42 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 



within the soil, such as size and grading of soil particles, presence of 

 organic matter, mineral salts, colloids, etc. Some of these properties 

 produce opposite effects. The following table shows the height to 

 M-hich capillary water rises in certain soils in one day. 



Table 11. — Height to ichich cfipiUary icatcr rises. 



Inchos. 



Llcrlit sandy soil ^ 14 



Gravelly soil . . Ifj 



Decomposed granite (loam) 21 



Heavy granite loam 16 



Heavy clay loam 11 



Heavy lava asli ' 16 



Pure sand 9 



In one day the capillary water moves upward about one-half its 

 apparent final limit. In three clays it moves about two-thirds its 

 apparent final limit. In some soils the movement upward in the 

 first two hours is as high as one-third of its movement for 30 days. 

 In pure sand it rises in one-half hour approximately one-half as 

 high as in' one day and in one day about 70 per cent as high as in 

 12 days. The movement is more rapid at the start in light soils, and 

 slows down rapidly. After three da3''s the movement is exceedingly 

 slow in pure sand. In heavy soils the movement is uniformly slower 

 and more sustained. In light soils the per cent of water in the soil 

 decreases rapidly with the height above the source, while in some 

 heavy soils the moisture content is strikingly uniform at different 

 depths. Horizontal capillarity occurs with much faster water move- 

 ment and greater uniformit}^ of moisture content. The speed and 

 distance varies with different soils and ranges from 7 to 33 feet in 

 the first 24 hours.^ 



The action of capillarity indicates that if water is to stand or flow 

 along a road embankment for even a few hours, regard must be given 

 to the height of the subgrade above the water line, and the lateral 

 distance of the water course from the road crust or foundation. 

 Assuming that the supply of free water will not endure more than 

 one day at a time, it is apparent that a height in the subgrade of 18 

 inches above the water line will insure that the moisture content of 

 the surface is not increased. Where the free water is present at a 

 considerable distance to one side this height may be safely reduced. 

 Shallow ditches a short distance from the center line tend, by hori- 

 zontal capillarity, to increase the water content of the subgrade at a 

 higher level than deep ditches similarly located. If the grade be 

 through standing water or against a continuous source of free water 

 so that the soil remains moist continuously, capillarity 'will lift the 



1 Capillary movement of soil moisture. Unpublished report of Riverside Exp., lOl.'i- 

 1917. W. W. McLaughlin. 



