The Short Straw
July 2016
Waterlogging – what can we do?
Lisa Castleman, Senior Land Services Officer (Cropping)
Riverina Local Land Services
We can see the water lying in the
paddock. We can see the crop going
backwards. Every time it looks like we
might get a drying day it comes in and
rains again.
What is actually happening under the
soil surface and what are the plant roots
doing? What is occurring to the
Nitrogen from the urea that you have
already applied?
The water fills up the macropores
between the soil particles before the soil
reaches the point of saturation. The
macropores allow for the usual
movement and storage of soil water.
The macropores are also where the
oxygen in the soil sits. When these
pores fill up then there is little
remaining space for oxygen gas and the
soil
changes
from
an
aerobic
environment healthy for plant roots, to
an anaerobic environment, not good for
soil microbes either. Overly wet or
saturated soils are detrimental to root
growth and function as the plant
cannot access the oxygen it needs
through the roots and respire, a time
when it will grow. The longer the soil is
water-logged, the longer the period of
poor growing conditions.
A healthy plant will be using water as it
photosynthesises. Other normal living
processes are for the plant to respire
and also transpire. Water in the roots is
pulled up through the plant by
transpiration (loss of water vapour
through the stomata of the leaves).
Transpiration uses about 90% of the
water that enters the plant. When these
usual processes cannot occur due to
water-logging, then the plant stops
using water and cycling water, which
further exacerbates the problem of
excess water ponded above or below
the surface.
A water-logged soil may have passed
the point of saturation in the profile
although soil constraints such as a sodic
layer (excess hydrophobic sodium ions)
or a compacted layer (heavy machinery
on wet soil) can slow or reduce
infiltration. Excess water that cannot
run off, then ponds.
Crop type
Some winter crops are more tolerant
than others. For instance, pulses and
canola crops are generally more
susceptible to waterlogging than any of
the cereals. Of all the pulse crops Faba
Beans are the most tolerant of
waterlogged conditions.
It is worth noting that rhizobia will not
survive in anaerobic, waterlogged
conditions either, which then has other
impacts such as nodules dying and
nitrogen fixation not occurring, which
results in the pulse crop not being able
to supply its own nitrogen.
Where crops were sown earlier or longseason varieties were sown in the early
part of the sowing window, then early
growth assists in less crop damage from
waterlogging. A plant in a more
advanced growth stage will also use
Waterlogging – what can we do?
more water, which further assists in
drying the profile out.
Severe crop damage
Crop damage is particularly severe if
plants are waterlogged between
germination and emergence.
For
paddocks
susceptible
to
waterlogging, plant them first with the
best crop choice for vigorous early
growth. It is a number of seasons since
waterlogging has been a major concern.
If the likelihood of waterlogging this
season was known then we could have
also increased the sowing rate to
protect against uneven germination.
Any seed sown into waterlogged
conditions has a good chance of
bursting
and
not
germinating
successfully.
However, if waterlogging delayed crop
emergence and reduced cereal plant
density to less than 50 plants/m2, then
plant numbers would suggest resowing.
The practicalities of a wet year mean
you probably won’t get another chance
to resow this season and lower soil
temperatures now would mean long
delays before emergence.
Weeds
Waterlogging depresses tillering when
plant growth slows or comes to a
standstill. While weeds can take
advantage of stressed crops, many
weeds will also suffer from waterlogged
conditions and not compete as
vigorously as usual with the crop. Weed
density affects a crop's ability to recover
from waterlogging. As a paddock dries
out then weeds compete for light,
water and nutrients including any
remaining soil nitrogen. Waterlogged
areas in a paddock can become weedy
after
crop
density
has
been
compromised and will require attention
to prevent seed-set.
Nitrogen fertiliser
Crops tolerate waterlogging better
where they had a good nitrogen status
before the waterlogging occurred.
However, applying nitrogen at the end
of a waterlogging period would be an
advantage to counteract the losses that
occurred
due
to
leaching
or
denitrification.
Denitrification is the process whereby
nitrate (NO3) is reduced to nitrite (NO2)
and then to a gaseous form of nitrogen
such as N2 or N2O (nitrous oxide). This
process is caused by denitrifying
bacteria, which use nitrate to respire
when oxygen is unavailable, such as in
water-logged conditions. A well-drained
soil will be more aerobic and nitrate
fertiliser will be more accessible for
plant growth.
NO3 ----------------> NO2 ---------------->
N2 denitrification
If the waterlogging has subsided and
the crop is salvageable then trafficability
Period of waterlogging
If waterlogging is moderate (7–30 days
waterlogging to the soil surface), then
nitrogen application after waterlogging
events when the crop is actively
growing is recommended.
However, if the waterlogging has been
severe and prolonged (greater than 30
days to the soil surface), then the
benefits of applying nitrogen after
waterlogging are questionable. An
assessment of plant density needs to
occur to check if there are sufficient
remaining live plants for a viable crop.
Water logging is a condition that slows
plant growth in winter and can
ultimately impact on crop yield. We
usually look forward to rainfall but
sometimes we can have too much of a
good thing.
Riverina Local Land Services
2
Waterlogging – what can we do?
If there is any opportunity to improve
drainage in the water-logged paddock,
you would take it, as waterlogging can
cause irreversible damage. But in a
dryland, broadacre situation there are
usually few options to fix it once waterlogging has occurred. An option to
explore could be ripping a drainage
channel through to lower ground to
allow water off the paddock and into a
watercourse or dam. This would only be
viable if there was somewhere for the
water to go on your property.
For a soil to dry out we rely on
evaporation (very low at this time of
year), water to drain deeper into the soil
profile, water use by the crop (impaired
by the actual waterlogging) and for no
new rainfall events to occur. Simply put,
the patches of water-logged crops in
our wetter paddocks are not able to use
any water until they dry out.
For further advice please speak to your
agronomist or farm advisor or contact
me at [email protected] or
0427 201 963.
Riverina Local Land Services
3