476 FIELD CROPS 



cleared for the farm operations. If the old piece was again 

 cleared after a few years, its original fertility would be found 

 to be largely restored, for the plants which grew on it during 

 the interval drew the plant food from the soil as it became 

 available and returned it with each recurring season. 



After a time, the practice became common of resting the 

 land for but a single season, allowing it to grow up to weeds 

 and then plowing them under. This was less expensive and 

 laborious than clearing new land, while its effect on crop 

 yields was nearly as good. As agriculture advanced, the 

 land was cultivated during this resting period to prevent the 

 growth of weeds and what was known as the ''summer fallow" 

 was developed. Still later, a cultivated crop was substituted 

 for the summer fallow, for land was constantly becoming 

 more valuable and it was not profitable to allow it to he 

 idle every alternate year. Crop rotation was thus eventually 

 developed. This same process of evolution from continuous 

 cropping to a systematic rotation of crops is repeated in 

 more or less detail in practically every newly settled countiy. 

 It is now taking place in a large part of our western territoiy, 

 though here the lack of rainfall may interfere in some degree 

 with the adoption of logical rotation systems. 



HOW ROTATIONS HELP 



650. Advantages of a Rotation. A rotation of crops 

 improves the physical condition of the soil, helps to con- 

 serve moisture and vegetable matter in the soil, lessens the 

 damage from insects and plant diseases, aids in the control 

 of weeds, increases crop yields, distributes the labor of crop 

 pro<iuction, and helps to systematize farm operations. 



651. Rotation Improves the Physical Condition of the 

 Soil. The roots of all plants do not penetrate the soil to the 

 same depth. Deep-rooting plants like clover and alfalfa 

 enter the lower layers of the soil. When their roots decay 

 they open channels for the passage of air and moisture and 



