The Value in Ground Truthing your EM Survey

We’ve talked about water holding capacity (WHC) previously, and following on from Nicole’s blog about visual soil assessments (VSA),  I’m wanting to shed a bit more light on how we go about putting water holding capacity values to your EM soil survey.

An EM survey shows you the differences in the EM value across the site where it is conducted. The values are relative to the rest of the values on the map and are not a specific water holding capacity. To get that you must ground truth the different EM zones and put values to the EM variation.

As a general rule we find that for every EM unit of change in your report, excluding the extremes at each end, the EM unit range is around 7-10mm of WHC difference in the top 700mm of the soil profile. I have to emphasise that this is a general rule and we have seen surveys where 1 mS/m (EM unit) has been worth 30mm of WHC difference over a map and others where the value is a lot less.

To find out the actual variation in WHC for your EM survey, Agri Optics select ground truthing sites and work alongside HydroServices who then go to the site with a neutron probe to ascertain the actual values at the given points. The objectives of this exercise are to:

1. Confirm the EM mapping reflects the differences in the soil moisture content.
2. Provide WHC assessments at key locations in key EM zones.
3. Confirm the EM map can be converted to a WHC zonal map for variable rate irrigation (VRI).

Agri Optics sends an EM map to HydroServices along with the GPS coordinate files so they know where to take the samples.

A shallow EM map with geo-referenced ground truthing sampling points marked.

Agri Optics also gives HydroServices the actual two soil profile depths (cm) and a spreadsheet with the EM values at the point locations.

Point #	Id	Lon	Lat	Zone Value	Shallow EM	Deep EM
1	Orange	175.xxx	-41.xxx	14.37	13.2	40.34
2	Orange	175.xxx	-41.xxx	14.37	12.95	35.73
3	Orange	175.xxx	-41.xxx	14.37	11.44	35.82
4	Yellow	175.xxx	-41.xxx	18.28	17.38	43.73
5	Yellow	175.xxx	-41.xxx	18.28	17.85	46.56
6	Yellow	175.xxx	-41.xxx	18.28	18.2	45.73
7	Green	175.xxx	-41.xxx	21.95	21.84	54.19
8	Green	175.xxx	-41.xxx	21.95	22.66	54.59
9	Green	175.xxx	-41.xxx	21.95	20.31	51.41
10	Blue	175.xxx	-41.xxx	25.81	27.01	62.05

With this data HydroServices then measure the soil moisture, calculate the full point, the stress point and the WHC at each site. This is then compared to the EM values in the different profiles and the correlations drawn up. HydroServices then write a report on their results for the client.

Graph showing the relationship between the soil moisture results and EM data.

From the data HydroServices collects Agri Optics can then generate a WHC map.

Normal EM zone maps can and are used in the majority of situations with great success for VRI, but by ground truthing the zones and generating a WHC zone map, the client is given an extra  level of information and certainty.

Ground truthing sites can also be used for moisture probe placement sites, as these sites have effectively been field calibrated by HydroServices for moisture probes. Agri Optics can provide you with AquaCheck soil moisture probes . I’ll go into more detail on this in a future blog, but more information is available now at www.agrioptics.co.nz/portfolio/aquacheck.

For more information on ground truthing contact Agri Optics or HydroServices.

Blog post written by Chris Smith from Agri Optics NZ Ltd. To find out more about EM mapping or any of the topics discussed in this blog call Chris directly, 021 796876.