Know your environment to get
the most out of nitrogen
G
ROWERS who want to get the most out of their nitrogen
applications need to focus on knowing their soils and
the climate, according to Queensland University of
Technology’s Peter Grace.
Peter says the first step in defining a nitrogen program is for
growers to observe how their soils respond to rainfall to better
understand how they are likely to lose nitrogen (see Types of N
loss page 10).
“With nitrogen application there are no set rules. Everything is
dependent on the grower’s soil type and both the seasonal and
short-term forecasts for the farm, so there is no hard and fast rule
for when or how much to apply,” Peter said.
“Sandy soils are far more likely to experience leaching, as the
coarse-grained particles facilitate good drainage, and because
nitrate is highly soluble, it will drain out of the soil profile with
the water. Clay soils on the other hand, are more susceptible to
denitrification because they’re more likely to reach saturation for
prolonged periods of time.”
He suggests monitoring soil moisture to increase
understanding of plant available water capacity and a soil’s
response to rainfall. Soils with a large plant available water
capacity – also called the soil moisture ‘bucket’ – will require
very heavy rainfall to reach saturation and for N to leach. Soils
with a small plant available water capacity will be more likely to
experience N losses after rain.
“By observing this response, growers can learn what level of
rainfall will cause denitrification or leaching, and avoid topdressing before such a rainfall event,” Peter said. “There are
complications for alkaline soils where the third type of loss –
volatilisation – can occur if there isn’t enough rain. That’s why it’s
so important that growers know how their soils respond and also
learn how to use weather forecasts.”
“If the longer range forecast is for a wet season, then the
yield potential is likely to be high, and also there’s likely to be big
rainfall events that may make it difficult to top-dress nitrogen
later in the cropping cycle. In this case, it’s probably a good plan
for the grower to put out a bit more nitrogen up-front. But in
an average or dry year, there’s more risk in paying for that extra
nitrogen up-front, so a grower may want to apply less up-front
and wait for a suitable forecast rain event during the season
when the crop is ready to take it up,” Peter said.
For short-term forecasts, growers can use their soil and plant
available water knowledge to wait for a rain event that will wash
the fertiliser in, reducing the risk of volatilisation, but not so much
rain that denitrification or leaching is a risk.
Peter recommends growers use the POAMA tools provided by
the Bureau of Meteorology to identify the most likely seasonal
conditions. While forecasts are never certain, they give the most
probable outcome so growers can use them to decide, based on
their soils and crop type, how to allocate their nitrogen budget.
Climate Forecasting: POAMA
Understanding soils and climate is the key to optimising N
efficiency according to Professor Peter Grace.
(Source: Sharon Watt, Porter-Novelli)
8 — Australian Grain
Seasonal climate forecasting has long been a controversial
topic for grain growers, and in the past forecasts have been
considered with low levels of confidence. But since the transition
to the Bureau of Meteorology POAMA model in 2013, seasonal
forecast accuracy has improved substantially.
POAMA, which has been supported by the GRDC through
the Managing Climate Variability (MCV) program, models the
climatic conditions based on the laws of physics, and on ocean,
atmosphere and land observations.
This is distinct from previous forecasting methods using
statistical techniques, which had a lower accuracy due to their
simplicity, and decreasing accuracy due to climate change altering
the usefulness of historical statistical relationships.
CSIRO Oceans and Atmosphere Flagship researcher Peter
McIntosh says that while climate forecasting will never be perfect,
it is already very useful. POAMA forecasts of above or below
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Water logged soils can result in significant N loss through
denitrification. (Source: Deanna Lush, AgCommunicators)
FREQUENTLY ASKED
QUESTIONS
I have a sandy topsoil, does that mean I’m safe from
denitrification?
A lot of soils in south-eastern Australia are duplex, so even if
a grower has a sandy or loam topsoil, if there is a clay sub-soil,
this will ‘block’ water flow and can still result in saturation and
therefore denitrification.
How much N can be lost to denitrification?
The N lost to N2O is easy to measure and is normally less
than 0.5 per cent of applied N. But the majority of loss is N2 gas
and since this gas already forms 79 per cent of the atmosphere,
it is very difficult to measure. Previous work has shown that for
every 1 kg of N lost as N2O, as much as 10–50 kg of N may be
lost to N2.
When should I top-dress urea?
This depends on the soil-type and forecast as a rain event
is required that is adequate to wash-in the fertiliser – but not
so much rain as to cause denitrification or leaching. The ideal
timing for application from trials conducted in the Wimmera
and Victorian high rainfall zone was to apply the majority
of the fertiliser to cereals at around GS30 (start of stem
elongation) to maximise nitrogen use efficiency.
What if a forecast rain doesn’t eventuate?
Growers with alkaline soils may lose N fertiliser via
volatilisation if there isn’t adequate rain to wash in the
fertiliser. These losses occur gradually for one to two weeks
after application for urea, so any rain within this time will
reduce the loss. In trials in northwest NSW, fallowed soils
experienced loss of 12 per cent after urea application, down to
five per cent in-crop. Previous trials have shown up to 18 per
cent loss from urea.
In acid soils volatilisation will not occur. So the only risk is
that if there is inadequate follow-up rain to give the expected
yield potential, the nitrogen application may not be costeffective.
10 — Australian Grain
median rainfall during the growing season are correct 60 to 80
per cent of the time over most of the grain growing regions, and
this level of skill can be very valuable over a number of years.
“Think of it as flipping a coin. Without additional knowledge,
you can only assume the coin is not biased, and is equally likely to
turn up heads or tails. POAMA gives information about how the
climate coin is biased each year, and in the long run this can be
very useful to a farmer,” Peter said.
The accuracy of the model varies both with location and
the time of year. Tasmania has lower accuracy as the weather
is influenced by more complex factors compared to the southeast mainland, which has higher accuracy. Forecasts improve
further into the calendar year due to influences of ENSO (El Niño
Southern Oscillation).
While POAMA was originally devised as a nine month seasonal
forecast, researchers later identified that it had good accuracy for
forecasting on multi-week timescales beyond traditional weather
forecasts.
“Weather systems can be predicted out to about 10 days.
Within this period, weather models can forecast where the fronts
THE MAIN TYPES OF NITROGEN
LOSS
Volatilisation
Ammonia volatilisation is loss as ammonia from ammoniumbased and urea fertilisers which have been broadcast and left
on the soil surface. The loss is caused by chemical reactions
between ammonium in the fertiliser and calcium carbonates
in the soil producing ammonia gas which is lost to the
atmosphere. Volatilisation only occurs in alkaline soils where
fertiliser is broadcast and there is no rain to wash it into the
soil. Soils with high clay content are a low risk of volatilisation
compared to sandy or loam soils.
Denitrification
When a soil is over 80 per cent saturated with water,
microorganisms in the soil convert soluble nitrate into gaseous
forms of nitrogen – di-nitrogen (N2), nitrous oxide (N2O) and
nitric oxide (NO). Di-nitrogen is the main gas lost, and can
equate to large losses of fertiliser.
As moisture content increases beyond the drained upper
limit (also known as field capacity), the loss of nitrogen
through denitrification increases. Heavy textured soils, such as
clays, are more susceptible to denitrification.
Leaching
This process is the loss of nitrate down through the soil
profile. Nitrate is highly soluble so when water drains through
the profile, nitrate travels with it. If water drains below the
rootzone of the crop, nitrogen leaching will occur. Once the
nitrate has drained below the effective crop rootzone it cannot
be recovered. Coarse textured soils such as sands are more
likely to experience nitrogen leaching than clays.
Immobilisation
While not a loss of nitrogen, a temporary unavailability of
N can occur when plant residues with low nitrogen content
breakdown and prevent the N from being available to the
crop. Immobilisation can be minimised by banding N away
from crop residues.
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Australian Grain — 11
and pressure systems will move. But beyond 10 days we can only
forecast how climate drivers such as ENSO change the average
behaviour of weather, and this is where POAMA comes in” Peter
said.
The latest version of POAMA, currently available from the
BOM website with a login, provides forecasts from one week up
to nine months.
The model is still improving, with MCV researchers including
Peter working on improving value from the model for grain
growers. In parallel, Peter is also working on a GRDC funded
project to incorporate POAMA into Yield Prophet.
“Currently growers can forecast yield based on previous years’
weather, or can choose only, say, El Niño years. Our project will
incorporate the actual POAMA predictions for the current season
and therefore produce 33 possible scenarios which will give
probability curves for grain yield. We’re currently developing this
system and hope to have it available for growers in a few years,”
he said.
MCV have measured the benefit of understanding forecasts,
modelling the yield growers would obtain with nitrogen
applications based on POAMA compared to no forecasting.
“Studies we’ve performed in Western Australia showed that
in the long run, using POAMA provided growers with a $50
per hectare benefit per year on average compared to using no
forecast. This is because improved knowledge of the seasonal
forecast allowed for more informed nitrogen applications,
providing the opportunity to maximise yield potential in wet
years, or save fertiliser costs in dry years,” Peter said.
Organic matter
Peter Grace says that the key to reducing the risk of nitrogen
loss is having adequate organic matter in the soil.
“Organic matter supplies native nitrogen to the plant in a form
that is slow-release. Because of the slow release, there’s less plant
available nitrogen at any time to be lost, so overall nitrogen loss
is lower. In sandy-loams, loams and clays, organic matter also
improves aggregation of the soil, which improves soil structure,
increases the plant available water capacity and improves
infiltration.
“If the water can get in and move into the profile and drain
relatively freely, the risk of denitrification is reduced,” he said.
Options to improve organic matter include pasture rotations,
reduced tillage, stubble retention, and where soils are very sandy,
clay spreading or delving can be considered.
“In essence, conservation farming practices will put any
grower well on the way to maximising N conversion into the
crop. By reducing the external nitrogen input, growers are saving
fertiliser costs and making it easier to balance the requirement
to add sufficient N to maximise crop yield with the limited topdressing opportunities in a season,” Peter Grace said.
More Information: Prof Peter Grace, 07 3138 9283, [email protected]
Dr Peter McIntosh, 03 6232 5390, [email protected]
Useful Resources:
GRDC Factsheet: www.grdc.com.au/GRDC-FS-PlantAvailableNitrogen
Learn about nitrogen mineralisation:
www.grdc.com.au/MR-UnderstandNitrogenMineralisation
Get started with soil moisture monitoring:
www.grdc.com.au/News-SoilMoistureMonitoring-0215
n
WHERE TO GET YOUR N: LEGUME ROTATION VS FERTILISER
Both fertilisers and legume rotations are known to supply
nitrogen to cropping soils, but which is the better option?
A GRDC-funded project has researched this question by
measuring inputs of fixed nitrogen (N) by different legumes,
monitoring legume induced changes in available soil N, and
comparing crop responses following legumes to applied fertiliser.
CSIRO Agriculture Flagship Research Director Mark Peoples
says the project found that both fertilisers and legume rotations
have their place in a cropping program.
Trial results
Various trials examined N fixation by pulse crops and pasture
legumes in southern and central NSW, the south-east region
of South Australia, and in both high and low rainfall zones of
Victoria over the past four years. These studies compared the
net inputs of fixed N, which represents the fixed N remaining in
legume residues at the end of the growing season.
“The trials demonstrated that, as expected, rotations that
Dr Mark Peoples says that both fertilisers and legumes
have a role to play in supplying N. (Source: GRDC)
12 — Australian Grain
resulted in highest returns of fixed N to the soil were brown
manured crops, and grazed or hay-cut legume pastures ,
since large amounts of N are removed from pulse crops in the
harvested grain,” Mark said.
An experiment undertaken near Junee in southern NSW
between 2011 and 2013 found that soil mineral N measured just
prior to sowing wheat in 2012 was 42 kg N per hectare greater
following lupins in 2011 compared to wheat when both were
harvested for grain. This figure rose to 92 kg N per hectare
greater than wheat if the lupins were brown manured.
Higher concentrations of available soil N persisted through
to the second wheat crop in 2013. The soil mineral N in 2011
was 18 or 34 kg N per hectare greater where lupins had been
harvested for grain, or brown manured respectively, compared
to continuous wheat.
Not all the N in legume residues became available or
was recovered by wheat the first year. In the Junee trials,
approximately 28 per cent of N fixed in the lupin residues were
taken up by wheat in 2012, compared to 47 to 59 per cent
of top-dressed fertiliser when it was applied at the start of
stem elongation (GS31), just prior to wheat’s period of peak N
demand. A further 11 per cent of the N in the 2011 lupin residues
was subsequently utilised by the second wheat crop in 2013.
In addition, losses of N are usually lower from legume sources
than from fertiliser (see ‘What are the types of nitrogen losses’
page 10), and more legume N remains in the soil, which will
further balance the initial lower recovery.
Legumes also have other benefits, including increasing
soil organic fertility, weed and disease management, and the
development of beneficial soil biology. Growers should consider
both options as both have a place within a farming system.
More information: Mark Peoples, [email protected]
March–April 2015