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Technical Reference
Measuring Soil Moisture
Whalley (1993), White, Knight, Zeggelin and Topp 1994) have
shown that there is a simple linear relationship between the
complex refractive index (which is equivalent to
)
, and
volumetric water content,
, of the form:
[3.]
This equation appears to work very well for most non-magnetic
soils and artificial growing media over a range of frequencies
between ~1MHz and ~10GHz.
The following graph shows composite data for a number of
different agricultural soils
You can see from this graph that the accuracy of the water
content measurements would be improved by using a different
calibration for each soil. However, the improvement would be
small (typically 2 or 3%), so a generalised
“mineral” calibration is
appropriate for a good range of agricultural soils.
Warning: This is not the case with clay soils, and a soil-specific
calibration may improve the accuracy by >10%. This is also true
of “organic” soils because that label covers a huge range of soil
types.
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