84 



WATER IN SOILS 



Capillary Water. Above the free water table in the soils and 

 in surface soils after rains, the capillary water may exist in large 

 quantities. Each of the many grains of soil has surrounding it a 

 film of moisture. These films about the grains are thick enough to 

 allow the moisture to readily move from moister to drier parts. 

 This process is called capillary movement. It may be illustrated 

 by the oil of a lamp moving upward through the wick to the flame. 

 A soft cloth placed with one end in a vessel of water and the other 

 end outside may draw the water out of the vessel. The cloth over 

 the edge of the vessel cannot be said to contain free water, because 

 it is so finely spread out in films over the fibers of the cloth. 



An ounce of sandy soil contains about sixty billion particles 



FIG. 65. Common lamp chimneys with cloth tied on to prevent the escape of soil 

 samples placed in them. This may be used to show the rate at which different soils will take 

 up rain water and their relative power of retaining water for use of crops. From left to 

 right they may contain coarse sand, loam, clay, and humus soil. 



and an ounce of clay loam contains about four hundred billion 

 particles. It is possible for a comparatively large amount of water 

 to be held in soils when we consider that the number of particles 

 is so great. The amount of capillary water which a soil may hold 

 depends somewhat upon the size and number of grains in it. A 

 clay soil will always hold much more capillary water than a sandy 

 soil, because of the greater number of particles in it (Fig. 65). 



The capillary film of water may readily move from soil grain 

 to root surface and root-hairs. It is in this condition chiefly that 

 plants make use of soil water. Free water must first be changed 

 to the capillary condition before field crops can use it. Capillary 

 water contains the plant food for roots to absorb. Soils wet with 



