318 LUTHER BURBANK 



able yield because the waste products of the first 

 crop are often not injurious to the second one. 



The fourth cause, which is far less common 

 than the others, is exhaustion from the soil of 

 certain elements necessary to plant growth. 



It is very seldom indeed that any one of the 

 elements necessary to plant growth is wholly 

 absent from any kind of soil. It does happen 

 sometimes, however, that an element is not 

 present in available form. The plant's roots, of 

 course, cannot take up certain elements that are 

 in such a form that they cannot be absorbed. 

 When the supply of material in the form that can 

 be used is exhausted, the plant does not thrive. 



Quite often the failure of crops when it can be 

 definitely attributed to the condition of the soil 

 is due to an unfavorable physical condition rather 

 than an unfavorable chemical condition. 



Rotation of crops often has an important and 

 essential effect upon the physical condition. 

 When alfalfa, cowpeas, clover, or some other 

 legume is grown, the roots grow deeply into the 

 soil and when another crop follows, the fissures 

 or canals opened up by these deep growing roots 

 are used by the roots of the new crop, besides 

 storing considerable nitrogen. In this way it is 

 much easier for the following crop to permeate 

 the soil where there is plenty of moisture. The 



