SOIL WATER 147 



The water that remains in the soil above the water table is capil- 

 lary water. It remains because the attraction of the soil particles 

 for it is great enough to overcome the pull of gravity. This water, 

 therefore, is a measure of the water-holding capacity of the soil and 

 since the total amount of surface of soil particles increases as their 

 size decreases we would expect a fine-grain soil to be capable of 

 holding more water than a coarse-grain soil. A coarse sand may 

 retain as little as 12 per cent of its dry weight of water while a loam 

 soil, containing both clay and humus, may retain 35 per cent or 

 more. Not all of this water is available to plants, however. The 

 total water in a soil is known as holard, while the part that is avail- 

 able to plants is called chresard, and that which is not available is 

 echard. The echard, then, is the water that clings so closely to the 

 soil particles that plants cannot get it. The echard as well as the 

 holard, varies in different soils. The coarse sand spoken of above 

 may have an echard as low as 1 per cent while the loam soil with a 

 power to retain 35 per cent of its own dry weight of water may have 

 an echard as high as 10 per cent. It is because of this difference in 

 the echard that plants often seem to get along pretty well in sand 

 which is so dry that it feels dry to the hand, a phenomenon that is 

 much less true of clay or loam. 



The echard, it must be said, is not determined entirely by the type 

 of soil since some plants can absorb more water from a given soil 

 than can others. It is usually necessary in ecological work, there- 

 fore, to determine the echard for the particular species being studied. 

 There are several ways of doing this, the most direct method being to 

 grow a plant in the soil until it wilts beyond recovery and to deter- 

 mine the amount of moisture that is then present in the soil. This 

 amount of moisture is taken as the wilting coefficient for the species 

 of plant, and the type of soil, used. 



After rain has ceased falling, evaporation may take place from the 

 surface of the soil. This reverses the direction of water movement 

 in the soil, since as the surface layers dry out water comes up from 

 below by the process of capillarity. It is well known that when a 

 small glass tube is lowered into a jar of water the water rises in the 

 tube some distance above the level in the jar. This is due to capil- 

 larity. If a larger tube is used the water does not rise so far because 

 the weight of the column of water partly overcomes the capillary 

 attraction between the glass and the water. Likewise, the distance 

 to which water will ascend in a soil depends on the size of the 

 soil particles since this governs the size of the capillary spaces 



