The Movements of Soil Water. 63 



or less. * The structure of the soil has also some effect on the 

 capillary pressure. Changes in the size and arrangement of the 

 interspaces will of course affect the nature of the water films in 

 them. But structure is itself a function of water content and 

 its mutual relations with capillary movement and capillary 

 pressures are too complex for analysis here. 



In this discussion capillary pressure and the height of 

 capillary rise must not be confused with the velocity of the cap- 

 illary movement of water. The finer soils show much greater 

 heights of rise but the frictional effects in the individual 

 interspaces are so great that the final height is very slowly 

 attained. Two or three hundred days may be required in a 

 clav as against five days or less for a sand. Similarly in the 

 establishment of all capillary equilibria. The finer the soil 

 the slower are all capillary movements through it. This becomes 

 of great importance when the availability of capillary water to 

 the plant is under discussion. 



In the whole of the above it has been tacitly assumed that 

 the absolute surface tension of the soil solution is always the 

 same and that the only variations in capillary pressure are re- 

 lated to differences in the soil or its water content. This is not 

 strictlv true. The surface tension of the soil solution is affected 

 both bv the temperature and by the dissolved matters which it 

 contains. Rise of temperature always decreases the surface 

 tension; a soil can hold more water when it is warm, and warm- 

 ing frequently causes a soil to lose water which was held by 

 capillaritv so long as the temperature was low. On the other 

 hand, most dissolved substances increase the surface tension of 

 water and increase it nearly in proportion to the amount dis- 

 solved. The soil solution has, therefore, a higher surface tension 

 than pure water. J Furthermore, if the solution in one part of 

 the soil is of higher concentration than that in another, and if 

 the two solutions are in capillary contact, the weaker solution 



*It is a curious fact that the capillarj' rise in the finest natural clays is never so great as 

 that obtained in fine granular powders (such as mill tailings) formed by artificial grind- 

 ing and composed of particles relatively much larger than the particles of clay. The 

 cause of this anomolous behavior is not fully known, but it is probably due to some 

 physical change produced by the wetting in the particles of clay. It is conceivable, 

 for instance, that the clay particles swell and close the water passages altogether. See 

 the investigations of Loughridge — Report Calif. Agr. Exp. Station, 1892-94; 91-100 



JThis is true only of clean, unfouled surfaces. Fretiuently, however, the suifaces of the 

 soil solution are fouled by thin films of one or another of the oily organic bodies of the 

 soil, and the effective tension of the surface is thus decreased. For this reason the 

 actual tension of the soil solution may be considerably lower than that of water. 



