THE FOEMS OF SOIL-WATBE 177 



diameter of a sand sample lie was able to calculate a theo- 

 retical flow which compared very closely to observed percola- 

 tions. In sandy soils low- in organic matter this law holds 

 in a very general way, but in clays it fails entirely. For 

 example, if such a law was in force a sand having a diameter 

 of .5 millimeter would exhibit a flow 10,000 times greater 

 than that through a clay loam with a diameter, say, of .005 

 millimeter; whereas the actual ratio, as observed experimen- 

 tally by King, was less than 200. Such a discrepancy is to be 

 expected as it is impossible accurately to apply mathematics 

 to soils carrying any appreciable amount of colloidal matter. 



Evidently, therefore, while it can be stated as a general 

 thesis that the flow of gravity water varies with the texture, 

 being much more rapid through a coarse than through a fine 

 soil, no law can be deduced for soils, since structure 

 exerts such a modifying influence. The percolation in a 

 heavy soil takes place largely through lines of seepage, which 

 are really large channels developed by various agencies. 

 If in the drainage of average soil, the farmer depended on the 

 movement of w^ater through the individual pore spaces, the 

 soil would never be in condition for crop growth. These lines 

 of seepage are developed by the ordinary forces that function 

 in the produ.ction of soil granulation, as freezing and thawing, 

 wetting and drying, lime, organic matter, roots, and tillage 

 operations. 



94. DetemiiiLation 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 means such data may be obtained. One method is to 

 saturate a soil 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. 



