INTRODUCTION 



In recent years, researchers in the environmental sciences have become interested 

 in developing a means of measuring and expressing the free energy status of water in 

 the soil-plant-atmosphere continuum. During the last decade a substantial amount of 

 research was devoted to this problem, and from this has emerged a whole new theoretical 

 approach based on thermodynamic principles and terminology. This research marked a 

 notable turning point in the science of soil-plant water relations; also, these studies 

 offered a fresh insight into the question of water and energy transfer. Certainly, 

 some of the most important contributions emanating from this work are the great number 

 of experimental techniques and methods now available for research in water relations. 

 The principal technique, thermocouple psychrometry , is used to describe the free energy 

 status of water in the soil-plant continuum in quantitative terms consistent with modern 

 thermodynamic theory. 



There is abundant literature dealing with the specific details of construction and 

 the theory of operation of these instruments; however, this literature is rather exten- 

 sive and specialized. It is the purpose here to briefly review the theoretical consid- 

 erations of water relations, to review the more pertinent aspects of thermocouple 

 psychrometry, and to describe in detail the construction, calibration, and use of a 

 rapid-response thermocouple psychrometer designed for measuring the free energy status 

 of water in the dynamic soil-plant complex. 



Measurement of the free energy status of water is essential to studies of the 

 relationships between water in the soil and in plants and its consequent loss to the 

 atmosphere. Water in the soil-plant-atmosphere continuum is dynamic and is rarely, 

 if ever, in equilibrium with water in adjacent locations. The driving forces respon- 

 sible for water transfer are gradients of decreasing free energy resulting from plant 

 transpiration, evaporation, vapor pressure gradients, temperature gradients, and various 

 other forces. Thus, water in the soil-plant-atmosphere continuum follows a gradient of 

 decreasing free energy from the soil, through the plant, out to the atmosphere. During 

 periods of high transpiration demand, particularly steep energy gradients may result 

 within very small distances between the soil and roots, within the plant itself, and 

 between the plant and surrounding atmosphere. Plant responses to water stress are more 

 closely related to the energy required to remove a unit of water from the soil than any 

 other single factor. Therefore, to be most useful, measurements of water in dynamic 

 systems should include as many aspects of the component factors controlling water 

 transfer as possible. 



There are a number of methods available for describing the water status in soils 

 and plants; these methods are mainly based on measurements of water quantity. However, 

 an alternate method based on determination of water-free energy also offers many advan- 

 tages. One of the most obvious advantages of this latter method is that of directly 

 expressing soil water in terms of the energy required for the removal of a unit of water 

 from the soil by plants. Measurements of soil water energy have merit because they are 

 more directly comparable among different textural classes. A given soil water energy 

 level has the same implications with respect to root absorption, evaporation, or other 

 forces, regardless of the physical properties of the soil. Also, recent advances in 

 techniques now permit direct measurements of the in situ energy status of water in soils 

 and plants with a minimum of disturbance. These measurements can be accomplished si- 

 multaneously in soil and plants and results are expressed in the same meaningful terms. 



