212 PLANT SOCIOLOGY 



soil, and gravitational water, which can drain oif . The attached water 

 is fixed either hygroscopically or by capillary forces; hence the terms 

 hygroscopic and capillary water. Vegetation is almost exclusively 

 concerned with capillary water the occurrence and distribution of 

 which depend mainly upon the structure of the soil. The capillary 

 water of the soil forms a thin, coherent water fihn around each solid 

 particle. This capillary water may move from particle to particle, 

 such movements tending to establish and maintain a uniform supply in 

 all adjacent portions of the soil. Atterberg's experiments with sands 

 of various grain size show that the height and rate of the rise of water 

 increase with a decrease of the grain size, reaching a maximum in the 

 case of soil particles of 0.02 to 0.05 mm. Finer grains are a hindrance 

 to water movement. 



Inasmuch as hygroscopic water cannot be taken up by the roots of 

 plants, and gravitational water flows off more or less quickly, only the 

 capillary water remains a permanent source of moisture for vegetation 

 and as a solvent and vehicle for mineral plant foods. Because of 

 these several functions, the capillary water plays an outstanding role in 

 the vital competition of plants and plant communities. The more 

 favorable the water supply the more intense is the competition; and 

 the less favorable the water supply the smaller the number of communi- 

 ties which are able to avail themselves of that supply. As a vegeta- 

 tional factor the water content of the soil often reaches a minimum and 

 exercises thereby a decisive effect upon the composition of vegetation. 

 The harmful effects of soil water when in maximum quantity are 

 discussed in the section on soil aeration. 



The water which is available for the vegetation is called "chresard" 

 by Clements and "growth water" by Fuller, in contrast to that which 

 remains fixed in the soil, which is called the "echard." The inter- 

 mediate stage between echard and chresard at which the wilting of the 

 plants sets in is known as the "wilting point," and its numerical 

 equivalent as the "wilting coefficient." Fuller (1914) has termed the 

 water above the wilting coefficient the "growth water," recognizing 

 that most plants continue to absorb small amounts of water from the 

 soil when it contains less moisture than that indicated as the wilting 

 coefficient. 



Available Water. — Determinations of soil moisture in order to be of 

 ecological significance should refer to the water available for the use of 

 plants; that is, the chresard or the growth water should be determined. 

 The total water content may be calculated by collecting samples of 

 soil in closed containers and noting the loss of weight after drying at 

 100° to 105°C. Then 



Total water content — wilting coefficient = growth water. 



