The effect of nutrient position and release rates in cereal growth

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					                                       DRAFT REPORT
The effect of nutrient position and release rates in cereal growth and nutrient
       Part 1 – Nitrogen product controlled environment studies 2006
                          Rob Norton, Will Hewitt and Peter Howie
            School of Agriculture and Food Systems, The University of Melbourne,
                            Private Bag 260, Horsham, Vic. 3401.

The GRDC supported project UM00023 (Synchronising nutrient supply and crop demand in
modern cropping systems) is investigating issues concerning nutrient synchronisation in
modern farming systems. The over-riding hypothesis is that matching the timing and rate
nutrient supply (N and P) to crops will improve the efficiency with which nutrients are used.
The project involves studies of nutrient dynamics in rotations (including nutrient balances)
and studies within a growing season. This second area involves estimating the temporal
variation in nutrient demand for N and P, as well as the strategies that can be used to alter the
rate of supply of nutrients to growing crops. Two reports have been prepared for discussion
on the factors affecting nutrient supply – one reporting the results of experiments on using
strategies to alter the release of N from urea and its effect on N uptake in wheat (Part 1) and
the other reporting results of phosphorus controlled environment studies (Part 2). In addition
to these studies, field experiments have been undertaken as well as modelling studies linking
the field and controlled environment experiments. Those components of this research will be
separately reported.

This DRAFT technical report has been prepared for circulation to participants in the Nutrient
Management Initiative and the GRDC for comment, as the main part of milestone 4 of the
UM00023 project. The objective of this component of the research project, which is still in
progress, is to investigate the effects of urea presentation, formulation and amendments on the
rate of N supply and plant uptake.

Urea is now, and in the future, likely to be the main N fertilizer source used for grain
production. Urea rapidly releases N as ammonia/ammonium which is then nitrified, and the
rate at which these processes occur largely determines the efficiency with which the N is
utilized by the crop. The rapid release profile of N from urea should be roughly matched to
the demand by the place to access the N. A lack of synchrony in supply and demand can
result in N losses and lowered N use efficiency.

Part of the underlying hypothesis for this part of the research is that urea too rapidly makes N
available in the soil solution, so that the excess N is then either leached or denitrified. Thus,
slowing the rate of N release, either physically (eg PCU/S coatings), by inhibiting urease
activity (Agrotain), by reducing nitrification (Entec) or by altering adjacent soil conditions by
acidification or raised CEC, all represent opportunities to better synchronise N supply with
crop N demand.

The objective of this component of the project was to evaluate a range of urea amendments
that are currently commercially available or are undergoing evaluation as release inhibiting
mechanisms for urea fertiliser. A total of 10 different biological and physical inhibitory
agents were evaluated under controlled conditions. Similar products were evaluated in field
experiments in 2005 and 2006 and these evaluations were undertaken in combination with

application strategies that could be used to manage release rates. The results of the field
experiments will be reported separately.

Products evaluated
1.       Standard granulated urea.
2.       Urea plus nitrification inhibitors (eg DMPP, Trade Name Entec), which slow the
nitrification process by interfering with the activity of Nitrosomonas group of bacteria that
transform NH3 to NO2- thus effectively slowing the formation of NO3-.
3.       Urea with urease inhibitors (eg nBTPT, Trade name Agrotain) which reduces
ammonia volatilisation potential by reducing the peak pH and ammonium levels in the soil
and by allowing more time for rainfall to leach surface applied urea into the soil (Schlegel,
Nelson et al. 1987).
4.       Polymer coating of urea fertiliser (PCU) involves the encapsulating of granules within
a polymer coating. The materials prevent the free dissolution of urea in the soil solution,
instead water must diffuse through the polymer membrane and cause the urea to dissolve,
which subsequently comes out through the polymer.
5.       The sulphur coating of urea (SCU) granules is designed to physically restrain the
release of nitrogen from the urea by preventing the free dissolution of the granule.
6.       Stable-U is a new stabilized urea fertilizer, which has not been reported in any of the
current literature published. This product is formed using a microchip-size granule of lime at
the core of each nitrogen fertilizer granule.
7.       The mixing of high CEC zeolite with urea aims to maintain the presence of N within
the soil through capturing – at least for a short time – ammonium released.
8.       Oil coated urea products have been tested in past studies as an inexpensive, effective,
biodegradable option for nitrification inhibition. The oil coating on the granule also provides a
hydrophobic coating to the granule which potentially slows the rate at which the granule can
be dissolved.
9.       Black Urea is a commercial product developed by Advanced Nutrients Pty Ltd and it
is a coating designed to reduce the leaching and volatilization of NH4+ as a result of the acidic
nature and high cation exchange capacity of the humic acid.
10.      Black urea plus zeolite, which would combine the features of both amendments.

Experiment 1
Aim - The aim of this experiment is to compare the effects of various urea amendants on the
rate of ammonium release under controlled conditions.

Methods - Columns 100 mm long and 45 mm in diameter were packed with 200 g of medium
textured sand and 10% clay. The tapered bottom of the cylinder was then packed with
glasswool. The column was packed to 75% capacity, the various urea fertilizer were added,
each supplying 2 g of N per column and then the final 25% of the medium was added. The
soil profile was then leached with a total of 40 ml of distilled water every second day for 17
days. The cores where then allowed to drain for 3 hours and the leachate was then collected.
These samples frozen then analysed for ammonium and nitrate.

Results – The results from the laboratory experiment were compiled to illustrate the
cumulative conversion of urea to NH4+ through the process of hydrolysis over the duration of
the experiment, to establish the rate of Nitrogen release for plant available uptake. Figures 1
shows the rate of ammonium release from the various urea treatments, compared to soil alone,
The release rate of all urea treatments over the first three days of the experiment was similar
nor could any significant difference be observed between urea treatments and the soil only
control. However after day 3, there were clear trends in the rate of NH4+ accumulation in the

leachates that indicate moderation of ammonium release in response to some of the treatments

There was very little nitrate measured in the leached solutions, and there were no differences
among the standard urea and the release moderating treatments and between the fertiliser and
soil only treatments.

Figure 1 Cumulative release rates of ammonium from various urea supplemented fertilizers over 17 days.

Conclusion – This experiment demonstrated potential of a range of these products to slow the
rate at which ammonium is released from urea granules. The humic acid and sulfur coatings
of urea granule both demonstrated slowed ammonium release by 14% and 25% respectively.
However, the largest effects were due to the urease inhibitor (80%) and the polymer coating
(90%). Given the experimental conditions, it was not surprising that the Entec treatment was
not significantly different to the straight urea.

Experiment 2
Aim – The aim of this experiment was to compare the effect of various urea amendants on the
early season supply of N to wheat grown in two different soils.

Methods - The experiment used the same 10 urea fertiliser treatments tested in experiment 1,
plus comparing two fertilizer placement techniques and two soil types. Soils from Hopetoun
(sodosol - sandy loam) and Kalkee (vertosol - clay loam) were dried and LEL and DUL
determined. Four litre free draining containers were filled with 4 kg of soil and the pots
watered to maintain soil water content at approximately 50% DUL.
The fertilisers were applied to the containers at rates equivalent to 45kg N/ha calculated from
the surface area of the container and the nitrogen content of urea and urea under each of the
treatments. Ureas were either incorporated throughout the soil profile to a depth of 5cm or
banded 2 cm to the side and 7cm below the seed.

 Wheat (cv Yitpi) was sown and plant dry weights and N contents were taken at DC15 (40
days after emergence) and DC30 (65 days after emergence). Additional measurements were
made of shoot number and leaf SPAD (chlorophyll) readings taken.

Results – Plant biomass and N contents were used to estimate N uptake per plant. These data
were then used to derive the proportion of the applied N recovered by the wheat plants at
DC15 (Figure 2) and DC30 (Figure 3). The values reported here are the means of the banded
and incorporated treatments. The Hopetoun soil had less mineral N at sowing than the Kalkee
soil, and so there was a larger proportion of the applied N taken up by DC15 in the Hopetoun
soil (Figure 2). However, by DC30, there were not large differences in N recovered between
the two soils.

Figure 2 The proportion of applied N recovered by DC15 from the various urea and amended ureas by wheat
grown in two different soils.

Figure 3 The proportion of applied N recovered by DC30 from the various urea and amended ureas by wheat
grown in two different soils.

On both soils, the PCU was effective at reducing the rate of N uptake by the wheat plants,
although there were differences at the sandier Hopetoun soil where Entec urea also slowed N
up take. The urease inhibitor showed little effect on either soil or at either harvest, despite the
effects seen in Experiment 1. This suggests that the effect of urease inhibition is relatively
short lived (<40 days).

Conclusion – While it is possible to alter N release rates from urea, there were fewer
differences seen on N uptake by wheat plants in this experiment. PCU was effective at
slowing N uptake, and there would appear to be soil type specific effects that moderate the
effect in other soils. This indicates that particular products will have specific situations where
they are most effective.

Experiment 3
In addition to the glasshouse experiments, N uptake has been assessed as part of the field
evaluations of the products tested above. Experiments were undertaken at Marnoo, SeaLake
and Inverleigh (2005) and Hopetoun, Kalkee and Inverleigh (2006) to investigate the crop
response to these products. Those results will be separately reported.

Future Research
Experiment 1 had provided a technique to assay N release rates for new products, and this
will be used to screen other materials as they become commercially available. It is clear that
Agrotain is likely to be most effective when topdressed rather than in furrow, so additional
research will be undertaken to evaluate it under those conditions. It is also clear than a
nitrification inhibitor would be most effective when soils undergo some waterlogging and this
hpothesis will be further tested.

The objective of the further research in this area is to identify those key factors that would
enable growers to identify the circumstances under which the particular products can provide


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