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Locally specific guidelines for grain growers in the Mallee
MANAGING MALLEE SEEPS
Dune seepage can turn productive cropping land into non-arable swamps called dune
seeps. This edition of Farm Talk presents 10 things to know about dune seeps.
1. What is a dune seep?
A dune seep is an area of saturated
soil caused by seepage from
sand dunes in Mallee dune-swale
landscapes. Researchers have found
saturated layers at depth (up to 10m
deep) below sand dunes by drilling
at several locations in the Mallee, a
result that was unexpected in a low
rainfall farming district.
Two factors cause a dune seep:
• Sand-dune soils with unused
water that drains down through
the soil profile, and
• A ‘perched water table’ where
water accumulates below
ground level upon a clay layer
with very low permeability.
Seeps form in lower-lying areas
where this clay occurs at shallow
depth and perched water is forced
to the land surface (Figure 1). This
is often in low-lying swales adjacent
to sand-dunes, but dune seeps can
also occur on slopes. In the Murray
Mallee, a geological deposit known
as ‘Blanchetown Clay’ forms the low
permeability layer. Similar materials
of similar age occur throughout
agricultural South Australia and the
Victorian Mallee.
2. How does a seep form?
Dune seeps form in subcatchments where there is a water
imbalance, caused by excess
water moving through sandy soils
beyond the reach of plant roots.
Initial development of seeps is
FIGURE 1: SCHEMATIC OF A DUNE SEEP
unpredictable, but seems to relate
to heavy rainfall years. A number of
seeps were reported to have formed
in the Murray Mallee after 2004 and
2010, which both had wet summers.
It is believed that the modern
practice of continuous cropping
combined with greatly improved
summer weed control has resulted
in a farming system that allows more
water to seep through sandy soils,
especially in years with wet summers
followed by another wet year.
3. What problems do seeps
cause?
Lower-lying parts of dune-swale
landscapes often constitute the
most productive land on Mallee
farms. When a seep forms, this soil
becomes waterlogged which reduces
germination and emergence, and
precludes heavy vehicle traffic,
meaning the land can no longer be
cropped. Over time, evaporation
increases the salinity of the seep
area, which further reduces soil
productivity (Figure 2). In addition,
bare soils can easily be eroded.
4. What are the options for
managing a dune seep?
There are three options when
faced with a dune seep. The first is
to increase plant water use across
the whole sub-catchment area and
so decrease seepage to the dune
seep area. This can be achieved, for
example, through the strategic use
of deep-rooted perennial plants or
the introduction of summer crops.
The second option is to only manage
the seep area, for example by
using suitable plantings to maintain
ground cover, minimise erosion, and
to prevent land degradation. The
third is to do nothing.
5. What will happen if I do
nothing?
One option for dealing with a dune
seep is to do nothing. This can result
in ongoing degradation of the seep
area from soil erosion and increasing
salinity. This can make a seep area
difficult to rehabilitate back to
arable land, and cause the area to
be lost permanently to productive
agriculture. In addition, since their
initial formation, many seep areas
have increased in size. It is possible
that the seepage trend could reverse
during a period of dry years, but at
this stage, this is an unknown.
6. How can I manage the seep
area?
Managing the seep as a separate
area to the rest of the paddock
means the land is lost to normal
cropping. Planting species that are
tolerant of waterlogged soil in and
around the seep establishes ground
cover that prevents erosion and
minimises the evaporation that can
lead to topsoil salinity. Once plants
are established they can potentially
be used for grazing, cut for fodder, or
can even play a role as a habitat to
promote beneficial insects. Mulch,
such as straw, can be spread over
a seep area to protect it, at least
temporarily, from land degradation.
Growers could consider perennial
vegetative cover such as tall wheat
grass, puccinellia or saltbush, or
other suitable shrubs or trees.
Advice should be sought from
appropriate native vegetation
experts or publications before
selecting native tree or shrub
species.
7. What needs to be done
to manage the seep at the
source?
To prevent excess water from
forming seeps, and to reduce the
impact of current seeps, the use
of rainfall by plants over the whole
source area needs to be increased
to decrease the amount of water
that drains beyond the plant root
zone. Options include improving
agronomic practices to grow
higher-yielding winter crops that
ABOVE: A badly degraded seep
make better use of soil water, the
incorporation of clay into sandy
topsoil to increase soil water holding
capacity, the integration of deeprooted perennial plants into farming
systems, the planting of summer
crops, or the introduction of pasture
phases into continuous cropping
systems. This method may require
significant commitment and changes
to current farming systems; advice
should be sought from appropriate
specialists.
8. What new methods are
being investigated?
Initial soil investigations and drilling
in several Mallee sub-catchments
have discovered that ground water
seepage is mostly at considerable
depth, with seepage along subsoil
layers playing a smaller role
(Figure 1). This demonstrates
the importance of selection and
placement of deep-rooted perennial
plants to resolve seeps. Researchers
are also investigating methods of
increasing soil fertility and water
holding capacity to increase crop
production and so reduce seepage,
including spading chicken manure
within sandy topsoils.
FIGURE 2: SALT ACCUMULATION IN A SEEP DISCHARGE AREA
9. USEFUL CONTACTS James Hall: 0477 400 092, [email protected]
MSF: 03 5024 5835, [email protected]
Natural Resources SAMDB: 08 8532 9100
10. MORE INFORMATION A full list of research cited in this Farm Talk is available at www.msfp.org.au
This Farm Talk was published in February 2016
Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of MSF or the Grains Research and Development Corporation (GRDC).
No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. MSF, GRDC and contributors to these guidelines may identify
products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may
perform as well as or better than those specifically referred to. MSF and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on
the information in this publication.
MSF would like to thank Juliet Creek Consulting, the Grains Research & Development Corporation, and
Natural Resources SA Murray-Darling Basin for their support in producing this edition of Farm Talk.