58 D.A.ROSE 



The second important component arises from the presence of such solutes 

 and is an osmotic or solute potential that becomes important for water 

 movement through semi-permeable membranes such as plant roots or a 

 water surface from which there is evaporation. Over the meniscus of a salt 

 solution the vapour pressure is less than it would be over a meniscus of pure 

 water or over a plane surface of solution, and the decrease below what would 

 exist over a plane pure surface is a measure of the total potential : 



ip — — In —cm (2) 



5 Po 



where tp cm is the total potential, R erg gm"^ °C "^ is the gas constant per 

 gm of water vapour, p is the vapour pressure over the meniscus, and po 

 is the value it would have over a plane surface of pure water at the same 

 temperature T°K. 



Note that p/p^ is the equihbrium relative humidity, never greater than 

 unity, and so (/» is either negative or zero. Thus tp has the same sign as h, and 

 may be split into components 



0= /j+7Tcm (3) 



where tt represents the osmotic potential. It too is negative. The osmotic 

 potential is proportional to the solute concentration m, so eq. 3 may be 

 rewritten 



ip = // + Amcm (4) 



where A = 4-73 x 10* for NaCl when m is measured in molal units. The partial 

 specific Gibbs' free energy of the water is then gip erg gm~\ while 

 Schofield's pF = logiQ { — ^)- 



The matric potential can be measured by tensiometer, suction plate, 

 pressure plate, or pressure membrane devices : the total can be measured by 

 freezing point determinations or by a variety of vapour pressure techniques 

 such as that of Monteith and Owen (1958). 



MOVEMENT OF WATER 



If a gradient of potential exists, water movement can be expressed by the 

 relation 



,=^4^=-Av^ (5) 



where Q/f gm sec ~* cross an area A cm^ normal to the direction of flow 

 when moving under the total water potential gradient V<A. Equation 5 



