314 ELEMENTARY STUDIES IN BOTANY 



drains away from the soil, retained more or less by a finely 

 pulverized soil, but it also evaporates from the surface. If 

 the surface becomes caked, it must be broken up and pul- 

 verized so that the soil can get a new grip on the water, or 

 the loss may be serious. Tillage, therefore, checks evapora- 

 tion as well as loss by drainage. In fact, in dry farming it 

 has been found that a shallow pulverized layer of soil acts 

 like a " mulch " or a blanket in checking evaporation. Of 

 course it is common to use an artificial " mulch " for the 

 same purpose, such as a layer of ashes or sawdust or leaves ; 

 but shallow tillage provides a natural mulch. 



22. Capillary movement of water. It must not be 

 thought that the only water available for plants is that which 

 is held by the soil particles with which their roots come in 

 contact. A most important fact is the capillarity of the soil. 

 This means that when films of water held by the soil particles 

 become thinner because some of the water has entered the 

 roots, there is a rearrangement of the water in all the neigh- 

 boring films. Water is always pulled away from a thicker 

 film toward a thinner one, and this starts a movement of 

 water from every direction towards the thinner films. There- 

 fore, as water is lost from the soil by evaporation or by pass- 

 ing into root systems, there is a movement of water from the 

 deeper parts of the soil. This is the so-called capillary 

 movement of water, which is in the main an ascending move- 

 ment. The available water for plants, therefore, is all the 

 water in the neighborhood that is free to move through the 

 soil. If the soil is in proper condition, this capillary move- 

 ment extends to the water-table, which means the level where 

 the ground begins to be saturated with water, usually because 

 it is held by some material through which it cannot pass. 

 This impervious material may be clay or rock. For this 

 reason, not only is the soil in which the plants are rooted to 

 be considered, but also the soil beneath, which is called the 

 subsoil. For example, it. is of great advantage to an open 



