STATE OF WATER IN THE 

 SOIL -PLANT- ATMOSPHERE CONTINUUM 



Theoretical Considerations 



Transpiration loss by a plant results in reduced turgor pressure, and, consequent- 

 ly, reduced potential energy of water in the leaves. This reduction of water creates 

 an energy gradient in the water column from the soil through the roots and xylem tissue 

 of the stem to the leaves. Actually, the transpiration process itself results from a 

 decreasing energy gradient of the water from the leaves to the surrounding atmosphere. 

 Thus, we can speak of a soil-plant-atmosphere continuum with respect to the gradient of 

 decreasing free energy in water from the soil, to the plant, and out to the atmosphere. 

 Evaporation from the soil surface also results from a decreasing energy gradient from 

 the soil to the atmosphere. The driving force for water movement is the decreasing 

 energy gradient of water in the system under consideration. Therefore, an understand- 

 ing of the processes of water transfer must begin with the energy relations of soil and 

 plant water in terms of thermodynamic principles. A brief review of these principles 

 is presented here, but the interested reader is referred to the following references 

 for a more rigorous mathematical treatment of the theoretical considerations of energy 

 relations of water: Slatyer 1967; Gardner 1965; Kramer and others 1966; Taylor and 

 others 1961; Taylor 1964, 1965; Taylor and Stewart 1960; Spanner 1964; and Briggs 1967. 



The concept of energy status of water in a system is best explained in terms of 

 free energy, or more correctly, Gibbs free energy (describes spontaneity). The free 

 energy of a component (water, in this case) in a system (soil or the plant) is an 

 expression of the capacity of the component to do work. The free energy of water de- 

 pends on the mole fraction of water available or the concentration of the water mole- 

 cules in the system relative to the concentration of other components in the same system. 

 Since the actual free energy is difficult to calculate, the free energy of water in the 

 soil or in plant tissue can be expressed as the difference between the free energy of 

 pure free water and the free energy of the water in the system at the same temperature 

 and pressure. The resulting net free energy of water has been referred to as the chem- 

 ical potential, or the more widely accepted term, water potential (Slatyer and Taylor 

 1960, Taylor and Slatyer 1962). 



The water potential is affected by factors that change the free energy of water 

 molecules in the system. The presence of solutes (ionized or nonionized molecules) , 

 colloids (clay or very large molecules), large particles such as clays, silt, and sand, 

 all decrease the water potential. The water molecules in the system, such as soil or 

 a plant, interact with these components and decrease the free energy of the water below 

 that of pure free water. Water potential can be defined, then, as the minimum addi- 

 tional work required to remove water from the soil (or any other system) in excess of 

 the work required to remove pure free water from the same location. On the other hand, 

 the partial specific Gibbs free energy, or the water potential of water in a multi- 

 component system, is an expression of the ability of a unit mass of water to do work 

 compared to the work that an equal mass of pure free water .can do. Since the presence 

 of other components (such as solutes) reduces the free energy of water, the potential 

 for water to do work in a multicomponent system is less than that of pure water, and 

 thus the water potential will be negative. 



The manner in which solutes and other components reduce the water potential in 

 soil and plants can be expressed in terms of their effects on the chemical activity of 

 water. "Activity" is a thermodynamic term for the tendency of water to react or move 

 in a system, and this value is equal to the relative vapor pressure of the water in the 

 system. Therefore, water potential is described in terms of the relative vapor pressure 

 of the water in the system to that of pure free water at the same temperature and pres- 

 sure, and can be written as: 



2 



