Nitrogen budgeting for crops

Understanding crop N requirements, and the dynamics ofN within the soil, is an important skill
for anyone involved in cropping.
Part 1
Choose a paddock where you have access to 2 years of cropping history – especially yield (and
hopefully protein) for last year (and maybe the previous year), possibly soil test N values, and N fertiliser used. If you don’t have
access to this information, I can supply some histories.
Use any one ofthe N decision support tools available‘, and estimate how much N
should have been applied to this year’s crop (eitherthis winters crop, orthe next summer crop). Show enough of your calculations so that
we can understand what you are doing, or describe/showthe set up values you use in the computer models. Ideally, you would also discuss ho
well the predictions matched the yield and protein results at harvest, but trimestertiming doesn’t let us do this, so you will have to
check the accuracy yourselves in December.
Part 2
Pick 3 areas/issues which can influence N dynamics and the accuracy ofthese decision
support tools, orwhich provide an important basis forthe tools. Briefly outline how each issue influences N dynamics and the prediction of
N requirements forthe crop with your chosen decision support tool.
Examples ofissues might be
influence of soil OM or soil type on
mineralisation rates,
importance oftaking enough cores when soil sampling,
effect offallow conditions on mineralisation,
effect of previous crop on residual N after harvest,
efflciency ofN use as affected by grain protein ortiming of application
in-crop mineralisation
potential N losses through denitriflcation or leaching
uptake ofN from different depths in the soil
“’The N
decision support tools can include:
Nbudget” produced by David Herridge. lfthey weren’t included in your notes, the booklet and
excel spread sheets are available

The simple N
budgeting approach from the Old and NSW DPl’s (which is described in flles
and a podcast on Moodle), and in “Soil Matters”

the N budget in Howwet, a
fallow modelling program based on the DPI approach and
available at
Yield Prophet (by
inputting soil test values, and looking at N response forthis year),
CSlRO’s Yield and N calculatorfor southern regions found

Whopper cropper- a modelling program based on ‘typical’ soils,
crops and N rates for each
part ofa region. It is easierto use, but much less speciflc than Yield Prophet, and doesn’t
include the
real time’ updates. Whopper has only been available on CD due to the huge
amount of data it contains, but it should be available
on-line by the end ofthe year.
Commercially available programs from Back Paddock, or Nutrient Advantage -these appear
to be similar
N budgeting approaches to most ofthe rest, but based on soil test values.
Something you may want to considerwith the different models is
how much they rely on soil test
information. lfthe information is available (and enough cores were taken to be accurate), then these
can be very useful. However, most paddocks aren’t sampled each year, so another method
which estimates N availability may have more impact
across the industry as it could be used more
frequently, and by more growers. David Herridge’s Nbudget, and the DPI N budget approach,
both able to estimate N fortheir calculations.
The “Managing Legume and Fertiliser N” booklet which is part of”Nbudget” is a
good source of
information forthis assignment, and it has a great list of references which may be of use as well.
Other references which
may be of use