EXTENSION COUESE IN SOILS. 27 
which their roots penetrate. After this has been partially dried out, 
as a result of the extraction of water by the growing crops, the water 
films 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 films 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 will 
rise above the surface level of the water in the containing vessel, but 
the column in the smaller tube will 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 all 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 
liquid surface beneath it. This elastic tension of a liquid 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 well; and (3) some liquids, such as 
those from manures and decaying vegetable matter, have been found 
