There
is a misconception that all soil (moisture) sensors are precise and tell you
the exact soil moisture content. Not
so. HydroServices runs a trial site
where a number of soil moisture sensors are installed for comparison – for both
the “calibrated” soil moisture content and any long term trends. A boring looking trial site because the sensors
are installed toward the small gap in the trees. In all there are 8 sensors – neutron probe,
Decagon 5TM and GS1, Acclima, AquaCheck and two (2) Aquaflex. The Decagon 5TM, GS1 and Acclima sensors are
installed at 10cm, the shallow Aquaflex on a slope from 10-25cm, and the
neutron probe and AquaCheck can measure at 15 and 10cm respectively.
Sensors
are provided with a factory calibration, usually one for silt loam, clay loam
and sandy loam soil types. These are
generic and may or may not truly measure the soil moisture content at your
location because (for example):
a) Your soil is unlikely
to be the same as the generic soil type;
b) The sensor is poorly
installed (especially if there is not perfect contact between the sensor and
the soil); and
c) The soil is loose
(cultivated) and perfect contact is not possible
While
the traces of soil moisture content are sort of similar, none (with their
generic calibration) read the same soil moisture content, as shown in the plot
of all sensors. (Note the GS1 Sensor is a recent addition and no
data is available for the dates compared).
The
only sensor that has been calibrated against true soil moisture content
(gravimetric laboratory analysis) is the neutron probe. Knowing that Field Capacity at this location
and soil type should be about 40%; only the neutron probe and Aquaflex measure
soil moisture content at this level. The
other three sensors measure field capacity 10%
less than the true field capacity. Disconcertedly two sensors measure soil
moisture content between irrigation events (the vertical rise in the traces) at
or very close to wilting point – approximately 17-18%. This is not the case; the pasture never died
nor showed any sign of being close to wilting point.
What
is to be taken home from the comparison? If
you want sensible and realistic soil moisture measurements the sensors must be
field calibrated. The simplest and
easiest way to field calibrate is by neutron probe – click on http://www.hydroservices.co.nz/index.php?option=com_content&view=featured&Itemid=308 for more details.
