50 



VERTICAL FARMING 



Movement of Capillary Water. The capillary or film move- 

 ment of moisture takes place in all directions, but its most 

 important direction is upward. When tight soils prevent the 

 downward percolation of free water, some is carried downward 

 by this pull which has been described elsewhere as the same 

 movement as the oil moving in a wick. As the amount of moist- 



TWO EXAMPLES OF CAPILLARITY 



ure that is held by capillarity is limited and if not replenished, 

 the supply within reach of the roots may soon be exhausted. 

 This available supply is partially maintained by the upward 

 movement of capillary water. As one point becomes dry, the 

 water is drawn from below by a constant pull on the thin film. 

 When capillary water moves from the more abundant supply 

 below, it brings with it soluble plant foods to assist in nourish- 

 ing the plant. 



The movement of capillary water is effected by several con- 

 ditions.. It is governed very largely by the texture of the soil. 

 The finer the soil and the more surface the soil particles expose, 

 and the more points of contact between the particles, the greater 

 is the pull. For example, a heavy clay soil containing 20 per 

 cent, of moisture may draw water from a coarse sand contain- 

 ing only 10 per cent. Sandy or coarse soils move water very 

 rapidly and in large amounts, but the movement in such soils 

 cannot take place over long distances. With clay, which has a 



