THE EFFECT OF DISSOLVED SALTS ON 

 WATER MOVEMENT 



D.A.Rose 



Rothamsted Experimental Station, Harpenden, Herts. 



If plants are to survive drought they must have a source of water available 

 near the roots, and to be of use the source must also be accessible. Because 

 root extension is not unhmited, at some stage the water must move to the 

 roots, and the flow wiU be governed by the product of a conductivity and a 

 potential gradient (cq. 5). Full understanding of 'drought resistance' needs 

 detailed study of both conductivity and potential, and in real soils there is 

 great complexity arising from sweUing and shrinking during wetting and 

 drying. In the present note the complexity is avoided by concentrating on 

 the behaviour of a rigid matrix. 



TOTAL POTENTIAL OF WATER IN A POROUS 



SYSTEM 



There are several possible components that can make up the total water 

 potential in a porous system, but only two arc important in the present 

 context. First there is the inatric potential, which the water possesses because 

 it is held in a framework of fme pores, and arises from capillarity (dominant 

 in wet media) and physical adsorption (dominant in nearly dry media). In 

 the wetter media there is a curved meniscus separating water from air, the 

 radius of curvature (r cm) being determined by the size of pore the meniscus 

 occupies. Standard theory shows that the hydrostatic pressure in the water 

 is less than that of the atmosphere by an amount zS/r in absolute units, 

 where S dyne cm"^ is the surface tension of water. This pressure deficit can 

 be defined as the matric potential in a convenient practical unit, the height 

 of water column h cm required to product this deficit, when 



I ^^ / \ 



n= — era (i) 



pgr ^ ' 



where p gm cm-^ is the density of water, and^ cm sec~^ is the acceleration 

 due to gravity. Note that the meniscus is a perfect semi-permeable mem- 

 brane : water molecules can pass across it freely but solute molecules cannot. 



