Nitrogen Management for a Dry Start in the Mallee in 2015
Therese McBeath, Rick Lllewellyn, Vadakattu Gupta, Michael Moodie, Bill Davoren
Here we use the Karoonda trial site to work through some figures on how we might estimate crop
nitrogen (N) requirement for 2015. These figures are to provide an example and will require
adjustment for use on other Mallee farms.
Setting the Scene
Soil Water and Yield Potential
Throughout the Mallee region, rainfall for August 2014–April 2015 was below average. For example,
at the Karoonda site the August 2014–April 2014 inclusive rainfall was 187 mm compared with
average 231 mm and 88 mm of that rain falling in January.
Karoonda Monthly Rainfall
140
120
Rainfall mm
100
80
2014
2015
60
avg rainfall
40
20
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 1. Karoonda Trial Site Average, 2014 and 2015 rainfall for Karoonda.
At Karoonda, plant available water at sowing was 40-60mm, swale to dune which with average
growing season rainfall gives an attainable yield potential of 2.3-2.7 t/ha.
Soil N
For district practice (50 kg diammonium phosphate (DAP)/ha at sowing) the soil N on our Karoonda
trial site has been tracking downwards, losing approximately 15-20kg Mineral N/ha in each season
from 2010 levels of 115kg N/ha/m with values of approximately 19 kg mineral N/ha measured in
March 2015.
In addition to fertiliser, the soil is a very important source of N for crops. There are a number of
reactions of N in soil over the summer fallow period that will influence the amount of mineral N
available for crop uptake early in the growing season. Under dry conditions, due to the low (or lack
of) microbial activity, biological mineralisation is reduced. This results in a reduced amount of
available soil N in the profile at sowing. Lack of adequate decomposition of last season’s crop
residues due to rainfall during the last 3 to 4 months could also lead to tie-up of N within stubble
following the season break resulting in nutrient deficiency early the crop, especially where the
applied fertiliser levels are low. In addition, dry soils reduce the ability of crop roots to explore the
soil profile for nutrients and therefore fertiliser nutrients placed near the seed become more
important for early growth. With only one major rainfall event in January/ Febuary and no significant
rainfall until April, Rhizoctonia inoculum levels have remained high. Therefore, N deficiency early in
the season due to low N mineralisation and little or no fertilizer application would exacerbate
rhizoctonia disease impact.
N management options
With lower yield potential and low starting soil N, choosing a N rate is going to involve some tradeoffs between the risk of over-fertilising if the season continues to have below average rainfall and
the risk of under-fertilising if rain does come.
At 50% fertiliser N use efficiency, the N requirement of a crop is approximately 40 kg/ha per tonne of
grain which can come from soil or from fertiliser. Each grower needs to identify what yield goal they
want to budget for and using knowledge of their likely soil N level estimate the amount of fertiliser N
required to meet that goal. With a yield target of 2 t/ha (adjusted down from the yield potential of
2.3-2.7 t/ha) and we have a further 20 kg N/ha of soil mineral N at seeding then we need 60 kg/ha N
from elsewhere. The CSIRO Generic N calculator estimates that 20kg N/ha (at 0.6% organic carbon)
will come from soil mineralisation through the season so this leaves a fertiliser N requirement of 40
kg N/ha.
The conditions appear to favour up-front application of N fertiliser in 2015 as the most efficient
application strategy. However, if you are on sandy soils that have a low soil test N and prefer to
apply N fertiliser in-season then these applications are going to be need to be earlier (eg. tillering or
before) to avoid a N deficiency lowering yield potential early in the season.
Remember that most calculations like this assume a high level of fertiliser efficiency (our N fertiliser
efficiency at Karoonda has been in the order of 25–50%). If fertiliser is applied in sub-optimal
conditions or there are other factors that prevent N from being used efficiently (for example disease,
weeds, other limiting nutrients) then using this amount of N fertiliser without treating the other
issues is not going to achieve the yield target.
For more information contact
Therese McBeath; [email protected]