20 THE CULTURE OF FARM CROPS. 



and unless it is restored by a course of good culture, by 

 plowing under green crops, or by furnishing manure to the 

 soil, or by cultivating such crops as clover and grass which 

 leave a large amount of vegetable matter behind them in 

 their roots, the land is gradually exhausted and becomes; 

 unable to produce profitable crops. A crop of clover has 

 been found to leave in the soil more than three tons weight 

 of roots, while the roots left by wheat do not amount to 

 one-fifth of this quantity. Hence we have the explanation 

 of the deep rich soils of newly cleared forest land, of 

 drained swamps, and of the western prairies, as well as of 

 the valuable effects of clover upon the land. 



