Sentek Volumetric Ion Content (VIC) Conversion to ECe Virginia Tech

Goals

To establish field calibration equations for a range of soil textures correlating VIC measurements of TriSCAN to ECe samples collected within the sphere of influence of the TriSCAN sensor. The main goal here is to establish a calibration library in IrriMAX Live for easy conversion of VIC into ECe (Electrical Conductivity equivalent of the saturated paste extract).

Salinity Units

Electrical conductivity has been customarily reported in micro-mhos per centimetre(μmho/cm), or in milli-mhos per centimetre (mmho/cm). In the International System of Units (SI), the reciprocal of the ohm is the Siemen (S) and, in this system, electrical conductivity is reported as Siemens per metre (S/m) or as deciSiemens per metre (dS/m) (same as mS/cm).

 

 

 

 

Method: Benchmarking Soil Salinity – TriSCAN Calibration

Material per Location

  • 3 x Drill & Drop Probes (60 cm).
  • 1 x Portable Salinity Meter (range : 0 to 6 dS/m).
  • Calcium Nitrate or another common mineral fertilizer.
  • Containment buckets for liquid fertilizer application.
  • 10 cm Sampling auger.
  • Plastic sample bags.

Note: Please find below the steps involved of conducting a TriSCAN calibration which is taken out of our TriSCAN Manual. I highly recommend proceeding along these lines. Soil samples need to be taken within the sphere of influence zone of the TriSCAN sensor and not 12 inches away!!! The soil sampler auger should almost touch the probe during soil sampling.

 

The TriSCAN sensor currently provides an output in VIC (Volumetric Ion Content). This is sufficient for the monitoring of trends in changing soil salinity. In many instances, however, it is important to be able to relate VIC to soil Electrical Conductivity of the soluble extract (ECe) to obtain a quantitative measure of soil salinity. The following procedure outlines a simple methodology for benchmarking different levels of VIC in relation to ECe.

The procedure involves installing 3 soil moisture probes in the soil to be calibrated. One probe is subjected to low EC irrigation water (ideally less than 0.3 dSm-1), the second probe to medium salinity (recommend 3 dS/m-1) and the third probe is subjected to high EC water (recommended 6 dSm-1). This benchmarking procedure may be performed at any stage of the growing season while the monitoring probe is in place but is most useful if done early in the cycle.

 

Sampling Method

Step 1

Insert 3 x 60 cm Drill & Drop probes (P1, P2 and P3) at 1 meter apart, into a representative soil site.

Step 2

Collect data from each probe location for at least 30 minutes prior to any salinity application. Use a logging interval of 5 minutes. Download the data and compare soil water content and salinity readings. Ensure that data sets from the 3 locations are similar. Data similarity will provide a good baseline for benchmarking. If data are very different (e.g. if soil water content varies by 20mm and salinity varies by 2000 VIC’s at comparable depth levels), then reinstall the access tube at a different location. Using replicates increases the data value for use and publication purposes.

Possible reasons for these differences are:

  • Soil disturbance (old backhoe pit).
  • Air gaps around the access tubes during installation.
  • Site polluted with fertilizer spill.

Step 3

Leave the logging interval of the probe at 5 minutes. Place a drum or bucket with the bottom cut out around each probe, with the base buried slightly to minimize lateral movement of water. Add 25 liters of normal irrigation water (low EC) around one probe (Probe 1 – P1). Apply this in stages so that there is no overflow and such that it is continuous, with no air infiltration.

Add 25 liters of irrigation water mixed to 3 and 6 dSm-1 respectively with fertilizer, to the other probes (Probe 2 – P2 and Probe 3 – P3). Only 1-2 cups of fertilizer are required for this volume of water (depending on N content). This amount added should be sufficient so that the wetting front moves past the 15 and 25 cm depth sensor on the probe. The VIC reading from the TriSCAN should be used to check this. A greater volume of salinity water must be added if the VIC does only respond by less than 100 counts increase after 24-48 hours.

Note: give it a day for the salts to move past the 15 and 20 cm sensor. If you see an insufficient VIC response apply another 25 Liter with the respective water salinity concentrations for all probes and wait another day to see if VIC has increased sufficiently. Target VIC values could be for example: 500 (P1), 3000 (P2) and 5000 (P3).

If the infiltration rate is slow, then use elevated water containers with fitted taps and drain the water to each probe adjusting the flow so that the head of water remains at a steady state (the picture below shows 2 buckets per treatment to replicate each treatment).

Step 4

Once you have waited 24 – 48 hours and the VIC data show a clear rise at 15 and 25 cm soil depth, you are ready to take samples. On the day of sampling, set the logging interval of the probe to 1 minute. Allow collection of data for 10 minutes before commencing sampling.

Step 5

Take 3 soil samples (1-2 cups each) per 15 and 25 cm sensor depth around each access tube. Note the exact time and sampling depth (so we can correlate the sampled soil salinity with the logged VIC data to establish the calibration equation.)

Record the average for each depth of these final VIC and moisture values for the 3 treatments.

Be accurate with taking soil samples at exactly the depths required. For example, when collecting for the designated 25 cm depth, auger down to 20 cm and remove 2 cups of soil not exceeding a depth of 30 cm. Collect soil samples from immediately adjacent to the access tube. Make sure that no topsoil falls into the augured hole. Place soil samples into a plastic sealable plastic bag. Note on the bag the salinity treatment, sampling depth and time, depth level, replicate.

Step 6

Send soil samples to a laboratory requesting ECe, CEC and soil textural data.

Other information required:

Please provide Lat and Long, soil texture and CEC (particle size analysis and CEC performed by a laboratory) of the data collection site. Also the soil type and its texture.

Results

 

Adjusting from Volumetric Ion Content (VIC) to ECe units

To determine the relationship between VIC and ECe, plot the VIC output recorded by the TriSCAN in the field (take the data point 2 minutes before the soil sampling time) against the measured ECe in Excel (see above). Perform a linear regression to derive the equation that best describes the relationship between VIC and ECe (below).

 

 

 

 

 

 

In this situation, the following relationship applies: ECe = 0.0012 x VIC -1.4351

This equation can then be applied to the data recorded by the TriSCAN sensors in the field to convert VIC to ECe using the data adjustment function in IrriMAX Live. Insert equation “Multiplier” and “Constant” as shown below.

The result is shown below. VIC data have now been converted to ECe measured in mS/cm.

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  • This field is for validation purposes and should be left unchanged.