The Substrate Moisture sensor (Teros12) from Metergroup measures the electrical conductivity (EC) of soil and substrate. In the platform, you will find ECb (Bulk EC) and ECw (Pore EC). ECb is the measured value and ECw is a calculated value from ECb. The agriculture industry standard, or what is usually meant by EC, is ECw (Pore EC). Read further to find out what the difference is between ECb and ECw. Please note that calibration and VWC values impact the ECw output value. Multiple calibrations are available to use and can be set within the sensor settings.

In this article:
Measured ECb (Bulk EC)
Converting ECb to ECw (Pore EC)
>25% VWC needed for ECw values

Measured ECb (Bulk EC)

The Substrate moisture sensor (Teros12) measures ECb (Bulk EC). This is the electrical conductivity of the bulk soil (soil, water and air) and is measured between needle 2 and 3. During measurements, the EC of the water is assumed to be its EC at 25°C. The ECb is factory calibrated to be accurate for soil and substrates. For salty soils, please follow cleaning instructions as measurements above 10 ds/m are sensitive to contamination of the electrodes.

Converting ECb to ECw

ECw is the electrical conductivity of the water in the soil pores. The agriculture industry standard or what is usually meant by EC, is pore water EC, which we abbreviate in the platform to ECw (water).

To measure ECw, tiny sensors would have to be inserted into microscopic water-filled pores. Obviously, it’s not possible to measure the EC of water on that scale. In fact, the only way to measure pore water EC is by extracting a soil water sample and measuring the EC of that sample.
So, how is 30MHz able to show ECw in the platform? We calculate the ECw by converting the measured ECb values. We do this by using the G. Amente, John M. Baker, and Clive F. Reece model:

Please note that calibration and VWC figures will impact the ECw output value. See this article to choose the calibration that works best for your substrate. 

>25% VWC needed for EC values

Metergroup indicates that the method for calculating EC pore water results in good accuracy (±20%) in moist soils and other growth media. In dry soils, where the VWC is less than 25%, the influence of the water on the result of the conversion becomes too big, and it was recommended to not calculate the ECw in soils with a VWC under 25% using this method. This is why 30MHz does not display the ECw when the VWC is below 25% and shows the following error in the platform: