CROP ROTATION 243 



crops that are not directly concerned with soil productive- 

 ness, and of these this book does not treat. In a number 

 of ways crop rotation may directly affect the soil, and these 

 will be discussed under several different heads. 



301. Root systems of different crops. — Some crops have 

 roots that penetrate deeply into the subsoil, while others are 

 only moderately deep-rooted and still others very shallow- 

 rooted. Among the deeply rooted plants are alfalfa, clover, 

 certain of the root crops and some of the native prairie 

 grasses. Among those having moderately long roots are 

 oats, corn, wheat meadow iescue and a few other grasses, 

 and among those having shallow roots are barley, turnips 

 and many of the cultivated grasses. 



As plants draw their nourishment from those portions of the 

 soil into which their roots penetrate, the deeper soil is not 

 called upon to provide food material for the shallow-rooted 

 crops, and the deep-rooted crops remove relatively less of 

 their nutrients from the surface soil. It, therefore, happens 

 that a rotation involving the growth of deep and shallow- 

 rooted plants effects, by utilizing a larger area of the soil, 

 a more economical utilization of plant nutrients than would 

 a continuous growth of either kind. 



302. Nutrients removed from soil by different crops. — 

 Some crops require large amounts of one fertilizing constit- 

 uent, while others take up more of another. For instance, 

 wheat crops are able to utilize the potassium and phosphorus 

 of the soil to a considerable degree, but have less ability to 

 secure nitrogen. They are usually much benefited by the 

 application of a nitrate fertilizer and leave in the soil a con- 

 siderable residue of nitrogen that may be available to other 

 plants. A number of other crops, as, for example, beets and 

 carrots, can utilize this residual nitrogen. 



Grasses remove comparatively little phosphoric acid. 

 Potatoes remove very large quantities of potash. A rota- 



