ROOTS 261 



ling water. It is evident that in considering the relation of 

 soil to plants, not only the surface soil must be considered, 

 but also the soil beneath (subsoil). For example, if humus 

 rests on sand, the water will drain away much more rapidly 

 than if humus rests on clay. 



It is necessary to understand the physical structure of the 

 soil in relation to water. However fine the particles of soil 

 may be, they never fit together in close contact, so that there 

 are open spaces everywhere among them. Immediately 

 after a soaking rain these spaces are full of water, but if the 

 soil is one that drains easily, the water gradually disappears 

 from the spaces, and the largeV ones are occupied by air. 

 In addition to this occasional supply of water, each particle 

 of soil is invested by a thin film of water, which adheres to 

 it closely, and which never entirely disappears even in the 

 driest soil. It is the water of the adherent films that enters 

 the roots, and not the " free water " that may occur in the 

 spaces between the soil particles. These spaces should be 

 kept free of water, that the air may " circulate." Roots are 

 living structures, and they need air just as the aerial parts of 

 the plant need it. This is the reason why good drainage is 

 necessary in a cultivated field, for drainage carries off the 

 free water which would drown the roots, but it does not 

 carry off the water of the films. The ideal arrangement is 

 for a well-aerated soil (with no free water) to rest upon a sub- 

 soil that holds water (like clay), so that the rain water may 

 '' soak through " the aerated soil, and yet be held near enough 

 to it so that the films may be supplied as they become thin. 

 It is this physical condition of the soil that the farmer must 

 look after with great care. 



But the soil is not merely a chemical laboratory supplying 

 certain substances, and a physical laboratory making water 

 and air available at the same time, but also an extensive 

 biological laboratory. In the chapter dealing with the 

 bacteria ( 37, p. 46), the extremely important work of soil 

 18 



