SOIL WATER 71 



soil. The annual amount of rainfall required for crop- 

 production is brought to a much higher figure by the loss 

 due to run-off and percolation. 



81. Capillary movement and plant Requirement. — We 

 have seen that there is a capillary movement of water from 

 the more moist to the less moist soil. As water is absorbed 

 by plants, the moisture content is reduced in the soil sur- 

 rounding the root-hairs by which the moisture is taken up. 

 Immediately a movement begins to establish equilibrium 

 in the water films and during the time the roots continue to 

 absorb moisture, the movement of capillary water goes 

 on. During the blooming period, plants must have very 

 large quantities of water if they are to develop fully and 

 produce large yields of grain. Capillary movement is 

 necessarily slow, especially in heavy loam and clay soils. 

 It is often impossible for the capillary movement to carry 

 moisture fast enough, except for short distances, to supply 

 plants adequately and the crop suffers for want of moisture. 

 In a dry season the capillary capacity of a soil is likely to 

 be of more importance than the rate of capillary movement, 

 as the supply is more easily available. Hence, in time of 

 drought a loam soil in good tilth is better than a sandy 

 soil. 



82. Optimum moisture for plant growth. — Plants wilt 

 for want of water at a moisture content somewhat higher 

 than that represented by hygroscopic moisture. They 

 show the pale color characteristic of too much moisture 

 when a soil is saturated. Before either of these well-known 

 signs of distress is shown, the plant may have too much or 

 too little water to allow of its maximum growth. The 

 optimum moisture content lies somewhere within the range 

 of capillary moisture. It is variously stated by different 

 experimenters to lie between 60 and 90 percent of the water 

 capacity of soils. Probably it varies with different soils. 



