THE WATER OF SOILS. 



225 



This table is very instructive in showing the great difference 

 in the rapidity of percolation in materials of uniform, even- 

 sized grains, as compared with such as contain particles of 

 many different sizes, in which the interspaces of the larger ones 

 are filled more or less closely by the smaller sizes of particles 

 (see chapter 7, p. 109). While it is true that we have no defi- 

 nite physical analysis of the soils here used, the differences are 

 so great as to be sufficiently striking. Compare the percolation 

 through the sand of .155 mm. uniform grain-size (a fine 

 sand), during the first half hour, with that through the sandy 

 loam during the first 21 hours. Twice as much water has 

 passed from the sand as from the soil in one forty-second part 

 of the time. Comparing similarly the finest sand, .083 mm. in 

 diameter, with the clay loam, we find the difference to be as 

 one to seventy-three. It is thus evident that but for the vari- 

 ously assorted sizes of the soil-particles, water would not be 

 held long enough to supply plant growth. 



Percolation in Natural Soils. In artificial percolation ex- 

 periments, as well as during a fall of rain, the gradual settling 

 of the fully wetted soil-column produces a compacting of that 

 portion of the mass, that increasingly impedes the downward 

 penetration. The effect of this under natural conditions is 

 readily seen in the fact that after the first, rapid absorption of 

 falling rain by the soil when in good tilth, there is a gradual 

 slackening of the process even when the rain is fine and slow, 

 causing a perceptible increase of the runoff until, should the 

 rain continue for some time, the absorption becomes so slow 

 as to cause all, or nearly all the water to drain off the surface. 

 The soil is then called " saturated," having really arrived at 

 that point right at the surface, and to a depth varying accord- 

 ing to the duration and amount of rain, and the natural per- 

 viousness of the land. 



When the rain ceases, the visible saturation of the surface 

 usually soon disappears in cultivated soils, and the zone of 

 saturation begins to descend. The progress of this descent 

 may be very strikingly observed in a series of holes (post- 

 holes) dug or bored across a ridge; as indicated in the sub- 

 joined schematic diagram, in which the successive dotted lines 

 represent the levels of the descending " bottom water " at suc- 



15 



