64 D.A.ROSE 



fertiliser in the soil, but the predictions of Phihp (1955), that vapour 

 conductivity is several orders of magnitude lower than liquid conductivity 

 in fairly moist soil, broadly confirmed by my current experimental work, 

 show that this effect is hkely to be small compared with Hquid movement 

 towards the roots. 



In more general terms the presence of solutes causes an osmotic potential 

 which tends to decrease the diffusion rate of water, this decrease being 

 proportional to the solute concentration and inversely proportional to the 

 diffusion coefficient of the solute molecules in the porous medium, thus 

 confirming the views of Schofield (1952). Extension of this work to 

 unsaturated media should provide information on the decrease of ionic 

 diffusion as moisture content decreases, which should be of value in the 

 study of movement of ions in the soil, the translocation of plant nutrients 

 being an important example. 



ACKNOWLEDGMENTS 



The author gratefully acknowledges the award of a research scholarship 

 from the Agricultural Research Council. He also thanks members of the 

 Building Research Station for providing stone samples, and H. L. Penman 

 for his help and encouragement. 



REFERENCES 



CuRRiE.J. A. (i960) Gaseous diffusion in porous media, Parts i and 2. Brit. J. appl. Phys. 



II, 314-324- 

 Edlefsen, N.E. & Anderson, A. B.C. (1943) Thermodynamics of soil moisture. 



Hilgardia, 15, 31-298. 

 Low, P.P. & Deming, J. M. (1953) Movement and equilibrium of water in hetero- 

 geneous systems with special reference to soils. Soil Sci. 75, 187-202. 

 Low, P.P. (1955) Effect of osmotic pressure on diffusion rate of water. Soil Sci. 80, 



95-100. 

 MoNTEiTH, J.L. &: Owen, P. 0.(1958) A thermocouple method for measuring relative 



humidity in the range 95-100%./. sci. Instrum. 35, 443-446. 

 Philip, J. R. (1955) The concept of diffusion applied to soil water. Proc. nat. Acad. Sci. 



India, 24A, 93-104. 

 Richards, L. A. & Weaver, L.R.(i944) Moisture retention by some irrigated soils as 



related to soil-moisture tension. J. agric. Res. 69, 215-235. 

 Robinson, R. A. & Stokes, R.M.(i955) Electrolyte solutions. London: Butterworths 

 Schofield, R.K.(i952) Private communication to Low& Deming (i953)- 



