

 increase the supply of organic matter and nitrogen ; and a small 

 grain crop, such as wheat, oats, barley, or rye, which is usually 

 a " money crop." Naturally, the rotation adopted will depend 

 upon many conditions. In the corn belt and in many sections 

 of the East where the practice of rotation is best established, 

 such rotations as corn, oats, and clover, or corn, oats, wheat, and 

 clover, or potatoes, wheat, and clover are common. In the 

 Southern states it may be such rotations as cotton, oats, and 

 cowpeas, or cotton, corn, oats, and cowpeas. In order to carry 

 out such a rotation it is necessary to have as many fields as 

 there are years in the rotation, each crop being represented on 

 one of the fields each year, and all fields following the rotation 

 regularly. Where a crop fails, it is usually wise to plant in the 

 same season a crop having an effect upon the soil which is 

 similar to that of the failing crop. Thus, if clover fails, another 

 legume adapted to the region should be substituted if possible. 



96. The importance of legumes in maintaining fertility. It 

 is possible to supply nitrogen to the land through legumes at a 

 cost of from 3 to 5 cents a pound, whereas in a commercial fer- 

 tilizer it costs from 1 5 to 20 cents a pound. Therefore every 

 successful rotation on the average farm must contain a legume 

 crop (Fig. 39) if the nitrogen supply is to be maintained cheaply. 

 The time may come when a cheap commercial source of nitro- 

 gen as a fertilizer will largely take the place of legumes in 

 supplying this element ; but such a cheap substitute is not 

 now available. Moreover, organic matter must be maintained, 

 and the legume crops furnish one of the most ready means 

 of supplying this. In case a man buys and uses much feed 

 and applies the resulting manure to the land, or in the com- 

 paratively few cases where barnyard manure can be obtained 

 from the city in large quantities, the necessity for legumes 

 is lessened. 



