140 Irrigation and Drainage 



is compact, so that each portion is part of one long and continuous 

 mass extending downward several feet before water is reached, 

 the surface tension of the water is not strong enough to maintain 

 the soil cavities full of water, and a part drains away downward. 



It is easy to demonstrate the nature of this action with a bit of 

 candle wicking 2 or 3 feet long, or with two or three folds of cot- 

 ton wrapping twine loosely twisted together. Placing this in a 

 basin of water and letting it become saturated, if it is then raised 

 out by both ends, holding it nearly horizontal and straight, the 

 water very soon ceases to drip from it ; but if it is allowed to sag 

 in the middle, the water will begin to drip rapidly, and will con- 

 tinue to do so until a new equilibrium has been reached. The 

 string will lose its water still more rapidly and completely if it is 

 simply suspended from one end, when it then represents the long- 

 est column of soil. 



How subsoiling decreases the capillary conducting power of 

 soils. When large open spaces have been developed in a soil by 

 any means, then every such cavity cuts off a part of the capil- 

 lary passageways through which the water might travel by capillary 

 conduction, thus making the amount of water which may move in 

 a given direction proportionally smaller. This being true, when 

 rain falls upon subsoiled ground it travels downward very slowly 

 through it until after the soil has become completely filled, and 

 drainage or percolation takes place. If, then, the shower is not 

 heavy enough to completely fill this subsoiled layer, it is nearly 

 all retained within it ; whereas, when the capillary connection is 

 good, then so soon as the surface layer becomes wetter than that 

 below, the water begins to move under the impulse of capillarity, 

 and will continue to do so until a balance has been reached. 



On the other hand, when the surface of the subsoiled ground 

 has become dryer through evaporation or by root action, water 

 from below will not enter it as rapidly as it will soil not so treated. 

 It is thus capable of acting as a deep mulch, to diminish the loss 

 of water by capillary movement upward. But should conditions 

 chance to be such that the whole root system of the crop has been 

 developed within this subsoiled layer, then a rapidly -growing crop 

 upon it might suffer for want of water when there was an abun- 



