134 FIRST PRINCIPLES OF AGRICULTURE. 



is the storage or vegetative period, and the second, the 

 period of seed-making. 



Root crops are all provided with tap or storage roots, 

 and hence they flourish well only in deep, mellow soils. 

 They are supplied with absorbing roots, which spring 

 mainly from the lower end, spread into the soil, and 

 gather the food. The chief supply of food is needed late 

 in the season, when the formation of tap-root is most 

 rapid. 



These plants cannot make ready use of the insoluble 

 mineral constituents of the soil ; hence, in order to insure 

 full crops, they must be liberally supplied with available 

 food. Of the three classes of fertilizing materials, the 

 phosphates are especially suitable for turnips, while the 

 slower-growing beets and carrots require an abundance 

 of nitrogen in quickly available forms. 



Tuber Crops. — The white potato tuber is not a root, 

 but an enlarged underground stem. The true feeding- 

 roots are produced by the underground portion of the 

 main stem. The extensive growth of the plant under- 

 ground requires that the soil shall be loose and open, 

 in order to permit the free entrance and circulation of 

 both air and water. Potatoes, like root crops, do not 

 possess strong foraging powers. The food must be in a 

 soluble and available condition, in order to insure maxi- 

 mum production. Where soils are suitable, potash seems 

 to be the ingredient especially useful in the manures 

 applied, as it is a potash-demanding plant. 



The sweet-potato tuber is an enlarged root, and not a 

 stem, as is the case with the white potato. The plant 

 is especially adapted to warm, dry soils; and, while it 



