A METHOD OF DETERMINING SOIL-MOISTURE CONTENT BASED ON THE VARIATION OF THE ELECTRICAL CAPACITANCE OF SOIL, AT A LOW FREQUENCY, WITH MOISTURE CONTENT

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Abstract
The electrical capacitance of moist soil of different textures and at different moisture contents was detd. with an A.C. Wheatstone bridge excited by a frequency of 1000 cycles, and with a specially designed probe-condenser of which the sensitivity to changes of soil moisture content was considerably enhanced by electrode polarization. The data were obtained on 5 Yolo soils covering a range of textures, extending from a moisture equivalent of 6.7 to 29.1. All the data thus far available indicate that the shape of the curve giving the dependence of electrical capacitance on soil-moisture, above the permanent wilting % over the range of readily available moisture to plants, does not (within the present exptl. error) depend appreciably on soil texture. Although a condenser of only one geometrical shape was used in reaching the foregoing conclusions, a condenser of almost any other shape should lead to similar results. In practice, the exact shape and design of the soil condenser would be detd. by the purpose for which it is to be used. A method, which seems to show promise, is suggested for using the capacitance method as a soil-moisture-content indicator whereby it would seem unnec- essary to calibrate the condenser over the entire range of soil-moisture content for every soil on which the method is used. A reply is made to a theoretical argument, presented elsewhere in the literature, leading to the conclusion that the capacitance of soil and of porous plaster condensers are unreliable indicators of soil-moisture content. The argument is shown to be based upon what appear to be experimentally unfounded assumptions and a seeming disregard of the effect of polarization in moist dielectrics and at electrode surfaces.