Home' Grower : June 2011 Contents The South Australian Grower -- June 2011
In the northern Adelaide Plains, almond
and winegrapes are winter dormant and
can especially benefit from rainfall-induced
Vegetable crops established during
autumn and winter may also get some
benefit from winter rainfall leaching.
Glasshouse crops can only benefit if the
rainwater is har vested and reused for
leaching irrigation between crops.
New glasshouse developments must
incorporate har vested stormwater in the
SIGNIFICANT rainfall events outside
dominant rainfall periods (Figure 1)
provide windows of opportunity for
'piggyback leaching', where irrigation is
applied to refill the soil within the
preceding 24 hours to facilitate leaching
under the subsequent rainfall event.
System turnaround times, water
availability and other factors may not make
irrigation prior to rainfall possible.
Instead, the reverse is used, where a
significant rainfall event is rapidly followed
by irrigation to promote leaching,
although the potential reduction in root
zone salinity is limited by quality of the
irrigation water source.
GOOD drainage through and beyond the
root zone is essential for effective leaching
management, which requires maintaining
good open soil structure.
Many natural water and reclaimed water
sources are dominated by sodium.
Continual displacement of calcium and
magnesium from clay surfaces by sodium
can cause serious loss of structure and poor
The condition is called sodicity and is
outlined in a later article.
DEVICES are available to log changes in
either the volume or tension of soil water,
from which the depth of irrigation and
depth of plant uptake of water can be
Tension is the energy by which water is
held in soil and is partially-determined by
Higher salinity means that more energy
has to be used by crops to take up the soil
In-situ probes can also monitor salinity
using a range of technologies.
'Artificial roots' or ceramic extractors,
such as the model shown, can be used to
draw out the soil water from which salinity
can be measured using a hand-held EC
A plastic syringe and hand pump is used
to form a vacuum and extract the soil
solution from the ceramic collector.
Soil water is collected at a similar range
of tensions that vegetable crops take up
water, which means the salinity of the
extracted water is similar to that to which
the crop root zone is exposed.
Field measurement of salinity requires
calibration with plant response via
obser vations and tissue testing.
When completed in conjunction with
soil moisture monitoring and soil testing,
the information can reveal whether the
crop is experiencing nutrient imbalance or
toxicity and/or high overall salinity
Soil and tissue testing is outlined in a
Daily soil water balancing tools, moisture
monitoring equipment and general field
obser vations can be used to associate
changes in root zone salinity with
A tool which enables calculation of the
daily water balance and leaching fraction,
following rainfall or irrigation, is available
from the PIRSA website.
Crop coefficients used to calculate water
usage assume that the crop is growing
under non-limiting conditions, which is
not the case for crops affected by salinity,
so field obser vation is essential to refine
A case study showing how the different
monitoring tools can be use to proactively
manage root zone salinity is outlined in
article 6 of this series.
SENSORS should ideally be located within
the upper and lower portions of the root
zone, with at least one sensor located
below the root zone.
Figure 3 shows approximate locations for
monitoring systems with four sensors.
The upper two sensors are located in the
active root zone where routine irrigation is
applied. Positioning them about two thirds
towards the edge of the wetted zone will
provide more representative data on soil
moisture levels and associated salinities.
Sensor 3 is located in the lower root
Routine irrigation may not always reach
this depth, resulting in accumulation of
The sensor is important in assessing
effectiveness of leaching irrigation and/or
rainfall events in removing salt.
The fourth sensor is positioned at a
depth that can detect deep drainage under
leaching irrigation and/or rainfall.
If the data is showing that the soil is
remaining continuously wet, it may be
indicating a drainage problem.
Proactive real-time monitoring of root
zone salinity is now a reality. Article 6
provides a case study to demonstrate the
use of monitoring tools.
The next article in SA Grower (July)
outlines how leaching is factored into the
• Anthony Fox is senior land management adviser,
Adelaide and Mount Lofty Ranges, NRMB, and
Jeanette Chapman is compliance officer for the
Environment Proection Agency.
• Details: 08 8523 7718 or
Next month: Targeting salinity management within
crop root zones
Figure 2: Salinity probes (left) Hydroprobe, salinity and moisture probe. Image Peter
Toombe, 2010 and (right) active solute extractor (http://www.Sentek.com.au, 2009).
Figure 3: Approximate location
of sensors. (Graphic: J
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