96 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



Capillary Rise of Water in Soils, — This is the upward move- 

 ment of water from the subsoil to the surface. This movement of 

 soil moisture concerns capillary water only. A good illustration 

 of this is the upward movement of water or oil in a lampwick. 

 SufHce it to say that the force which causes this rise of water either 

 in the soil or a lampwick is " capillarity.'^ 



When capillary water rises in soils it does not fill the pore 

 spaces in between the soil particles, but rises or moves from soil 

 grain to soil grain in films which surround the soil particles; thus 



capillary water may move not only upward, but 

 in all directions, from a greater to a less amount 

 of soil moisture. It is important that the pore 

 spaces do not all become filled with water, 

 because it is through these openings that air 

 enters a soil; and air is one of the factors 

 determining fertility. 



Factors Influencing Capillary Rise of Water. 

 — There are several factors influencing the rise 

 of water in soils, but we shall consider here only 

 those which are of practical iiiiportance to the 

 farmer; viz., soil texture, compactness or firm- 

 ness of soil, and obstruction. 



Influence of Soil Texture. — Soil texture has 

 a decided influence on the rise of soil moisture. 

 In a fine textured soil water rises much higher 

 but more slowly than in one of coarse texture 

 (Figs. 40 and 41). Capillary water cannot rise 

 upthroughgravel or coarse sand, thus a seed bed 

 having a coarse sand subsoil is supplied with no 

 appreciable amount of moisture from the subsoil through capil- 

 larity. A seed bed of silt loam underlaid by silty clay, on the 

 other hand, may be supplied with considerable capillary water 

 from the subsoil to a depth of from four to five feet during a drought. 

 King concluded that moisture may rise in silt loam soils from depths 

 of ten feet in forty-five weeks. Another investigator has shown 

 water to rise in a very heavy soil to a height of only forty-six 

 inches in twenty-eight weeks. Since in a humid climate a drought 

 seldom extends over a period of six weeks, a seed bed of silt loam 

 on silty clay subsoil would probably not secure any moisture 

 from depths greater than four or five feet. Since in most soils in 

 humid climates roots of farm crops extend to depths of three to 



Fig. 40 — Diagram 

 showing how capillary 

 water rises m soils — ^in 

 films from soil particle 

 to soil particle S, soil 

 particles 



