An
Electro-magnetic (EM) sensor generates a constant electro-magnetic field that
penetrates into the soil profile. It measures the bulk electrical conductivity
of the soil profile. As we conduct an EM survey the sensor is taking
readings at two different depths simultaneously. These two depths are known as
the ‘Shallow EM’ and the ‘Deep EM’. The depths the DualEM reads depends on the
height the machine is off the ground. With our EM setup we are reading the soil
profile depth of 0-40cm for the shallow EM and the deep EM at a soil profile of
0-125cm. So the deep EM values are the same as the shallow plus another 85cm
deeper. This is why the deep EM readings are always higher than the shallow as
it is reading that extra 85cm.
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Figure 1: Shallow EM
of the same area with values from 1.6-27 EM units (mS/m)
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Figure 2: Deep EM
survey values varying from 16-43 EM units (mS/m)
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Generally speaking and depending on what part of the country
you are in and the time of year the survey is carried out amongst other things,
we would class a range in EM in the shallow profile of 1-3 units as low
variability, 4-8 units as moderate variability and over 8 units range as high
variability in the shallow layer/soil profile. In the deep EM/soil profile
layer a range of 1-6 would be low variability, 6-15 moderate variability and
over that high. It is often dangerous to generalise like that, but it gives you
an idea of the type of ranges we look at, and as previously stated there are a
lot of other factors that determine if the readings are low, medium or high
variability. You also have to look at the distribution of the values as well,
if the majority of the values are within a certain range and a few rogue values
outside that but on a minimal area of the total, then the range in variation
may not be as much as it first looks. How much the variability is costing you
in terms of blanket irrigation applications compared to variable rate irrigation
applications be it water, seed or fertilizer is a subject for another day!
Also of
note as an aside; is that with all our readings legend red is the lowest value
and dark blue the highest. This does not mean that red is bad and blue is good,
there are a number of factors that determine that, and there is also a climatic
influence on what is good or bad, in a dry year the heavier EM areas would be
better where you don’t have VRI, and in wet years the lighter areas with more
free draining soils would be more favourable. Likewise in peaty acidic soils
with high organic matter they would show through as the high blue EM areas and
clay soils would show up as the lighter red soils on the map. So it is always
worth taking samples as you conduct the survey to see what is affecting the
readings.
For more information on EM Surveying please contact us at
Agri Optics NZ Ltd.
Chris Smith.
