by Gaylon S. Campbell
People have measured the water content of soils and other porous materials for a very long time, and the ideas surrounding that measurement are easily understood. Water potential is a more recent concept, and, in spite of its importance, is still not well understood by many soil and plant scientists. The main component of the soil water potential, the matric or capillary potential, was first described by Edgar Buckingham almost 100 years ago. Buckingham recognized that gradients in water potential are the driving forces for water movement in soil, and that components of the total potential could be balanced against each other. He made the first attempts to determine the relationship between water content and water potential in soils by balancing the matric potential against the gravitational potential in vertical soil columns which he stood in containers of water. He was correct in assuming that the matric potential would equal the negative of the gravitational potential (which he computed from the height above the free water surface) once the columns were at equilibrium. He could not have known then that equilibrium would have taken many decades.
More than a decade passed before significant progress was made beyond Buckingham's experiments. In the 1920's and 1930's two lines of research produced the main tools used by soil physicists for water potential measurements for the next 70 years. L. A. Richards, working in the laboratory of Willard Gardner at Utah State University, and later at the U. S. Salinity Laboratory, developed the idea of using a semipermeable porous ceramic to balance a pressure potential against a matric potential. At equilibrium the matric potential could be determined from the balancing pressure potential. The tensiometer and the pressure plate apparatus both came from this work. At about this same time George Bouyoucos at Michigan State University equilibrated gypsum blocks with soil and measured the water content of the gypsum to determine water potential. He measured the change in water content by measuring the electrical resistance of the gypsum block.
Again a decade or more passed before there was significant additional progress. In the late 50's two scientists, L. A. Richards in the U. S. and John Monteith in Britain published papers describing a thermocouple psychrometer for measuring the water potential of soil samples. Those early devices were eventually developed into the Tru Psi thermocouple psychrometer system sold by Decagon for the past 15 years. They were also the precursors to the new WP4 Dew Point PotentiaMeter now being introduced by Decagon.
Somewhat later, in the early 70's, Claude Phene and Stephen Rawlins, of the U. S. Salinity Laboratory, invented an improved matric potential sensor. The principle of operation of the sensor was similar to that of the Bouyoucos block, but they used a porous ceramic for the standard matrix, and used heat dissipation to determine its water content. These changes eliminated problems with dissolution and solute sensitivity that plagued gypsum block sensors. Both the ceramic and thermal sensor were improved upon by C. Calissendorff and myself at the Washington State University soil physics laboratory. These sensors are part of the ThermoLink system for field measurement of matric potential.
The development of accurate and reliable sensors has been a difficult and time consuming activity, but the dream of a century of soil physicists is finally near reality. The WP4 can measure the water potential of any sample between O and -40 MPa (saturated to air dry) in less than 5 minutes, with an accuracy of 0.1 MPa or better. This provides a host of opportunities to researchers ranging from rapid and accurate moisture characteristics over the entire plant growth range to studies of seed zone moisture in arid environments. Matric potential sensors with the ThermoLink can be used for in situ water potential measurements. Their range is -0.01 to -100 MPa. The response is approximately proportional to the logarithm of the water potential (linear with pF), so readings in the wet range have a resolution approximately equal to a tensiometer; readings in the mid range have a resolution comparable to a thermocouple psychrometer, and in the dry range the sensors respond to changes in atmospheric humidity. The sensors therefore offer a convenient, reliable and trouble-free method of monitoring water potential in the field.