Studies of the Dielectric Method of Measuring Soil Moisture

DURING the past 15 years a number of investigators have reported successful attempts to use the high dielectric constant of water as a basis for measuring soil.moisture. References to this literature have been given by Anderson (i). In spite of the promising results obtained, • none of the methods described has come into general use. A review of the literature suggested that this was due to the wide .variations in technics reported and the lack of comparative tests. A study has been undertaken whose purpose is to compare the various types of equipment and answer some of the more important questions regarding the basic principles involved. The points under consideration may be broadly classified as follows: (a) Importance of using between the condenser plates a medium having fixed porosity; (b) relative merits of condenser plates in direct contact with the soil water versus condenser plates insulated from the soil water; and (c) choice of frequencies of alternating voltage used for measuring capacitance. ' The present paper reports progress to date on this study. Only partial solutions to the problems listed above can be given at this time. The use of a fixed porous medium between the condenser plates has one purpose, to make possible a calibration of the unit which will apply to all soils. Obviously, then, if such a calibration can be accomplished without the use of such a medium, it can be eliminated. The method proposed by Anderson will, as he stated, require further tests. Since it depends on polarization capacitance which, in turn, depends on the use of bare electrodes, it therefore involves the question as to whether or not the electrodes should be insulated. Only one investigator (3) has reported studies in which a condenser with insulated plates was used. Dr. R. W. Cummings, who made some tests with this instrument, informed the author that, whereas good results were obtained in the laboratory using isolated portions of soil, he was not able to get readings in the field when the condenser unit was buried in the ground. In order to test this type of equipment more thoroughly, certain modifications were made so that consistent readings could be obtained in the field. On the assumption that stray capacities were causing erratic operation of the instrument, a soil unit (condenser) was'evolved in which the two plates are concentric and have an air core. This limits the flow of current to the medium between the two plates. Then a portable capacity -bridge was built in a metal cabinet to provide complete shielding. The shield is carried at ground potential while readings are being taken, thus preventing variations in capacity to ground from the bridge components. Construction of the soil unit is shown in Figs. I and 2. The inner plate is a piece of sheet copper, insulated with Dow-Corning 993, one of the thermoplastic silicone varnishes. Around this plate a thin layer of plaster of paris is cast, using a mold which leaves a spiral groove on the outside of the casting. In this groove is wound enamelled copper wire which forms the outer plate of the condenser. This arrangement permits good contact between the porous medium and the soil, yet maintains the concentric arrangement of the two plates. A rough calibration (capacity vs. pF) has been made and indicates that, as far as porosity is concerned, the plaster of paris is a suitable material for use as a dielectric medium. Previous attempts to use ceramic clay were unsuccessful due to the small size of the pores. A preliminary check on the stability of the equipment was made as follows: The soil unit was placed in a No. 2, tinned can and a loam soil was packed