REASONS FOR CROP-ROTATION 509 



diseases that affect more than one species of plant, as 

 does the beet and potato scab, there is need for special 

 care in arranging the rotation. Such considerations 

 may frequently make it desirable to change the plan of 

 a rotation. 



Another feature of the relation of crop rotation to 

 plant diseases is that the more thrifty growth obtainable 

 under rotation assists the crop to withstand many dis- 

 eases. 



363. Loss of plant-food from unused soil. A system 

 of crop-rotation permits a more constant use of the 

 land than is possible with most annual crops. As a 

 soil bearing no crop upon it always loses more plant- 

 food than one bearing a crop, it is thus possible, by a 

 well-chosen rotation, to save plant-food that would 

 otherwise be lost. 



364. Accumulation of toxic substances. That the 

 soil frequently contains organic substances that exert 

 an injurious effect upon the growth of certain plants 

 is indicated by recent experiments and was surmised 

 by some early writers upon the subject. De Candolle 

 was probably the first to advance the idea in 1832. 

 He suggested that at least some plants excrete from 

 their roots substances that are injurious to themselves, 

 although harmless or even beneficial to other plants. 

 This he considered one of the reasons for the failure of 

 many crops to succeed when grown continuously upon 

 the land, while that same soil may be productive under 

 a rotation of crops. Liebig, in his first report to the 

 British Association in 1840, made a similar statement. 



Recently, Pouget and Chonchak, working with alfalfa 



