238 SOILS: PROPBBTIES ANB MANAGEMENT 



its percentage of moisture when capillarily satisfied, are 

 known, the following formula may be used : — 



T^ , . . 1 1 ['percentage of pore space 



rercentage or air space when 1 I . , & rr r\ 



•n -1 ^ ^. J ^ = < — (percentage or H2O 



capillarily saturated 



I X ap. sp. gr.) 



Percentage of free water pos- 1 ^ [ percentage of air space 

 sible J 1 ap. sp. gr. 



163. Value of studying flow and composition of gravita- 

 tional water. — While the determination of the possible 

 free water that a soil will hold is of little real value, a 

 knowledge of its movement and its composition is of 

 vital importance. It has already been shown how the 

 rate of movement of such water is a factor in efficient 

 drainage. The amount likely to be thus lost is of interest 

 in plant production from two standpoints: first, the 

 role that water plays as a food and a regulator; and 

 secondly, the losses of nutritive elements that always 

 occur with drainage. It is quite evident that these 

 questions should be studied only on soil in a normal field 

 position. Consequently two methods of procedure are 

 open -the use of an efiicient system of tile drains, and 

 the construction of lysimeters. 



164. The study of gravity water by means of tile drains. 

 --" In the first method an area should be chosen where 

 the tile drain receives only the water from the area in 

 question and where the drainage is efficient. A study 

 of the amounts of flow throughout a term of years will 

 yield much valuable data concerning the factors already 

 discussed. An analysis of the drainage water will throw 

 light on the ordinary losses of plant-food from a normal 

 soil under a known cropping system. The advantage 



