The major argument for measuring the soil salt concentration and water content at the same position is their high correlation and spatial variability. Time domain reflectometry (TDR) was used for simultaneous measurement of soil water content, θ (derived from the soil dielectric constant ε), and bulk soil electrical conductivity, σa (from the attenuation of a transmitted pulse), for uniform and layered soil profiles in the laboratory. The purpose of the study was to test the different concepts of travel time and attenuation by using independent estimates of θ and σa. Both parallel (two‐rod) and simulated coaxial (three‐rod) probes resulted in essentially the same ε. The three‐rod probe measurements were easier to interpret and did not necessitate an impedance‐matching transformer. The TDR measurements of θ for layered profiles were not always accurate, especially when wet soil was overlying dry soil, due to erroneous interpretation of the TDR trace. In this case, the pulse travel time should be measured at the inflection point, which is sometimes difficult to identify, and not at the minimum of the TDR trace. This study reports a new method of calculating σa, which is based on a direct measurement of the transmission‐line load by TDR. This new method, which is simpler than the previous ones and independent of multiple reflections, was found to correlate better than the previously published methods with another, independent method for σa measurement (four‐electrode technique).

Soil electrical conductivity (ECa) by the four electrode method was studied in the laboratory as a function of soil water electrical conductivity (ECw) and water content in six soil types. A linear relationship between ECa and ECw was valid only at salinities > 4 mmho/cm. At very low salinity levels the contribution of the surface conductance was not constant and had a higher contribution than ECw to measured ECa. A method for calculating the partial contribution of surface conductance to ECa as a function of ECw and clay content is applied including a correction for dependence on the soil water content.

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Close, direct, and accurate monitoring of the plant water status may serve as a practical (irrigation scheduling) and a research (climateenvironmental induced physiologic changes) tool. Methods for highfrequency capacitance measurement (e.g., time domain reflectometry [TDR]) possess the potential for high resolution dielectric measurements with minimal dependence on properties of the measured matrix. The objective of this study is to test the accuracy, response time, and sensitivity of the TDR methodology in measuring changes in water status in a mango (Mangifera indica L., Cultivar 'Kent') tree stem exposed to several perturbations concerning water, salinity, fruit load, and radiation. Under several induced stress conditions, stem and root zone water content (u) and electrical conductivity (s) were simultaneously measured. Our study is distinct in its detailed and frequent measurements of stem water content (u stem) using short (29-70 mm) TDR probes in trees growing in a detached medium. We have found that u stem response to root zone applied salinity and water stress were negative and positive, respectively. Stem electrical conductivity (s stem) was primarily dependent on u stem and only negligibly on stem cells salinity. The u stem response time to water application was~4 h. Two practical outcomes of our study were: (1) Because the salt content of the tree cells only slightly affected s stem , stem resistivity measurements could be used to represent dielectric changes, and (2) quite short probes could be used to include young trees of slim tree branches.