THE SOIL AND ITS IMPORTANCE TO PLANTS 



25 



may even do much harm by washing away a portion of the soil 

 itself. The water which does enter the soil may either percolate 

 downward between the j^articles under the influence of gravity, 

 or may be hold in the soil by capillarity. 



Percolating or gravitational water passes downward rapidly 

 if the soil particles arc coarse, more slowly if they are finer, until 

 it arrives at a level where all the soil spaces are filled with standing 



/^;? / Transpiration 



Run- off 



Zone of 

 ■Capiltaru 

 Wafer 



Fig. 13. — The various fates of rain-water which falls upon the soil. It may 

 run off without entering the soil; it may be evaporated from the surface; it may 

 enter the roots and be transpired from the leaves; it may be held in the soil by 

 capillarity, or it may percolate downward to the water-table. 



or hydrostatic water. This level is known as the wafer-table. 

 Its position at any given point determines the height at which 

 water will stand in a well dug at that point, and its distance 

 below the surface varies from place to place and is subject to 

 much fluctuation. A similar saturated condition occurs in the 

 upper soil layers after heavy rains, but persists there for only a 

 short time. When water has percolated downward to this level 

 it is often beyond the reach of roots, and is thus quite unavailable 

 to plants. 



Capillary water is water held in the soil by the force of capil- 

 larity. Common observation teaches that when an object 

 (such as one's hand) is immersed in water and then lifted out 

 again, some water still adheres to its surface in a thin film, or 

 "wets" the object. This is due to the fact that there is greater 

 attraction between the surface of the object and the water than 

 is exerted by the force of gravity or the cohesion of the water 

 particles themselves. Any material with a large amount of 



