CROP ROTATION AND TOXIC MATERIALS 511 



excretion from the plant roots of substances which, 

 while more or less toxic to other plants, are especially 

 so to plants of the same species. 



Although there are yet many phases and details of 

 this subject to be worked out, there seems to be some 

 relation between the presence in the soil of organic 

 substances poisonous to plants and the continuous 

 growth of one crop; and this may be considered to be 

 one reason for the benefit derived on some soils, at least, 

 from the practice of crop-rotation. 



