The robust and accurate HydraProbe probe is the optimal choice for your soil measurement campaign of the parameters soil moisture, temperature and electrical conductivity. With the patented measurement principle of coaxial impedance dielectric reflectometry and the operating frequency at 50 MHz, the HydraProbe offers excellent accuracy, exceptional comparability of values between different probes (+/-1.2%), virtually no drift and allows soil measurements over a wide range of soil types and salinity. The 4 measuring rods made of hardened stainless steel are stable and corrosion resistant. The probes can be easily connected to all standard data loggers (SDI-12, ASCII RS485, Modbus RS485) or for mobile applications with HydraGO and the free app the data can be stored directly on your mobile device.

Measuring principle

Standing electromagnetic waves at a radio frequency of 50 MHz are reflected from the soil, and the return signal is processed to generate soil moisture related variables such as the real dielectric permittivity. A separate thermistor in the base plate of the probes measures soil temperature. By separating the real part εr from the imaginary part εi of the real dielectric permittivity, the HydraProbe is less affected by variable salinity, temperature and mineralogy of different soil types than TDR or FDR soil sensors.

References

The HydraProbes are particularly often used in long-term and spatially extensive monitoring projects and are, for example, part of the standard instrumentation of several national and state measuring networks in the USA, e.g:

• U.S. Climate Reference Network (USCRN), for the prediction and evaluation of dry periods/droughts
• in 460 stations of the SNOTEL measuring network (snow telemetry) of the Natural Resources Conservation Service (NRCS)
• Soil Climate Analyses Network (SCAN) of the U.S. Department of Agriculture, for referencing satellite data and monitoring drought stress and climate change

Background

The Stevens HydraProbe is different than other soil sensing methods. It characterizes the ratio of the amplitudes of reflected radio waves at 50 MHz with a coaxial wave guide. A numerical solution to Maxwell’s equations first calculates the complex impedance of the soil and then delineates the real and imaginary dielectric permittivity (Seyfried 2004, Campbell 1990). The mathematical model that delineates the real and imaginary component from the impedance of the reflected signal resides in the microprocessor inside the digital HydraProbe. These computations are based on the work of J. E. Campbell at Dartmouth College (Campbell 1988, Campbell 1990, Kraft 1988). 

The HydraProbe from an electric and mathematical prospective can be referred to ratiometric coaxial impedance dielectric reflectometer and works similar to a vector network analyzer at a single frequency. The term “ratiometric” refers to the process by which the ratio of the reflected signal over incident signal is first calculated which eliminates any variability in the circuit boards from one probe to the next. This step is performed on several reflections. The term “coaxial” refers to the metal wave guild that get inserted into the soil. It has three outer tines with a single tine in the middle that the both receives and emits a radio frequency at 50 MHz. “Impedance” refers to the intensity of the reflected signal, and “dielectric reflectometer” refers to a reflected signal that is used to measure a dielectric.

Basing the soil moisture calibration on the real dielectric permittivity instead of the apparent permittivity has many advantages. Because the HydraProbe separates the real and imaginary components, the HydraProbe’s soil moisture calibrations are less affected by soil salinity, temperature, soil variability and inter sensor variability than most other electronic soil sensors.

Measuring the electrical conductivity in soil, how and why?

The salt content of a soil is important for the water absorption and health of plants, as well as for soil fertility. Electrical conductivity (EC) serves as a measurement parameter for the salt content. A distinction is made between:

• Bulk EC - total conductivity of the soil, which is made up of the conductivity in soil, pore water and soil air and which can be measured in-situ with appropriate sensors
• Pore water EC - the conductivity of the salts dissolved in the soil water. The pore water EC cannot be measured directly, but can only be calculated using equations such as the Hilhorst equation, or determined by sampling soil water using suction probes.

Soil moisture measurement and EC influence each other. A high EC content can influence the moisture measurements and lead to errors in capacitive moisture sensors. HydraProbe soil moisture measurement is less sensitive to salinity than other capacitance-based probes. On the other hand, the measured EC value of the soil will change drastically with water content. The effect of salinity on water availability to the roots of a plant is large. If the electrical conductivity of the soil water changes, the water requirement will also change. Excessive fertilization, poor drainage and saline irrigation water, but also natural factors, can lead to an undesirable accumulation of salts in the soil. This type of salinization affects more than 16 million hectares of agricultural land worldwide.

By monitoring the electrical conductivity and soil moisture at different depths, the salt content in the plant's root zone can be controlled by means of suitable irrigation control. This effectively prevents the leaching of nutrients on the one hand and the accumulation of salts on the other. By measuring both dielectric permittivity components (real & imaginary), the Hydraprobe probe offers the possibility to approximately calculate the EC of the soil pore water using the Hilhorst equation. With the Hydraprobe Professional, this calculated pore water EC is output directly, which allows a better characterization of trends and causes of nutrient enrichment, drainage and the characterization of high salinity soils.

• Brochure Stevens Water HydraProbe