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Inaccurate TDR Readings
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Last Updated
10th of January, 2012

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1.        Is the correct rod size selected?

2.        Are the rods fully inserted in soil? (sensor block touches the soil)

3.        Is the clay content high? (>30% Clay)

4.        Is the soil salinity high? (>2 dS/m)

5.        Is the soil high in organic matter? There is no clear value for this category. But, soils in production will probably be under 8% OM. To solve this problem, perform a site-specific calibration.

6.        Check calibration.  In air, the meter should read VWC = 0%.  In saturated sand, the meter should read a VWC of about 40%.  In water, the meters with the original white block will read VWC  100-120%.  For the current meters with the purple or black blocks, the reading in water depends on the rod size.


      3.8 cm rods > VWC  65 - 70%

7.5 cm rods > VWC  75 - 80%

12 cm rods > VWC   70 - 75%

20 cm rods > VWC   60 - 65%

Often, a site-specific calibration can be done to address this problem if the issue is with clay, organic matter or salinity. Sometimes, RWC mode can be used if information about wetness state of soil is more necessary than the actual VWC itself.


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