Home' Grower : April 2011 Contents The South Australian Grower -- April 2011
Water quality and leaching
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Water quality effects
under irrigation can at best
maintain soil salinity close
to the salinity of the infiltrating
Water quality becomes an
issue when the salinity of the
source water used for leaching
irrigation exceeds the critical
tolerance level of the crop
being irrigated. When this
occurs, minimising yield loss is
the management goal. Each
crop type will have an upper
limit to water salinity that
enables an economic yield to be
Table 1 below compares the
quality of the major water
sources used on the Northern
Adelaide Plains relative to the
threshold values of crops grown
in the district.
THE first article in this series looked at the different types of salt and
solutes that may be found in soils, measuring salinity, and crop
tolerance to salinity. In part 2, ANTHONY FOX and JEANETTE CHAPMAN
assess leaching and the potential limitations of different water sources.
Table 1: Salt
tolerance of crops
grown on the NAP
compared with the
ranges in salinity of
major water sources
salinities: Ayres and
MOST irrigators on the NAP have access to
groundwater with salinity at or above the
tolerance level of the crop being irrigated.
Reclaimed effluent is also used on the NAP.
Salinity is cyclic from a minimum level dur-
ing winter to a maximum allowable level of
1500 mega litres (Figure 1). Depending on
crop type, planting date and time to har vest,
salinity of the reclaimed effluent may be at
or above tolerance levels for some or all of
the growing season.
By contrast, rainfall with its low salinity is
the only water source able to reduce root
zone salinity below critical tolerance values
of salt-sensitive crops.
Make full use of rainfall to
Maximising leaching efficiency during rainfall
events is outlined in article 4.
Avoid forming salt barriers
between the root system and
fresher water being
accessed by the plant.
Rainfall acts as a 'full coverage' system,
although the physical structure of plants
enables part of the intercepted rainfall to be
channelled towards the base of the stem. Crops
irrigated with drip and micro sprinklers or spray
emitter types will potentially be exposed to two
different wetting patterns, 'full coverage' as a
result of rainfall, and partial coverage when
irrigated (Figure 1).
SALT is transported in water as dissolved
solutes. Salt will therefore move into the
root zone with the applied irrigation water
and will stop moving when the water front
stops moving. Thus salt will accumulate at
the wetted edges.
Field wetting patterns are strongly influ-
enced by irrigation system types and mainte-
nance, full and partial surface coverage, and
soil textural changes within the root zone. It
is important that managers determine field
wetting patterns relative to location of plant
This key strategy is especially important
when establishing young plants, to avoid
them becoming dehydrated.
Transportation of salt
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Weevils, other grain beetles and grain moths
can all be killed by spraying your grain as you turn
it or move it between storage bins. And the spray
will prevent new insect attack for up to 6 months.
If you want to feed your grain to stock or deliver
it for sale, the withholding period is just 1 day after
application. Which means you can feed it to stock
straight out of the holding bin or silo.
Natural pyrethrum, is registered for this
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The cost at the 2-3 months protection level is
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Just measure the amount of grain passing through
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then calibrate your sprayer to deliver 1 litre of spray
solution per tonne of grain. Apply the spray directly
onto the grain as it falls into its new storage location.
There are a few other factors you may wish to find
out about: what to spray to eliminate insects in the
bin or silo before you add grain, use of long lasting
dusts for the top surface during long storage and
fumigation options. Please phone our entomologist
for a quick discussion.
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