EXTENSION COXJKSE IN SOILS. 27 



whicli their roots penetrate. After this has been partially dried out, 

 as a result of the extraction of water by the growing crops, the water 

 fihns are reduced somewhat in thickness and therefore have acquired 

 greater tension and have the power of drawing up some of the moisture 

 in the thicker fihns of the soil below. This capillary rise of water 

 undoubtedly causes an important addition to the available supply. 

 This movement of water varies greatly, however, in soils of different 

 texture. It is of importance in coarse or sandy soils only when the 

 ground-water level is within 10 or 12 feet of the surface, while in 

 heavy clay soils it may come from considerably greater depths. 

 The capillary movement is not rapid, but it is much faster in sandy 

 than in clay soils. In the case of rapidly growing crops, especially 

 on clay soils, in which the rate of capillary rise is slow, the water 

 supply furnished in this way is altogether inadequate to maintain 

 growth after the moisture in the surface soil has been reduced to the 

 lower limits of good growing condition. It is, nevertheless, an 

 important addition to the moisture already held in the soil. 



Capillary rise of water in soils is illustrated by holding two glass 

 tubes of very small but different-sized bores perpendicular, with the 

 lower ends under the surface of water. In both tubes the water wiU 

 rise above the surface level of the water in the containing vessel, but 

 the column in the smaller tube wiU stand the higher. This rise of 

 water in capillary tubes is due to two forces: (1) The attraction of 

 the glass for water, which causes the water to creep up the tubes a 

 little above the general level of the water surface within the tube; 

 and (2) the tension, or stretch, which is on the surface of aU liquids. 

 If a dry needle is carefully placed upon a smooth surface of water, 

 the needle will float, but can be seen to be causing a stretch of the 

 Hquid surface beneath it. This elastic tension of a hquid surface 

 causes the surface within the tubes to tend to form a plane. The 

 simultaneous action of these two forces noted will cause the water to 

 rise within the tubes until the weight of the water therein equals the 

 force of tension of the surface films. The column of water in the 

 smaller tube, being the lighter, will rise to the higher level. 



In soils, the openings between particles, or pore spaces, serve as 

 capillary tubes, and the perpendicular rise of water behaves in 

 accordance with the laws of capillarity. Fine-textured soils, there- 

 fore, have a higher rise of water from this cause than soils of coarse 

 texture, although the rate of rise is much slower in the former. 

 There are, however, other factors of practical importance affecting 

 capillarity in soils which should be noted: (1) Some mineral salts in 

 solution strengthen the surface tension of water and add somewhat 

 to the rise of water in soils; (2) heat reduces the strength of the sur- 

 face film of water and other liquids as weU ; and (3) some liquids, such as 

 these from manures and decaying vegetable matter, have been found 



