Research in soil-plant water relations during the last two
decades has successfully addressed itself to the problems of expressing water status in soils and plant tissues in terms of free
energy. The initial impetus was provided by Spanner (1951}
in his classical work on the psychrometer. During the ensuing
years the science of water relations was provided with a new
theoretical approach based on thermodynamic principles and
terminology. This was complemented by a substantial amount
of research on basic techniques. The result has been a rather
sophisticated technology which provides a means of describing
the free energy status of water in natural systems in quantitative
terms consistent with modern thermodynamic theory. This
marks a significant turning point, because now we are capable
of expressing the flow of water in the soil-plant continuum in
meaningful terms of energy status. The recent comprehensive
review by Barrs (1968) cites over 50 references on the use of
the Spanner psychrometer and related vapor exchange methods,
and a great many other useful references on various techniques
for evaluating other aspects of plant water status. However, with
this and other reviews (Boyer, 1969; Brown, 19TH; Pack 1968,.
1969; Rawins, 1966) there still appears to be a need for a practical guide on the use of thermocouple psychrometers and other
methods of measuring water potential and its components
