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ASSESSING SOIL WATER CONTENT USING COMPLEX PERMITTIVITY

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Transactions of the ASAE.  VOL. 43(6): 1979-1985 . (doi: 10.13031/2013.3104) @2000
Authors:   W. S. Rial, Y. J. Han
Keywords:   Electrical permittivity, Dielectric constant, Soil water content

For most temperate zone food and fiber plants that require medium amounts of water (mesophytes), it is crucial for plant health and growth to know the volumetric water content in the growing media. Understanding the basics of the complex electrical permittivity of wet agronomic soil can lead to the development of a real-time, relatively low-cost, reasonably accurate, dielectric soil-moisture probe. Theoretical and practical concepts are presented concerning the nature of the complex permittivity and dielectric constant within an operating frequency range that can make development of a low-cost probe feasible. The real component of the complex permittivity contains water content information while the imaginary component contains soil electrical conductivity information. A commercial soil-moisture probe operating at 50 MHz was used to provide the temperature-corrected real component of the complex permittivity in custom-made, 85/15 (85% graded sand and 15% pulverized peat moss) golf course greens-mixture soil (soil especially formulated for use on golf course greens). Gravimetric water-content and real dielectric constant measurements were made simultaneously in the greens-mixture soil for developing an empirical equation with desired absolute error of less than 0.015 cm 3 /cm 3 between the permanent wilting point and field capacity values, and error less than 0.03 cm 3 /cm 3 between zero water content and the saturation value. When compared with the results using a time domain reflectometry (TDR) calibration equation, it is possible that a simple electrical capacitance/dielectric probe may perform as well as the more expensive TDR equipment. The probe was able to estimate the volumetric water content with reasonable accuracy in the presence of a 1:1 electrolyte (KCl) at concentrations from zero to 0.01 M.

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