210 FIELD, FOREST AND FARM 



They accordingly belong to the class of plants that 

 impoverish rather than enrich the soil in which 

 they grow. 



"It is impossible, thus, except by a ruinous ex- 

 penditure of fertilizer, to raise a crop of grain every 

 year on the same land. But if we should let pota- 

 toes succeed wheat, and wheat succeed potatoes, 

 what would be the result? The latter crop, deriv- 

 ing a large part of its nourishment from the air, 

 would flourish in soil comparatively exhausted by 

 wheat; and on having its leaves and stalks turned 

 under it would give back to the soil a part of its 

 former fertility. Wheat could then be successfully 

 raised again on the same land. 



"This practice of raising successively on the same 

 land different crops as little harmful to one another 

 as possible and capable of utilizing to the utmost 

 the dressing put on to the land, is nothing but that 

 very rotation of crops that I have already told you 

 something about. Its purpose is to economize fer- 

 tilizer and at the same time to secure an uninter- 

 rupted succession of crops. The underlying prin- 

 ciple consists in making an enriching plant succeed 

 an impoverishing one; that is to say, a plant with 

 luxuriant foliage is made to succeed one with scanty 

 foliage. The chief enriching plants are clover, lu- 

 cerne, sainfoin, potatoes, turnips, and beets. 

 Cereals, on the contrary, are all impoverishing 

 plants. It is a general custom to raise on the same 

 land a more or less extended series of different 

 crops, the series running four, five, or six years, or 



