EFFECT OF DISSOLVED SALTS ON WATER MOVEMENT 59 



defines k cm sec"\ the capillary conductivity of the water in the medium, 

 hi soils free from solutes there is general agreement that both hquid and 

 vapour move under a potential gradient V<A=v/j because 7r=o, vapour 

 movement having been treated theoretically by Phihp (1955). When 

 solutes are present, however, there are differences of opinion as to their 

 effect on water movement. 



Edlefsen and Anderson (1943, pp. 278-280) consider that an osmotic 

 potential gradient along a soil column will lead to water movement 

 before the solute concentration gradient has been brought to zero by ionic 

 diffusion. In contrast, Richards and Weaver (1944) observe that, if a tensio- 

 meter is fdled with distilled water and the manometer allowed to attain an 

 equihbrium reading with the porous cup standing in distilled water, then 

 almost no change in pressure (less than 0-003 atm) is registered when 

 saturated NaCl solution is substituted for the distilled water surrounding the 

 cup. They state that 'in the absence of semi-permeable membranes moisture 

 flow is produced primarily by gravity and gradients in soil-water tension, 

 and not directly by solute concentration gradients'. Schofield (1952) also 

 expressed the view that 'two distinct processes are involved in water move- 

 ment, (a) bulk flow of the solution governed by its viscosity and one aspect 

 of the geometry of the pore space, and (b) diffusion (of the water and solutes 

 in opposite directions) governed by the diffiision coefficient and a different 

 aspect of the geometry of the pore space'. Low (1955), from a thermo- 

 dynamic treatment of the equihbrium and movement of soil water, 

 predicted that osmotic potentials would act as negative hydraulic pressures 

 in influencing water diffusion. His equation was tested on clay suspensions 

 and shown to be vahd, but the measured osmotic potentials were much 

 lower than theoretically calculated values. 



The following analysis assumes that vapour moves under the total water 

 potential gradient V >Jj, but that hquid moves only under a matric potential 

 gradient V h. 



THEORY OF THE DIFFUSION CELL 



Consider the system ABODE (Fig. la), consisting of a porous block BCD 

 between air gaps AB, DE, with water potentials ip^, ip^, W, ^g, ifj^ imposed 

 such that )Ai>i/'2 >^>h>'p4. and</ri + )A4 = )/'2 + 'A3 = 2'^. In the steady 

 state under these conditions the flux of water is uniform, either as vapour 

 distilling across the air gaps AB and DE or as hquid diffusing through the 

 porous sample BCD. In practice, A and E are pads of fflter paper saturated 

 with NaCl solutions of concentration m^ and ttiE respectively, and BCD 



