MAINTAINING SOIL FERTILITY 307 



crop and a cleansing crop, and give as wide varia- 

 tion in habit of growth and food requirements as is 

 practicable. This general rule is necessarily sub- 

 ject to many exceptions. Sometimes the whole 

 scheme may be upset by economic exigencies, as 

 the relative value of the different crops, the imme- 

 diate need of the farmer of money, fluctuation in 

 the price of live-stock feeds, etc. 



TYPICAL SYSTEMS OF ROTATION 



The number of years that a rotation may last 

 varies from two to eight or even more. "Four 

 course" rotations, lasting four years and including 

 four crops, are most common. As a rule, the poorer 

 the land, the shorter the rotation. Fixed rotations 

 are not as common in the United States as in Great 

 Britain. Many good farmers habitually change 

 crops upon their land with quite satisfactory 

 results, without following any definite system. 

 The only sort of rotation followed by many farmers 

 is to alternate a grain crop with a green crop, and a 

 cultivated crop with an uncultivated crop. As 

 this embodies two of the most important principles 

 in crop rotation, one will not go far wrong in follow- 

 ing these simple rules, even though specific crops 

 are not assigned in the rotation. Any number 

 of exigencies may arise that may make it desirable 

 to modify or depart from a system of crop rotation ; 

 but it is well to have some definite system in mind 

 and follow it as closely as possible. 



A few examples of common systems of rotation 

 in the United States will show now the principles 

 outlined above are applied. 



1. Potatoes, winter wheat, clover. 



This rotation is frequently used when the money 



