1. General Description

The soil sensor measures soil temperature, soil moisture, soil bulk electrical conductivity, complex dielectric permittivity of soil, and electrical conductivity of the "water pores" contained in the soil hole. Both soil conductivity and pore water are automatically compensated for temperature.

The soil sensor bases its measurements on the physics and behavior of an electromagnetic radio wave reflected off the ground. Based on the measurement of the ground impedance through frequency domain reflectometry, the probe determines the dielectric permittivity. The complex dielectric permittivity allows the sensor to simultaneously calculate soil moisture and soil electrical conductivity. The sensor uses a frequency of 50 MHz, which minimizes the effects of texture and salinity, making measurements accurate on most hydroponic substrates.

The electrical conductivity of "pore water" is calculated using algorithms based on soil moisture and soil electrical conductivity values.

2. Structural Components

This sensor is designed to remain buried in the ground for many years. The probe is made up of three main components:

Marine grade stainless steel sticks set.
Nylon. The outer shell is made of this material.
Epoxy resin. It provides internal electronic protection as well as a ruggedly constructed design for the probes.

3. Specs

MEASUREMENT	ACCURACY	RANGE
Real dielectric permittivity	土1.5% or 0.2	1 to 80 where 1 = air, 80 = distilled water
Soil moisture for mineral soil	土0.03 max	From completely dry to fully saturated
Bulk electrical conductivity	土2.0 % or 0.02 S/m	0.1 to 15 mS/cm
Temperature	土0.3ºC	-10º to +55ºc

4. Measurements, Parameters and Data Interpretation

4.1 Soil temperature

The user can observe the soil temperature in both Celsius and Fahrenheit. The soil temperature varies in a range from -10ºC to +55ºC.

5.2 Soil moisture

The soil sensor provides soil moisture measurements in units of water fraction by volume (wfv or m3m-3), that is, a percentage of water in the soil that is displayed in decimal form. For example, a water content of 0.20 wfv means that one liter of soil sample contains 200ml of water. Accurate measurements of soil moisture in the range of 0 to 0.5 wfv. Outside this range the values are not accurate.

5.3 Soil bulk electrical conductivity

The probe measures in situ electrical conductivity in units of millisiemens per centimeter in a range of 0 to 15 mS / cm. This electrical conductivity is compensated by the temperature of the soil.

5.4 "Pore water" electrical conductivity

The soil sensor provides electrical conductivity measurements of the "water pore" found in the soil pores. In this way, the user can obtain the electrical conductivity of the water in the root zone after irrigating the soil. This measurement depends on the humidity of the soil and the electrical conductivity of the soil, according to the algorithm used. This electrical conductivity is also compensated for by temperature.

6. Installation

The soil sensor must be installed horizontally. It is buried about 15 centimeters below the surface and at a distance of about 5 centimeters from the stem. This sensor analyzes the soil so its placement is very important to obtain the expected values. Once installed, the sensor must be connected to the station by inserting the connector in its position and threading the piece to adjust it.

6.1 Warnings when installing the sensor

DO NOT remove the sensor from the ground by pulling the cable.
DO NOT use excessive force to drive the probe into the ground as the sticks could bend. If the probe has difficulty entering the ground due to rocks, simply relocate the probe to a slightly adjacent area.
While direct bury cable is very durable, it is susceptible to abrasion and shovel cuts. The user must be very careful NOT to damage the cable or probe if it is necessary to excavate the probe for relocation.
DO NOT install the probes where they can be run over by tractors or other agricultural equipment. The sensor may be tough enough to survive being hit by a tractor if it is buried; however, the compaction of the soil column by the weight of the vehicle will affect the hydrology and therefore the soil moisture data.
DO NOT place more than one probe in a bucket of wet sand while recording data. More than one sensor in the same compartment while powered can create an electrolysis effect that can damage the probe.

7. Maintenance. How to check the sensor

Before contacting the manufacturer for a sensor malfunction, perform the following the troubleshooting steps:

Make sure the connections to the data logger are correct and secure.
Make sure the batteries in your data logger are not dead or weak.
Check the sensor cables for nicks or cuts that may cause a malfunction.
If a probe appears to be malfunctioning, use a clean plastic container filled with distilled water and make sure the probe is completely submerged (If distilled water is not available, the user can use bottled mineral water for this test)
1. If the probe produces a soil moisture value of 80 +/- 5 in distilled water (or bottled mineral water), the probe is NOT malfunctioning.

8. Calibration

8.1 VWC

Each probe is factory calibrated to measure dielectric permittivity accurately in the range of 1 (air) to 80 (water). Numerous researchers have studied the relationship between dielectric permittivity and volumetric water content (VWC) in soil. As a result, numerous transfer equations that predict VWC from the measured dielectric permittivity. The probe uses the Topp equation (Topp et al. 1980) that fits most of the mineral soil.

Reference

Topp, G.C., J.L. David, and A.P. Annan 1980. Electromagnetic, Determination of Soil Water Content: Measurement in Coaxial Transmission Lines. Water Resources Research 16:3. p. 574-582.

8.2 Pore water and electrical conductivity

The probe measures in situ electrical conductivity in units of siemens per meter. Soil electrical conductivity is indicative of dissolved salts, dissolved solids, and fertilizers (McBride 1994). For many applications, it is advantageous to know the electrical conductivity of the solution contained in the soil pores, which is a good indicator of the solute concentration in the soil. The probe estimates the conductivity of the solution contained in the soil pores according to the work of Hilhorst (2000).

Reference

Hilhorst, M.A. 2000. A pore water conductivity sensor. Soil Science Society of America Journal 64:6 1922-1925.