Agriculture Reference
In-Depth Information
Box 2.2
(continued)
variationsandmountedinfrontofatractororplacedonarubberorPerspexsled
drawnbyanall-terrainvehicle(igureB2.2.1B).AGPSmountedonthevehicle
enables the position of the sensor to be tracked as it is moved over the land.
The EM38 works by EM induction, whereby a low-frequency, oscillating
electric current in a transmitter generates small, fluctuating currents, with
associated magnetic fields, in the soil. The ratio of secondary to primary magnetic
field strengths is directly proportional to the
EC
of a roughly cylindrical soil
volume below. These magnetic fields are detected by receivers built into the
instrument. The current “standard” EM38-MK2 has two receiver coils, separated
by 1 m and 0.5 m from the transmitter, providing
EC
data from depth ranges of
1.5 m and 0.75 m, respectively, when positioned in a vertical orientation, and 0.75
m and 0.375 m, respectively, when in horizontal orientation (see
www.geonics.
com
and
www.farmingit.com.au
).
A related technique is that of measuring soil electrical resistivity (the
reciprocal of conductivity). A direct electric current is applied to electrodes built
into the coulter wheels of a cultivator drawn behind an all-terrain vehicle. The
potential difference between the electrodes is related to the soil's resistivity.
In Australia, the EM38 is used to measure vineyard soil variation. The results
have been calibrated against salinity, clay content (also reflecting water content),
and soil depth (e.g., where a sharp change from soil to rock occurs within the
depth range of the instrument). When the EM or a resistivity instrument is used
in an existing vineyard, care must be taken that the signal is not distorted by steel
posts and wires. This is not a problem for the EM38 when the row spacing is more
than 2.5 m, provided the instrument follows the midline between rows, and it is
less of a problem for electrical resistivity measurements because the electrodes are
closer together than the transmitter and receiver in an EM38.
Commonly, a series of preliminary holes is “augered” to determine whether
a full survey with sensing devices and profile pits is needed. If a full survey is
deemed necessary, the first step may involve remote sensing, such as with air
photos or satellite images, or with techniques such as gamma radiation and
laser-imaging radar. Traditionally, stereoscopic analysis of air photos (giving
a three-dimensional picture) has been used to map surface topography and
vegetation, both of which can indicate soil changes, but nowadays a real-time
kinematicGPSisusedtoconstructadigitalelevationmodelofthesite(see
figure 1.15, chapter 1).
The next step usually involves proximal sensing of the soil with an EM38,
as described in box 2.2. At its most basic, an electromagnetic (EM)-derived map
shows the pattern of soil variation that can be caused by a number of factors. If
several soil properties (e.g., soil depth, clay content, and salinity) are involved in
determining the EM signal, the main value of the map is in indicating where
to sample to cover the full range of soil variability, thus greatly improving the
