Nitrogen strategies in cropping
03 Nov 2023
Tim Bartimote - Cropping Ag Advisor, Central West Local Land Services
Nitrogen is arguably one of the most important inputs to our farming systems, and one of our biggest costs.
The ‘risk’ associated with its use often means that it gets underused. Studies suggest that the yield gap in the Gilgandra shire is in the order of 38%1. That is the difference between water-limited yield potential and actual yields (Table 1). Under application of nitrogen is the biggest contributor to the yield gap2.
The GRDC ‘RiskWi$e’ project is looking at the risk-reward of various nitrogen management strategies in an effort to address the yield gap caused by nitrogen underutilisation.
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Table 1. Estimated yield gap between water-limited potential and actual yields at Gilgandra trial site.
To consider these questions a little further, Central West Local Land Services has teamed up with Grain Orana Alliance (GOA) to establish a site near Gilgandra to look at some of the pros and cons of various nitrogen strategies. Similar trials are being conducted across Australia under the GRDC funded ‘RiskWi$e’ project.
In our Gilgandra demonstration, 4 strategies have been implemented and each one aims to maximise yield while also trying to minimise risk in different ways.
1. Moderate seasonally responsive strategy
Enough nitrogen is applied to meet the water limited yield potential in a decile 5 season. A decile 5 season is one where 50% of years had more available water and 50% had less. Essentially this strategy aims to feed enough nitrogen for an “average” year.
The strength of this strategy can also be considered its weakness. By aiming for the middle ground every season, producers will be less likely to overcapitalize in fertiliser investments during poor seasons. While at the same time this strategy prevents producers from making the most of good seasons where extra application of nitrogen is justified to capitalise on increased rainfall.
In an environment where winter rainfall is more consistent, and the average amount of rain is not far from the historic median across the years, this strategy might be considered. However, this would likely not be applicable in a location which might be better defined as either ‘boom or bust.’
2. Financial replacement strategy
This strategy is driven by the financial situation at the end of the previous year. Where a fixed amount is allocated to fertiliser purchases based on a percentage of the gross income from the preceding season.
Its basis is found not in the production needs of the crop necessarily but rather in the availability of finances. Simply, this strategy seeks to prioritise minimising financial risk over potential production when it comes to nitrogen.
Advantageously, the fertiliser bill for the upcoming season can be locked in prior to sowing since it is determined by the gross income of the last harvest. This also aids in planning and sourcing fertiliser when prices may be more suitable. This is very important as the price of fertiliser is a key factor in determining how many kgs of nitrogen are applied per ha with this strategy. By having a fixed amount to spend, production may suffer when prices are unavoidably high or when production in the previous year was low.
3. Physical replacement strategy
A significant proportion of nitrogen is removed from the paddock with harvested grain.
For example, wheat exports close to 20 kg N/t and canola removes close to 35 kg N/t. Therefore, another way to approach nitrogen replacement is to apply the equivalent in nitrogen fertiliser of grain nitrogen removed at harvest in the previous season. This assumes that losses are small and that other sources of nitrogen tie-up (like cereal stubble which is about 5 kg N/t) are less significant or at least will become available to crops at some later stage.
Historically, the general rule of thumb is that roughly 50% of applied nitrogen is recovered by the crop (grain+straw), with some recent research arguing for as low as 35% (see Armstrong et al. 2021). The rest of the nitrogen is then either stored in the soil or lost through various mechanisms.
Due to these inefficiencies simply adding what was exported via the grain may not sufficiently make up for what was taken off the paddock the following year.
For example, if a 3t/ha wheat crop removes about 60 kg N/ha then 130 kg urea/ha is required to replace it. However, only 30 kg nitogen (50%) of what is applied will likely end up in the crop of that year.
Therefore, if another 3t/ha crop is grown at a similar quality the crops will have to source nitrogen from what’s already in the profile. So having an accurate understanding of nitrogen losses and current nitrogen levels is helpful for this strategy to be successful.
A physical replacement strategy also means that costs will be proportionate to the degree of cash flow each season since fertiliser expenses will be low after a poor season and high after a good season.
4. Nitrogen banking strategy
Nitrogen banking seeks to maintain or build nitrogen within the soil profile to a certain level.
Much like a savings target in an actual bank account, a nitrogen banking strategy will not just make up for withdrawals but add extra to ensure the target amount is reached.
This strategy argues that fundamentally soils feed plants nitrogen, not bags of urea. At first, the upper limit of stored nitrogen a soil profile can hold is identfied through APSIM modelling. Then through ongoing urea application, the level of nitrogen is maintained or built to this predetermined value. Hence, feeding nitrogen into the soil profile should in turn feed the crop whenever it is required.
Practically, this may look like adding more fertiliser than is needed by the crop in a single season because various significant stores of nitrogen in the soil are lacking.
This strategy can provide producers with a proactive approach to nitrogen and therefore open opportunities to purchase nitrogen at low prices when demand isn’t as high. However, nitorgen applications will likely be more consistent and perhaps higher overall due to a change in mindset from a crop focus to a soil focus for an overall crop benefit.
This demonstration will seek to compare each of these strategies by monitoring overall yield and changes in nitrogen over time.
The site will also be open for inspection at upcoming field days and results included in future updates.
For more information about nitrogen application strategies please contact your local Central West LLS Ag Advisor.
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