162 THE STUDY OF PLANT COMMUNITIES • Chapter VII 



spaces become refilled with air. If the soil remains sat- 

 urated with gravitational water for several days, serious 

 injury to root systems may result from lack of oxygen 

 and accumulation of excess carbon dioxide. Hence gravi- 

 tational water is of little direct value to most plants and 

 even may be detrimental. 



2. Capillary water is held by surface forces as films around 

 the particles, in angles between them and in capillary 

 pores. Immediately after gravitational water has drained 

 away the capillary water is at its peak, and a soil is then 

 said to be at its field capacity. Much of this film water is 

 held rather loosely and is readily available to plants, but 

 some of it, which is held by colloidal material and which 

 is in the smallest pores, is relatively unavailable. It is in this 

 connection that the size of particles becomes important. A 

 cubical sand grain one millimeter on the edge has a surface 

 of only 6 square millimeters, but if it were divided into 

 cubes of colloidal size, 0.1 micron on the edge, the total 

 surface resulting would be 60,000 square millimeters. The 

 increase in surface and angles between particles would 

 thus increase tremendously the total capacity for holding 

 capillary water. However, the water available to plants 

 does not increase proportionally, for the greater curva- 

 ture of the films and the sharper angles sufficiently increase 

 the force with which water is held to materially increase 

 unavailable water. 



3. Hygroscopic water is held in a very thin film on the sur- 

 face of particles by surface forces and moves only in the 

 form of vapor. The moisture remaining in air-dry soil is 

 usually considered as hygroscopic and is, in general, un- 

 available to plants. Distinction between this and capillary 

 moisture is difficult, for exposure of soil to increasingly 

 moist atmospheres may increase the water content even 

 to saturation. 



4. Water vapor occurs in the soil atmosphere and moves 

 along vapor pressure gradients. It is probably not used 

 directly by plants. 



Origin of Soil Water.— Precipitation in the form of rain, hail, or 



