ROTATION OF CROPS 189 



Different crops have different systems of 

 root development ; in some, e.g. mangolds, 

 the roots go deep into the soil, while in 

 others, e.g. barley, the roots are disposed 

 within comparatively few inches of the 

 surface of the ground. If one were to grow 

 only shallow-rooted crops, practically no 

 use could be made of the plant food lying 

 deep in the subsoil. On the other hand, 

 if one confined one's choice to deep-rooted 

 crops, the plant food present in the surface 

 soil would be insufficiently utilized. But by 

 alternating, either annually or periodically, 

 deep-rooted and shallow-rooted plants the 

 whole body of soil is laid under contribution 

 to provide nourishment, and not only so 

 but plant food drawn from considerable 

 depths by such a crop as mangolds or wheat, 

 will, when the crop is consumed and the 

 residues returned to the land as manure, 

 become available for the use of such a shallow- 

 rooted crop as barley. 



While all crops make use of the same kind 

 of plant food, their requirements in the 

 matter of quantity vary within very wide 

 limits. As regards nitrogen, we find that a 

 crop of cereals or of potatoes removes from 

 the land only about 50 Ib. per acre, whereas 

 turnips and swedes require twice as much, 



