238 SOILS: PROPERTIES AND MANAGEMENT 



its percentage of moisture when capillarily satisfied, are 

 known, the following formulas may be used : 



Percentage of air space when 1 J^ rcenta S e of pore space 

 capillarily saturated " (percentage of H 2 O 



J I X ap. sp. gr.) 



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

 sible J ( 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 efficient 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 



