THE FORMS OF SOIL WATER 237 



recommended for a sandy soil. A rational understanding 

 of the movements of gravitation water is clearly necessary 

 in the installation of tile drains, not only that the system 

 may be fully effective, but also that a minimum effective 

 cost may be realized. 



161. Determination of the quantity of free water that 

 a soil will hold. — While there is no particular advantage 

 in finding the quantity of gravitational water that a soil 

 will hold, since a normal soil should never remain saturated 

 for any length of time, it is nevertheless of interest to 

 know by what methods such data may be obtained. One 

 method is to saturate a column of known weight, and 

 then, by exposing it to percolation, measure the amount 

 of water that is lost. The gravitational water can then 

 be expressed in terms of dry soil. The disadvantage in 

 this method lies in the fact that it is extremely difficult 

 to free a soil entirely of air, so that a determination made 

 in this way would yield low results. Again, a very long 

 time must elapse before a soil will give up all its gravita- 

 tional water. King * found that with even a sand the 

 draining-away of the free water continued over a space 

 of two and one half years. It must also be noted here 

 that because of the lessening of the capillary water as a 

 column of soil is ascended, the space for possible free 

 water increases, thus accounting for the ready entrance 

 of rain into a soil which on the average may contain a 

 relatively high water content. 



162. The calculation of the free water of a soil. — A 

 more nearly accurate idea of the possible free-water 

 capacity of soil may be obtained by calculation. If the 

 absolute and the apparent specific gravity of a soil, and 



1 King, F. H. Physics of Agriculture, pp. 134-135. Pub- 

 lished by the author, Madison, Wisconsin. 1910, 



