SOIL WATER, DRAINAGE, AND IRRIGATION 297 



the surface of compact soils, the water from below, if there 

 is any, immediately comes to the surface. Water passes 

 readily through relatively solid soils. 



If an extremely fine-drawn glass tube is placed with one 

 end in a dish of water, the water will rise some distance in 

 the tube. This action is known as capillarity. The fine 

 spaces between the walls of the soil particles act as capil- 

 lary tubes, and the soil water passes upward if the soil is 

 compact enough to have the spaces continuous. 



The action of the soil may be illustrated by using a piece 

 of compressed sugar. Touch one end of the lump to a liquid, 

 and the liquid soon goes through the entire lump. In the 

 same way in dry weather compact soils lose their deep soil 

 water through evaporation. Another experiment, with two 

 lumps of sugar, will suggest a method of cultivating soils so 

 as to hold moisture. Allow a second lump to lie loosely 

 upon the first, which just touches the liquid, and the liquid 

 will pass into the second very slowly if at all. The so-called 

 capillary connections within the lumps of sugar are not con- 

 tinuous between the two; there are no continuous lines along 

 which the water passes. In the same way constant surface 

 cultivation of the soil keeps the capillary spaces broken, and 

 water does not pass upward through the soil rapidly. If 

 the soil is deeply plowed at the beginning of the season and 

 thoroughly cultivated on the surface throughout the season, 

 there is (1) a reservoir for catching water and (2) a device 

 for preventing its rapid evaporation. 



301. Natural drainage of soil water. Water may be drained 

 from soil by surface drainage or by underground drainage, 

 and either may take place naturally or artificially. When 

 rains or snows leave more water on the soil than it will 

 absorb, the water flows away toward regions of lower levels. 

 When very heavy rains fall in a short time or when snow 

 that lies upon frozen or rocky soil thaws rapidly, the run-off 

 may be very great. When rain falls slowly, the soil will take 



