164 THE STUDY OF PLANT COMMUNITIES • Chapter VII 



two or three days after a rain leaving the soil water content at 

 field capacity. 



A simple explanation of the movement of capillary water may 

 be entirely adequate for most ecological purposes. Since capillary 

 water forms a continuous, thin film around soil particles and in 

 the small spaces and angles between them, it is obvious that sur- 

 face tension of the water creates inward pressure in the film and 

 that water, therefore, tends to move from regions with thicker 

 films to regions with thinner films. An explanation with broader 

 applications considers- the difference in attraction for water be- 

 tween two portions of soil having different moisture contents and 

 expresses this attraction or force as capillary potential— -that is, the 

 force required to move a unit mass of water from a unit mass 

 of soil. Various methods of measuring this force indicate that the 

 potential is directly related to the water content and that there is 

 no change in the state of water as moisture content is reduced 

 from field capacity to an oven-dry condition, but merely an in- 

 crease in energy required to move it. On this basis, the boundaries 

 between gravitational, capillary, and hygroscopic water are too 

 indistinct to be recognized. That these boundaries are indistinct is, 

 in fact, true regardless of the point of view. Such relatively simple 

 considerations seem entirely satisfactory for an adequate under- 

 standing of plant-water relationships, although recent studies of 

 soil moisture by soil physicists have become increasingly technical. 



Movement of capillary water is closely related to soil texture. In 

 wet soils, it is rapid in sand and slow in clay, but the rate is re- 

 versed as soils dry out. Capillary rise, or the distance that capillary 

 force will move water, is much greater in clay than in sand al- 

 though the rate of movement is less in clay. The rate is surpris- 

 ingly slow at all times and probably is quite insufficient to main- 

 tain an adequate film on the soil particles from which a root is 

 removing water. The water coming to a root by capillary action 

 does not at all equal the amount made available in new films that 

 the root contacts because of its elongation and production of new 

 root hairs. When soil water is below field capacity, capillary move- 

 ment is probably insufficient to replace the film on particles from 

 which roots of an actively transpiring plant are removing water. 

 The continuous elongation of these roots with the production of 



