SOIL AS WATER RESERVOIR 23 



the usual quantities of water added in irrigation seldom, 

 if ever, are sufficient to bring the soil beyond the first or 

 second foot to maximum capillary saturation. 



The quantity of water that may be applied in any one 

 irrigation depends somewhat upon the nature of the soil, 

 yet varies within rather narrow limits. It is seldom pos- 

 sible to apply at one irrigation less than 2 inches, and 

 practically impossible to apply more than 10 inches, 

 unless the soil be very gravelly. The practical limits are 

 yet narrower; a light irrigation is about 3 inches, a heavy 

 one about 8 inches; and an average one from 5 to 6 inches. 



One inch of water is equivalent to 3 per cent of soil 

 moisture to a depth of one foot. A heavy irrigation of 

 8 inches would, therefore, raise the soil moisture in 1 

 foot of thoroughly dry loam soil to 24 per cent, or maxi- 

 mum capillary saturation. If the soil, at the time of 

 irrigation, contains 12 per cent of moisture, one such 

 heavy irrigation would raise 2 feet of soil to full capillary 

 saturation. Since the moisture in the upper 8 to 10 feet 

 is concerned in plant-production, the full soil column 

 under such irrigation is far from saturation. In fact, a 

 loam soil, containing 12 per cent of moisture, will con- 

 tain, after receiving a heavy irrigation of 8 inches, not 

 more than an average of 15 per cent to a depth of 8 feet; 

 while the full saturation is about 24 per cent. With a 

 medium irrigation of about 5 inches the unsaturated 

 character of the soil would be still more marked. 



22. The movement of soil moisture. The water 

 added to soils by irrigation, instead of saturating the 

 upper soil a foot or two, distributes itself with great 

 regularity to considerable depths in the soil. All soil 

 moisture is acted upon by the two chief contending 

 forces of gravitation and adhesion. Gravity tends to 



