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Converting Bulk EC to Other Forms of EC for Salinity Sensors

December 31, 2014 10:28 am

Salinity sensors such as Decagon Devices GS3 and 5TE measure bulk electrical conductivity (EC) of the soil. Yet scientists often need to know soil water EC, saturation extract EC or 1:5 mass ratio EC.


Figure 1. The GS3 and 5TE sensors measure EC as well as temperature and volumetric water content (VWC).

The conversion from bulk EC (ECb) that the GS3 or 5TE sensor measures directly to other meaningful forms of EC like soil water EC (ECw) or saturation extract EC (ECe) is not particularly precise or robust. Trying to get to a 1:5 mass ratio EC is likely even more difficult. That said, there is still useful information in the ECb signal. If you want to try to get an idea of that 1:5 number, here are some thoughts on how best to do that.

The first conversion that needs to be made is from ECb to ECw, and this conversion is best made at high water content. If you use the Hillhorst technique that is offered in the GS3 and 5TE manual and in DataTrac 3, then the ECw value in high water content soil should be reasonable accurate. Then, you would need to use information about the volumetric water content and the dry bulk density of the soil to dilute that ECw number to the ECw number for a 1:5 mass fraction. That should be a fairly straightforward manipulation. One thing to note is that even if there is some inaccuracy in the 1:5 number that results, you should be able to adequately track changes in that 1:5 number over time if you always use the values at high water content to do the calculations. In other words, the precision should be good even if the accuracy is not.

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