Decagon Devices 5TE Operator's Manual

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5TE Operator’s Manual3.Theory9εb is also measured by the 5TE. Raw VWC counts can beconverted to bulk dielectric by the 5TE dielectric calibra-tion:Finally,εσb=0 is an offset term loosely representing thedielectric permittivity of the dry soil. Hilhorst (2000) recom-mended thatεσb=0 = 4.1 be used as a generic offset. However,our research in several agricultural soils, organic, and inor-ganic growth media indicates thatεσb=0 = 6 results in moreaccurate determinations of σp. Hilhorst (2000) offers a sim-ple and easy method for determining for individual soil types,which will improve the accuracy of the calculation of σp inmost cases.Our testing indicates that the above method for calculatingσp results in good accuracy (± 20%) in moist soils and othergrowth media. In dry soils where VWC is less than about0.10 m3/m3, the denominator of equation 1 becomes verysmall, leading to large potential errors. We recommend thatσp not be calculated in soils with VWC < 0.10 m3/m3 usingthis method.Pore Water vs. Solution ECAs noted in the previous section, pore water electrical con-ductivity can be calculated from bulk EC using the sensor-measured dielectric permittivity of the medium. However,pore water EC is not the same as solution EC. Pore waterEC is the electrical conductivity of the water in the porespace of the soil. One could measure this directly if the soilwas squeezed under high pressure to force water out of the50Rawbεε = (3)