> = RT In e_ 



e 



(1) 



where , 



-ty is the symbol for water potential, expressed as a negative value, 

 R is the universal gas constant , 

 T is the absolute temperature in °K, 



e is the actual vapor pressure of the water in the system, 

 e^ is the vapor pressure of pure free water. 



This equation implies that water potential is expressed in energy units, such as ergs 

 mole 1 , but these units are somewhat inconvenient to convert from energy units to 

 pressure units (atmospheres or bars), in which case, equation (1) becomes: 



RT , 

 -il> = — In e 



V e 1 J 



o 



where , 



— 3 " 1 



V is the partial molal volume of water (18.015 cm. mole ). 



(See Appendices 1, 2, and 3 for energy-pressure conversions, definition of units, and 

 equivalent relative vapor pressures for various solution concentrations.) 



Components of Water Potential 



There are a number of component forces affecting the water potential, the most 

 important of which are: 



Osmotic potential {ty ) : the osmotic component (it) of the total water potential (ip) reduces 

 the chemical free energy of a solution as a function of the presence of dissolved sub- 

 stances in the solution. Dissolved substances, such as salts, sugars, and other solutes, 

 are either ionized in solution or have an asymmetrical distribution of surface electri- 

 cal changes so that they attract the highly polar water molecules. The attractive 

 forces of these oppositely charged ions and molecular surfaces are reduced by the high 

 dielectric constant of water. The force of this attraction, resulting in neutralizing 

 ion-dipole bonds, reduces the chemical-free energy or water potential of the solution 

 below that of pure free water. 



Matric potential (tj; m ) : the matric component (m) reduces the water potential as a func- 

 tion of capillary or colloidal adsorptive forces by soil particles, cellular colloids, 

 and cell walls. In normal, nonsaline soils the energy status of water is more closely 

 related to the thickness of the absorbed film of water on the soil particle than to any 

 other factor. The force of adsorption between the matrix surface and the water mole- 

 cules reduces the chemical free energy or water potential below that of pure free water. 



Pressure potential (i^p) : the pressure component (p) may raise or lower the water poten- 

 tial depending on whether the molecules are subjected to pressures above or below 

 atmospheric pressure. Under either natural conditions or simulated conditions in the 

 laboratory, only atmospheric positive pressures will be of concern. Under atmospheric 

 pressure conditions, the effect of the pressure component on an open system, such as 

 soil, is zero. Positive pressure against the vacuolar membrane and cell wall in plants 

 (turgor pressure) resulting from hydrostatic forces of water will add free energy to 

 the system, and the water potential will be increased. At wilting, the pressure com- 

 ponent will approach, or reach, zero and will not affect water potential appreciably. 



3 



