Nitrogen fertilisers — improving efficiency and saving money

Nitrogen is critical to plant growth and reproduction. Pasture and crop growth will often respond to an increased availability of soil nitrogen. This situation is often managed through the addition of nitrogen fertilisers.

Nitrous oxide is a powerful greenhouse gas and accounts for 5 per cent to 7 per cent of global greenhouse emissions with 90 percent of these derived from agricultural practices. Nitrogen based fertilisers and livestock manure (urine and dung) are the key sources of nitrous oxide emissions on farms.

Greater efficiency in the capture of nitrogen in products has the greatest impact on reducing nitrous oxide losses, as well as reducing ammonia volatilisation to the atmosphere and nitrate leaching and runoff to groundwater and waterways. Improved nitrogen use efficiency (NUE) has both productivity and profitability benefits.

Nitrous oxide is most likely released from warm, waterlogged soils where there is excess nitrogen in the form of nitrate. Volatilisation of nitrogen as ammonia can also lead to indirect nitrous oxide emissions through redeposition contributing to excess nitrate elsewhere in the landscape.

Farmers can save money, boost pasture and crop production and reduce nitrous oxide losses by carefully planning and implementing best management practices with regards to the 4 Rs — the ‘right’ rate, source, timing and placement of nitrogen fertilisers to match plant needs.

Follow the 4 Rs:

• Right product
• Right rate
• Right time
• Right place

Management options

Research has estimated that usually 40 percent to 60 percent of nitrogen inputs into cropping and grazing systems, respectively, is lost to the environment. By improving agricultural practices we can reduce these losses, improve productivity and save money.

Match nitrogen supply to crop/pasture demand by:

• using soil or plant testing to assess plant available nitrogen supply. Apply nitrogen fertiliser rate based on target yield and crop or pasture nitrogen requirement over the growing season.
• accounting for soil moisture availability and seasonal forecasts for more timely and calibrated fertiliser decisions support.
• using industry-relevant decision support tools (such as Yield Prophet in Grains, Dairy nitrogen fertiliser Advisor, GrazFert for beef and sheep).
• avoiding high application rates of nitrogen in any single application (never exceed recommended rates, split applications may be more effective and adjust rates according to rainfall and temperature).

Time fertiliser application to minimise nitrogen loss:

• Where possible, align nitrogen fertiliser applications with crop and pasture demand. Crop/pasture demand is highest when growth rates are highest.
• Avoid applying nitrogen fertiliser to warm (>10°C) waterlogged soils.
• Avoid tillage under wet conditions.
• Consult a 7-day weather forecast to identify risks of soil saturation and if likely delay nitrogen fertiliser application.
• In summer, avoid applying urea fertiliser after irrigation as this is likely to increase volatilisation losses.
• Minimise the length of fallow when converting long-term pasture to crops, especially in high rainfall zones and irrigated crops.

Determine and improve plant access to nitrogen by improving soil health and nutrient status — see next section on Soils. Adding nitrogen to soils that have inherent limitations to plant growth is unlikely to result in higher productivity and financial gain.

Choose the best type of nitrogen:

• Avoid Nitrate based fertilisers which are more prone to losses.
• Enhanced Efficiency Fertilisers, for example coated for slow release, or with nitrification inhibitors may better match the fertiliser supply and plant demand for soil nitrogen.
• Chemicals can be added to fertiliser (inhibitors) which can reduce nitrate leaching and ammonium volatilisation. However, it is recommended to seek expert advice when choosing inhibitors.

Incorporate fertiliser at the top of raised beds or ridges to avoid concentration and losses in furrows and wet areas.

Estimate the methane and nitrous oxide emissions on your farm using a greenhouse gas accounting tool (go to the links to appropriate tools for your type of enterprise).

Summarised version of a Nitrogen cycle in a grazing/cropping system. The Victorian Resources Online website includes a detailed animation explaining the N cycle.

Improving nitrogen fertiliser use efficiency

There is a growing body of evidence that indicates that significant amounts of applied N fertiliser remains unaccounted for under certain cropping systems and conditions. Unfortunately, this N is often irretrievably lost from the cropping system, representing both a significant cost to growers and the environment.

Agriculture Victoria research scientists have been at the forefront of research into N fertiliser management and N₂O emissions in Victoria's cropping industry for over a decade. A variety of field experiments have been established on farms across Victoria's cropping zones to help better understand the issue.

In the high rainfall zone (HRZ) of south-western Victoria, Agriculture Victoria researchers have recorded losses of up to 85 per cent of the N applied in situations where large amounts of N fertiliser were applied at sowing. In some trials around Hamilton, applying N at sowing to soils already naturally high in N, gave no significant increases in crop yield response. Effectively, what this means is that applying N fertiliser in these situations was both a waste of time and money.

In 2017, Agriculture Victoria researchers completed a collaborative, three-year research project into N use efficiency in key Victorian cropping zones.

Lead researcher, Professor Roger Armstrong stated that:

A real standout result of the project was the high number of instances where N fertiliser response was limited. Reducing current rates of fertiliser N input, particularly in the HRZ, where paddocks have experienced a long history of legume pastures, can have minimal impact on productivity while saving growers money. However, of real concern to growers is the large losses of N fertiliser that we recorded. Fertiliser N recovery in the crop, plus that remaining in the soil at harvest, averaged only 71 per cent; it varied from 63 per cent in the HRZ to 76 per cent in the low/medium rainfall regions.

This research indicates that the best strategies to reduce both fertiliser costs and N₂O emissions in these systems are to:

• increase crop utilisation of soil N ('soaking up' excessive N)
• reduce fertiliser inputs, via better predictions of current soil N status using deep soil sampling prior to sowing.

Currently, growers and their advisers can only guess the likely amount of In-Crop Mineralisation (ICM) that is occurring in these soils when making predictions about likely amounts of fertiliser that will need to be applied to meet predicted crop demand. However, simple and rapid soil tests are being developed that will allow an accurate assessment of potential N mineralisation rates before sowing.

Two soil tests (Hot KCl and Solvita) show promise as predictors of ICM, both of which can perform better than some of the current 'rules of thumb' used by advisers across the regions but are not currently available commercially.

Nitrogen fertilisers and nitrous oxide

Nitrous oxide (N₂O) is emitted from soils, N fertilisers and stock effluent. Sometimes called 'laughing gas', N₂O is no laughing matter. Nitrous oxide can have significant impacts on our environment. It's a powerful greenhouse gas that's around 300 times more effective in trapping heat than carbon dioxide and it persists in our atmosphere for up to 114 years. Nitrous oxide also has the added downside of being an ozone layer destroying gas.

Nitrous oxide emissions represent a loss of valuable N from soils that would otherwise be available for plant growth. Nitrogen is critical to plant growth and reproduction. Production agriculture requires higher levels of N than are normally found in native soils. Hence, the addition of N fertilisers.

Although some N₂O production is a natural part of the N cycle, levels of N₂O emissions are greatly affected by the way we manage our soils and fertiliser input. High levels of N₂O emissions usually indicate overuse of N fertiliser. Unfortunately, there is increasing evidence that the relationship between N₂O emissions and increasing N input is an exponential, rather than a lineal relationship for most crop types.

Excessive levels of N can also result in leaching of nitrates into water systems, both above and below-ground. Nitrogen rich leachates are a key culprit in algal blooms and dissolved oxygen depletion, which is toxic to wildlife.

Nitrogen-based fertilisers and livestock waste (urine and dung) are the key sources of N₂O emissions on farms. In 2007, Australian N₂O emissions from agricultural soils were estimated at 20.2 million tonnes of 'carbon dioxide equivalent' or 85.9 per cent of all anthropogenic N₂O emissions. Between 1990 and 2007, N₂O emissions in Australia rose by 24 per cent and this increase is largely attributable to the increased application of nitrogenous fertilisers. That's why the race is on to better understand how nitrogen fertiliser can be better managed while finding cost-effective ways to reduce N₂O emissions.

Agriculture Victoria Researcher, Professor Roger Armstrong, explains some of the key Victorian N₂O research findings:

'Overall we have found that N₂O emissions are quite low in low and medium rainfall areas, particularly if N isn't applied during periods of temporary waterlogging. The key thing is to remember that N₂O emissions are closely tied to anaerobic soils that occur during waterlogging events.'

High levels of N₂O emissions in cropping soils indicate excessive fertiliser use (especially nitrate-based fertilisers) which is amplified on waterlogged sites with high levels of organic labile soil carbon, clay soils and warm clay soils (with temperatures above 15°C.

Soil texture also influences N₂O production since pore size influences drainage and N₂O emissions peak when water filled pore space of soils is around 70 per cent.

Not surprisingly, the 'perfect storm' of conditions suited to high N₂O emissions are most often found on irrigated and HRZ sites.

Ammonium Sulphate Ammonium Sulfate Crystal Powder Agriculture Grade

Best management practices to reduce emissions and improve efficiency

Thanks to the wealth of research undertaken in Victoria and other major cropping areas in Australia and overseas, we now have some pretty solid 'rules of thumb' around best management practices for N fertilisers.

A handy way to remember these is to think about the four Rs, which are the:

• right product
• right amount
• right timing
• right placement.

The 'four Rs' are summarised in the following video.

Right product

Less N₂O is emitted where ammonium (NH₄⁺) is the main form of N or where the rate of NH₄⁺ conversion to nitrate (NO₃^- ) in the soil is slowed. Try to use fertilisers that don't supply N in the form of nitrates — for example, N supplied as ammonium or urea will result in a slower rate of nitrate production, especially in waterlogged soils, and therefore less N₂O emissions.

Consider using nitrification inhibitors, which slow the conversion of ammonium to nitrate). But the price and logistics must be considered for your situation.

Slow release products, where the release pattern matches plant demand for N, are not currently cost-effective in many cropping situations but may become more viable as the technology develops.

Aim to provide N from organic matter sources, for example, manure, compost and pasture legume or pulse crop residues. However, adding high rates of organic matter adds significant amounts of readily mineralisable N as well as lots of labile carbon (C) and can exacerbate N loss. In cropping, the more realistic option is to utilise more legume residues for N supply this will provide a more moderate rate of N and C.

If you're using manures, get a nutrient analysis to determine both organic and total N and make allowances in your fertiliser budget.

Right rate

The higher the rate, the greater the likelihood of emissions, so make sure you test your soils to determine both the existing mineral N content and then allow for the amount of soil N that could be mineralised during the growing season. Once you have estimated the crops' N demand, subtract the measured N supply from the soil via your soil test and the estimated N supply from mineralisation to determine the N fertiliser rate you need to apply.

The optimum N fertiliser rate depends on the type of crop, the soil type, farming system and the crops' growth stage. Aim to match the rate closely to a conservative and realistic estimate of your crops projected needs:

Sample soils to determine mineral N content wherever there is uncertainty around the quantity and location of N in the crop root zone. In annual crops, sample close to planting as feasible, or soon after crop emergence, especially where high levels of residual N exist, for example, from previous N-fixing crops residues. Depending on crop root depth, sample soils at depths 0 to 10 cm and 10 to 60 cm (and down to 60 to 90 cm if roots extend to that depth).

Also, check that other factors such as other nutrients, disease or soil constraints (such as acidic soils) aren't limiting your crop's growth and ability to utilise any extra N applied. This is where precision cropping techniques (such as variable rate technology, pH mapping) and disease assessments can help get more accurate fertiliser application and avoid wasting inputs that aren't needed.

Determine and improve plant access to N by improving soil health and nutrient status. Pouring additional N inputs onto soils that have inherent limitations to crop growth such as high salinity is unlikely to result in financial gain.

If available, using appropriate, industry-relevant decision support tools (for example, Yield Prophet in Grains) and seasonal forecasts for more timely and calibrated fertiliser decision support. Knowledge of moisture status and soil N reserves and supply must be taken into account.

Consider all other non-fertiliser N sources, for example, soil mineral N, organic N mineralisation from previous crop residues, manures and irrigation water.

Consider subsoil limitations such as salinity, acidity or high boron that might restrict your crops ability to use N effectively.

Right time

Matching the timing of fertiliser application to coincide with your crops' changing demand will maximise crop N uptake and minimise your fertiliser costs. Always aim to avoid applying fertiliser when soils are waterlogged or likely to become so, for example, before forecast heavy rain. Consider that:

• crops usually only use small amounts of N in early growth stages so if you apply most of the crops projected N needs at or close to planting, N losses are more likely
• sample soils and, if you're able to, apply 'in-crop' N fertiliser rather than 'up-front' N to better match your crops' demand. Supplying N fertiliser at crop development stages during high N uptake periods can increase N uptake by the crop, provided sufficient follow up rainfall occurs. Most crops require less than 20 per cent of their total N requirement by first flowers².

If you're converting long-term pasture to crops, minimise the length of fallow, especially in HRZ and irrigated crops. Long fallows lead to rapid build-up of N in the nitrate form and subsequently, much higher N₂O losses compared to short fallows before cropping. Avoid conversion in high rainfall years — for example, when La Niña is forecast.

Right place

Use the best technique for placing N fertiliser in the right place that maximises crop N uptake. Apply fertiliser close to the active root zone or where rain will move fertiliser to the main root zone. Generally, it's best to place your N into the soil. This helps to ensure N is more accessible to the crop roots, because crops can't access N if it is stranded in dry topsoils. It also tends to reduce losses of N by ammonia volatilisation from some soils.

In soil types that are prone to erosion or volatilisation, such as soils with highly alkaline topsoils that can occur in parts of the Wimmera and most of the Mallee, apply fertiliser into subsoils and aim to place it as deep below the composting crop residue as possible, may help to improve crop uptake of applied N.

Use tillage practices and systems such as controlled traffic farming to maximise water and N uptake by crops, by avoiding compaction and waterlogging and maintain good drainage. Where waterlogging results from sealing or dispersive topsoils, practices such as the following can be effective:

• retention of soil cover
• maintaining soil organic matter
• gypsum or lime application
• reducing cultivation.

Remember, too, that improved surface drainage, minimising tillage and controlled traffic practices also help by improving soil structure (resulting in better grain yields) while reducing N losses occurring during waterlogging.

Don't forget to make sure you've checked and measured the accuracy of your fertiliser spreader and operator and avoid double-ups and drainage lines — unless you want to watch money go down the drain.

Other management techniques to improve N use efficiency and reduce N₂O emissions include:

• Use deep-rooted follow up crops to 'mop up' residual N in high N situations with sufficient rainfall.
• Switch off fertiliser applicators at end rows and other turn around points.
• Protect stored fertiliser from moisture.
• Avoid/pick up fertiliser spills.
• Ensure your fertiliser operators are adequately trained or accredited in handling and spreading fertiliser.
• Use GPS enabled dataloggers to identify high and low yielding zones and adjust your inputs for best N use efficiency.
• Use crop varieties that are able to make the most of available N.

Future research into N use and management is continuing to ensure Victorian farmers can not only reduce greenhouse gas emissions, but also ensure fertiliser costs are minimised.