Home' Grower : August 2011 Contents 14
The South Australian Grower – August 2011
Case Study: Using logged soil water and salinity
monitoring data to predict salt movement and
risk of uptake
IN the last of this six-part series. ANTHONY FOX and JEANETTE CHAPMAN focus on groundwater salinity above the crop
threshold. Water allocation only covered nominal crop water requirements. Leaching management aims to minimise damage
to the developing canopy early in the season, and to take opportunities to ‘piggyback’ leaching either side of significant
rainfall events during summer. The objective of opportunistic leaching is to reduce salinity within the upper active root zone.
Harvest occurs in February-March.
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Interpreting Changes in Soil
SOIL water monitoring devices are
traditionally used for irrigation
scheduling. The refill point set at a
nominal level of deficit in soil stored water.
Soil water monitoring charts can also
provide a means of identifying the:
•Depth of wetting following irrigation
hence the zone in which salt associated
with the applied water is deposited.
•Depth of uptake of water by the active
root zone and whether it corresponds to
the zone in which salt is potentially
•Changes in the amount of water taken
up by the crop between irrigation. Higher
rates of uptake will promote higher root
zone salinity unless compensated through
more frequent and/or higher application
This information can be used to fine
tune calculated leaching irrigation volumes
(article 5) or make smarter use of limited
water allocation by focusing on minimising
salt accumulation within the upper active
root zone, and minimising the need for
uptake of more saline water from the lower
root zone (article 3).
Soil tension data is shown in Figures 1
and 2. Tension is the energy by which the
soil water is held and is measured in
kilopascals (kPa). Since tension is time
dependent it is displayed on the vertical
axis with time shown on the horizontal
axis. Wet soil has tension near zero, with
values becoming more ‘negative’ as the soil
dries. In the example, the zero is
deliberately located at the top of the Y axis
so that the slope of the plotted line appears
to fall away as the soil dries, and rises as
the soil rewets (as often taught in school).
Different coloured plots correspond to
sensors located at var ying depths.
Identifying potential salt build-up and
associated risk of uptake by the crop root
THE depth of irrigation is indicated by
the lowest sensor displaying a ‘vertical’
increase in slope. Figure 1 shows that
irrigation consistently reached a depth of
60 centimetres. This means salt
transported from the upper portion of the
root zone (article 3) will be deposited in a
zone of soil around 60cm depth.
Uptake of soil water by plants has a
distinctive stepped pattern as most water is
lost during daylight hours. The depth of
crop water extraction is given by the
deepest sensor displaying the stepped
pattern, which again occurred at 60cm.
This means that salt deposited around this
depth is likely to be taken up by the crop.
The stepped pattern will follow a
relatively straight line until the readily
available water is used up at which point
the slope will become more horizontal.
This was exhibited by the sensor at 60cm
(olive chart) after 2 days and potentially is
a sign that salt accumulating at that depth
is making the water more difficult to
extract (higher energy -article 2).
▲ Figure 1: Recognising depth of irrigation and crop water use on a tension soil water
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