ROTATION OF CROPS. 85 



chemical elements the proportion in which they use 

 these varies greatly. So, also, the quantity and pro- 

 portion of these they leave on the soil, in the stubble 

 or refuse, differs much. 



2. The range of the roots and the power of assim- 

 ilating plant food differs much in different crops. 



3. Farm crops differ much in the length of time 

 required to come to maturity, and in the time of year 

 in which they make much of their growth. 



An ordinary grain crop will take from the soil much 

 less potash than will a crop of clover, potatoes or any 

 of the root crops. A crop of oats will take more pot- 

 ash than will one of wheat or Indian corn. A crop of 

 clover will take considerably more phosphoric acid 

 than will a grain crop, while one of mangels or tur- 

 nips will take considerably more than will the clover. 

 Obviously there is an advantage so far as the supply 

 of these substances is concerned in alternating crops 

 which take less with those which take more. 



The red clover plant, in a suitable soil, sends its 

 roots down to a greater depth than the mass of the 

 roots of such a grass a^ Kentucky blue grass. A long, 

 large mangel or sugar beet sends its roots much 

 deeper than a round turnip. The strong growing, far- 

 reaching roots of corn go deeper than those of oats. 

 The deep and shallow-rooted plants will obtain much 

 of their mineral food at different depths. A well 

 established crop of red clover would obtain a large 

 part of its food from a depth scarcely reached by a 

 crop of white clover or potatoes. 



The unusual depth to which clover sends its roots, 

 and the large size of these roots, which greatly in- 

 creases the stock of vegetable matter left in the soil, 



