206 THE PRINCIPLES OF 



An inspection of a table of analysis of plants indicates a 

 general resemblance in the constituents they require. They 

 perhaps do not take up the substances in the same pro- 

 portions, but there is a fair general resemblance in the 

 composition of both the soft tissues and the ash of plants. 

 But there is a very great difference in root distribution, so 

 that we must look for the explanation of the value of the 

 rotation of crops partly in certain facts which belong to 

 chemistry, that is, the exhaustion of certain available 

 materials required for a particular plant; but we must also 

 look at the question from the physiological point of view, and 

 remember that the root distribution, i.e. the section of soil in 

 which the root labours, and the amount of absorbent surface 

 - which the root is able to present to the soil, has a great deal 

 to do with the advantages of a rotation of crops. To grow 

 wheat or any other crop year after year upon the same land, 

 thereby removing from the soil identically the same sub- 

 stances, and exposing it to exactly the same root distribution, 

 taking nourishment from the same section, must be an 

 economical mistake. It would be a flagrant blunder, even if 

 carried out with a commercial profit. Much wiser would 

 it be to interpolate a crop of beans, or to take a crop of 

 clover, in which case you would introduce a plant which asks 

 not only for a different amount of plant food, or a different 

 proportion of plant food, but which likewise seeks that 

 nourishment in quite different layers and sections of the 

 soil. 



These are sound reasons why a rotation of crops is in every 

 respect to be recommended, and there are certain subsidiary 

 reasons which are less direct, but which are at least to be 

 taken into account. A proper rotation of _crops undoubtedly 

 ensures the proper cleaning of the ground. It is exceed- 

 ingly difficult to keep land clean under continuous corn 

 growing, and they find this a serious item of expense at 



