222 SOILS. 



lowed to soak into an air-dry soil column without sensible shock 

 or motion, from a constant level, we obtain the nearest ap- 

 proach to a definite determination of the relative permeability 

 of soils to water under the conditions usual in the arid region. 

 A number of determinations thus made is tabulated in the dia- 

 gram given below, which embodies the observations made by 

 Mr. A. V. Stubenrauch 1 in connection with a more extended 

 investigation. 



As these experiments were made with soils not in their field con- 

 dition, but gently broken up with a rubber pestle, a standard of com- 

 pactness was established by weighing the quantity which could con- 

 veniently be settled into a tube space of 100 centimeters capacity by 

 tapping the sides and bottom of the tube, without touching the soil 

 itself. In this way the following standards were established : For the 

 University Adobe soil, 140 grams ; for the Yuba loam soil, no grams ; 

 for the Stanislaus sandy soil, 170 grams. Tubes i^ inches wide were 

 used, and the soils were introduced in bulk, inside of a cylinder of stiff 

 paper upon which previously to rolling it up the soils had been thoroughly 

 mixed. After introducing the soil-filled paper roll it was gently with- 

 drawn, leaving the soil column in the tube as uniform as before ; a con- 

 dition almost impossible of fulfilment when the soil is introduced piece- 

 meal. The tubes were, of course, left open at the lower end, using a 

 wire netting to keep the soil column in, so that the air could escape 

 freely before the descending water column. 



The results thus obtained do not, of course, apply directly to the 

 same soils undisturbed in place in the field ; where, moreover, the air is 

 confined by the wetting of the surface and thus directly opposes pen- 

 etration of the water. Still, they doubtless give a correct idea of their 

 relative permeability for water when in the tilled condition. The water 

 level was automatically maintained at the depth of half an inch above 

 the surface of the soil columns. Pore-spaces given are calculated from 

 volume-weight and specific gravity. 



This diagram shows plainly that there is no direct relation 

 between the total pore-space in a soil and the facility of water- 

 penetration. The highest pore-space, in the fine-grained allu- 

 vial loam, allows more rapid percolation than the heavy clay or 

 adobe soil, but is greatly exceeded by the coarser sandy soil. 

 In all it is very apparent that the downward movement slows 

 down as the water descends, doubtless because the great fric- 

 tion in a longer column gradually diminishes the effect of hy- 



1 Rep't Calif. Exp't Station for 1898 to 1901, p. 165. 



