Choosing a nitrogen strategy

Key messages:

• It’s important to always have a nitrogen application strategy for your crops, but be prepared to be flexible and change in response to the seasonal conditions  
• There are many nitrogen strategies, including seasonally responsive, replacement and nitrogen banking strategies, which each have their advantages and disadvantages, so it’s important to work with your agronomist to determine which one will best suit
• Deep soil nitrogen testing is an essential tool to understand the nutrients you have in the ‘bank’.

Nitrogen is a key nutrient for plant growth and is essential for producing plant proteins and chlorophyll. However, using nitrogen efficiently can be tricky in broadacre cropping. Deciding the rate and timing of nitrogen is one of the most critical challenges grain growers face in achieving maximum returns. This is because over-application is expensive and can also result in environmental losses through leaching and de-nitrification, while under-application can mean that crops aren’t able to reach their full yield or protein potential. With the shortage of urea last year, many wheat crops were ‘under-cooked’, indicated by low protein levels, which suggests nitrogen may have been a limiting factor in these crops.  

For this reason, it’s important to have a nitrogen use strategy to maximise productivity and profitability, while minimising environmental losses. Having a fertiliser use strategy will also assist with planning, allowing for early ordering of product to mitigate delays with supply shortages, which has been an issue in recent years.  

Despite the rainfall received in late May, it has been an extremely dry autumn and early winter across the Riverine Plains region. In most areas, crops will behind where they have been in the past few years, so the volume of nitrogen application and timing must be considered carefully with advice from your agronomist. 

Picking a long-term nitrogen strategy

There are multiple approaches farmers can take when it comes to managing their nitrogen inputs in the long term, including seasonally responsive strategies, replacement strategies and N-banking. Each has various advantages and disadvantages, with different farming businesses being better suited to a particular strategy, based on their key focus.  

Moderate seasonally responsive strategy

Approach: Apply enough nitrogen to meet the yield potential in an average (decile 5) season.
Advantages: Minimises over-investment in poor seasons.
Disadvantages: Limits potential gains in above-average seasons.
Best suited: Regions with consistent winter rainfall.

Financial replacement strategy

Approach: Allocate fertiliser budget based on a fixed percentage of the previous year’s gross income.
Advantages: Reduces financial risk and allows for pre-planning.
Disadvantages: Potentially limits nitrogen application when prices are high or after a poor harvest.
Best suited: Farms prioritising financial stability over maximum yield.

Physical replacement strategy

Approach: Replace the nitrogen removed by the harvested grain from the previous season.
Advantages: Aligns nitrogen application with actual nutrient removal.
Disadvantages: Inefficiencies exist due to nitrogen loss and variability in nitrogen recovery by crops.
Best suited: Farms with detailed knowledge of soil nitrogen levels and losses. 

Nitrogen banking strategy

Approach: The aim is to strategically apply higher levels of nitrogen, above anticipated crop demand, and leverage carry-over effects into subsequent seasons. In turn measuring residual nitrogen is also important to prevent excess application, whilst ensuring there is enough nutrients available for crop production and soil microbial practices year-round. 
Advantages: Ensures a steady nitrogen supply, potential for cost savings through bulk purchasing.
Disadvantages: Requires higher and more consistent nitrogen application.
Best suited: Farms focusing on long-term soil fertility and nitrogen availability.

Learn more about Riverine Plains project work on nitrogen banking and other nitrogen strategies 

Nitrogen budgeting

Setting a nitrogen budget can help farmers determine their crop nitrogen demand to achieve a target yield and protein. A key part of nitrogen budgeting involves estimating the existing nitrogen supply in a paddock, with the following tools and data points helping farmers develop these estimates.

Tools - important to utilise multiple sources for accurate estimation, including:

• Crop rotation
• Fertiliser application history
• Deep soil nitrogen (DSN) tests
• In-crop normalized difference vegetation index (NDVI) testing
• Shoot density
• Paddock history - a note of caution, paddock history alone may be misleading and should be used in conjunction with other measurements. 

Data points:

• Previous crop yield 
• Grain protein
• Soil moisture levels 
• Rainfall
• Deep soil nitrogen testing 

Deep soil nitrogen (DSN) tests are one of the most important tools to calculate existing nitrogen levels in soil. Using DSN testing prior to sowing mean that you have access to accurate nitrogen levels, which can give a baseline starting point to make fertiliser decisions based on expected yield.  

If low nitrogen levels are detected before sowing, growers may need to adjust the nitrogen program. Monitor low nitrogen paddocks closely and apply nitrogen early if required, or as conditions allow. 

Nitrogen timing

Nitrogen application before early stem elongation (GS30) can increase tiller numbers and dry matter production, which can be especially useful when growing dual purpose crops. Note, paddocks should not be grazed by stock for at least 14 days after urea application, sometimes longer if conditions are cold. It is important to consult your nutritionist or stock agent when determining the withholding period to prevent nitrate poisoning.

Nitrogen application at stem elongation (GS31-37) is recommended as this is the peak growth period, with most of nitrogen at this time contributing to yield.  

Any nitrogen applied after head emergence (GS59) will increase grain protein whilst also maintaining yield in high production years. Nitrogen use efficiency is often reduced with higher temperatures and under low rainfall conditions, so careful assessment of potential return versus risk of high environmental losses is required. Grain protein levels of less than 10.5 – 11 percent suggest that nitrogen was limiting. 

Source: No-Till Farmer

Nitrogen use efficiency

With higher input prices over the past few years, it’s important to ensure that you are getting the highest return from your fertiliser investment.  

Nitrogen use efficiency can be improved by spreading: 

• at the right time - prior to periods of rapid growth
• paddocks free from soil problems e.g. acidity, salinity, sodicity, compaction
• where there are no other limiting factors to plant growth such as root diseases (e.g. crown rot), leaf diseases (e.g. stripe rust or Septoria), weed and pest pressure. 

Higher risk of losses from volatilisation occur when there is:

• enough rain to dissolve the urea but not enough to wash it into the soil
• air temperatures above 18°C
• alkaline soils
• wind
• soils with low cation exchange capacity (CEC) - the CEC of a soil depends on clay %, the type of clay, soil pH and amount of organic matter
• warm waterlogged soils, above 10°C, will also increase incidence of de-nitrification losses.

Other sources of nitrogen

Nitrogen can come in forms other than synthetic fertiliser. Animal manure, where available, is a great alternative to synthetic fertilisers, while incorporating legumes in the rotation can also contribute significant amounts of nitrogen to the following crop, as well as providing other benefits. 

Summary

Ensure you are collecting soil, yield and protein data, and using the tools that are available, to inform your nitrogen decision making.  

When topdressing urea, timing is key. Remember these rules of thumb:

Applying nitrogen before GS30 increases tiller numbers and dry matter for dual-purpose crops.
Applying nitrogen at GS31-37 aligns with the peak growth period, with nitrogen going mainly to yield potential.
Applying nitrogen after GS59 will increase grain protein, but requires careful assessment due to efficiency concerns. 

Always aim to spread when conditions are favourable to minimise losses and maxmise nitrogen use efficiency.  

Sources & further reading

• NSW LLS - Nitrogen strategies in cropping
• Agriculture Victoria - Nitrogen fertilisers improving efficiency and saving money
• CropSmart - Revisiting your nitrogen strategy
• Incitec Pivot Fertilisers - Mangaging nitrogen torpdress
• GRDC - Managing nitrogen for high crop yields and sustainable farming systems
• GRDC Groundcover - Nitrogen banking a long term approach
• Incitec Pivot Fertilisers - Maximise your nitrogen use efficiency
• NSW LLS - Grazing winter wheat
• Riverine Plains blog - Getting the most out of your deep nitrogen soil testing
• 'Optimising soils and available water to improve drought resilience'