APPENDIX 



Appendix 1 



Conversion of energy units to pressure units. The expression of chemical potential 

 in energy units (ergs mole" 1 ) is inconvenient for discussions of soil-plant water rela- 

 tions. Therefore, we ordinarily deal with pressure units; the units of equation (2) are 



erg mole 1 



erg cm. -3 = dyne cm. 1 cm. 3 = dyne cm. 2 



cm. 3 mole" 1 

 and : 



1 bar = 0.987 atm. = 10 6 dyne cm." 2 = 10 6 erg cm." 3 



Appendix 2 



Definition of terms commonly used in water relations: 



e - The vapor pressure of water in the system under consideration at tempera- 

 ture T, in mm. Hg 



e - The saturated vapor pressure of pure free water at temperature T, in mm. 

 ° Hg 



A^ - Relative water activity at temperature T 



R - Universal gas constant, expressed as: 



pressure units = 0.08205 liter atm. mole" 1 °K _1 

 energy units = 8.3143 X 10 7 erg mole -1 °K" 1 



= 8.3143 joules mole" 1 °K" 1 



= 1.987 cal. mole -1 °K" 1 



T - Absolute temperature °K (0°C. = 273.16 °K) 



V - The partial molal volume of water (18.015 cm. 3 mole l ) 



\p - Total water potential, defined as the capability of the water in the 



system to do work compared with pure free water at the same temperature. 

 Since the soil (or plant) water has a lower free energy (ability to do 

 work) than pure water, it will be designated as a negative energy or 

 pressure . 



\p - Osmotic or solute potential, resulting from the presence of dissolved 

 solutes which lower the free energy of water. This quantity will be 

 designated as a negative pressure. 



\\> - Matric potential, resulting from the presence of colloids and adhesive 

 surfaces of the soil matrix, and is a negative pressure. 



^ - Pressure on turgor potential, resulting from the effects of positive 

 ^ pressure greater than atmospheric pressure, and is designated as a posi- 

 tive pressure. 



2 3 



