Improving nitrogen use efficiency from timing and placement of nitrogen in high rainfall cropping

Take home messages

• The concept of deep banding nitrogen at sowing is not designed to replace in-season applications in the HRZ but provide an alternative management technique to mitigate the risk of sub-optimal fertiliser applications, poor crop uptake and nitrogen losses.
• Placing urea in a tight band below the soil surface creates a slow-release effect which could be considered a cheaper and more available option to some fertiliser nitrogen inhibitor products.
• However, in seven trials conducted in 2018-2020 across southwest Victoria and west Gippsland, improvements in wheat yield, protein and nitrogen use efficiency through banding urea at sowing were not consistent, compared to conventional fertiliser applications.
• Care needs to be taken when considering placing large amounts of urea below the seed, especially extremely sensitive crops such as canola. An absolute separation of at least 3.5cm below the seed row is required.
• A practical constraint to the technique is that most current seeding systems are incapable of  such separation of seed and fertiliser. An application of banded urea may be required prior to the sowing operation.

Introduction

Fertiliser applications amount to a considerable variable cost in grain production and are frequently rising as growers continue to push their production systems for higher yields, while soil organic matter is declining or at best staying the same. The importance of obtaining the highest level of efficiency from fertiliser is therefore paramount, not only to reduce costs but also to minimise nutrient losses and impacts on the environment.

In high rainfall cropping, large applications of fertiliser (mainly urea) are surface applied onto crops from mid tillering to early stem extension. This growth stage typically occurs in the winter months when soils are often at field capacity and waterlogged. This can result in a reduction in nitrogen use efficiency through denitrification of the applied urea which in turn releases nitrous oxide, a gas that has 310 times the global warming potential of carbon dioxide. The loss of nitrogen to the environment also reduces the amount available for plant uptake. Trafficability issues in the high rainfall zone (HRZ) especially during winter and early spring can also delay fertiliser applications. Deep banding a portion of the crops nitrogen requirements at sowing could be an option to reduce nitrogen losses and mitigate the risk of being unable to apply fertiliser at the optimum time.

Nitrogen Use Efficiency

Urea undergoes through a conversion process where it is ultimately converted into the plant available form of nitrate (urea > ammonia > ammonium > nitrite > nitrate). Losses through volatilisation, denitrification and leaching can be avoided by slowing down these conversions and holding nitrogen in the ammonium form for longer. It has been found that by placing urea in concentrated bands below the soil surface, the conversion of fertiliser nitrogen into plant available nitrate is slowed, and resulting in a slow-release effect of fertiliser nitrogen (Wetselaar et al. 1972).Previous research has also found that by placing the band at a depth greater than 7.5cm below the soil surface, volatilisation losses are reduced to almost negligible amounts (Rochette et al., 2014).  If this practice can supply the nitrogen when required, and result in fewer losses, an improvement in nitrogen use efficiency would be expected.

Practical Implications

Cropping in the HRZ can present many challenges and trafficability under wet conditions is a common constraint. Fertiliser inputs are typically required through the winter months when soils are often at field capacity and paddocks are regularly too wet to traffic, resulting in delayed applications of fertiliser, or not getting on to paddocks at all. Banding a portion of the crop’s nitrogen requirements at sowing could be used as a tool to mitigate this risk. The banded application is not designed to replace in-season applications altogether but provides an alternative management technique that can mitigate the risk of sub-optimal timings and reduce nitrogen losses during a period when soils may be at field capacity. With its slow-release effect, it could also be a cheaper and more accessible option than some nitrogen inhibitor products available.

A practical consideration for this management tool is the need to separate the urea and seed to avoid fertiliser toxicity. Unless modifications are made to seeder configurations to increase the distance between seed and fertiliser boots, the practice of banding urea would require an additional pass with the seeder prior to sowing. This is not always practical or economical. However, as a management tool it could be considered across certain areas of paddocks or whole paddocks that are known to become wet and with reduced trafficability in-season, rather than becoming a whole farm adoption.

It is essential to reiterate the importance of crop safety here when seeking to band large applications of nitrogen below the seed. Care must be taken to ensure seed/fertiliser separation is maintained to prevent damage to emerging seedlings. If urea is to be banded below the seed an absolute separation of 3.5cm below the seed row is required (Norton & Desbiolles, 2011). See GRDC fertiliser toxicity fact sheet.

Method

From 2018-2020 seven wheat trials were established across southwest Victoria and west Gippsland to compare different rates of nitrogen deep banded at sowing at varying depths below the seed, as displayed in figure 1.

Figure 1. Visual representation of urea deep banded below the seed zone at 2, 7 and 12cm depths. The big circles represent the slow-release band of urea.

Initial trials established in 2018 and 2019 compared four depths below the seed and four nitrogen rates, as outlined in Table 1. In 2018, the trials evaluated the use of deep banding nitrogen alone, with no additional in-season urea applied. This was modified in 2019, where each treatment received additional top-dressed nitrogen in-season.

As more results were obtained the 2020 trials were revised again, and only two depths, 2 and 7cm below the seed were trialled and three rates of banded urea, 0, 50 and 100 kg N/ha. An additional factor was also added combining the practices of 2018 and 2019 where treatments compared additional top-dressed urea at two to three timings from stem elongation, versus deep banding alone.

Table 1. Trials established in 2018 and 2019 compared deep banded urea at four depths and four nitrogen rates. In 2020, three rates of banded urea were compared at two depths, with and without the addition of in-season top-dressed fertiliser.

*In 2019, all treatments received top-dressed urea in season.

Nitrogen use efficiency was assessed through percentage of fertiliser recovery, using the difference method:

PFR = (NF)-(NC)/R

PFR = percent fertiliser recovery

NF = total crop N uptake from fertilised plots

NC = total crop N uptake from unfertilised (control) plots

R = rate of fertiliser N applied

Summary of results

Depth of banded nitrogen

The depth of banded urea did not appear to influence final grain yield across each of the three years of trial data. These trials however were only conducted with wheat. Results may vary between crop types, and canola is known to be sensitive to fertiliser toxicity. It is also important to note that while no crop damage was observed in these situations with the high rates of nitrogen deep banded at 2cm below the seed, there are several variables that may change this, such as soil moisture and texture, and it is not a recommended practice, particularly at the higher rates.

Rate of banded nitrogen

Varying grain yield and protein responses have been observed by rate of deep banded nitrogen. A summary of results is below:

• Where no in-season fertiliser was applied and crops relied on deep banded nitrogen alone, the trend across seasons showed an increase in grain protein as rate of banded nitrogen increased, however yield results varied.
• In 2019, one of the sites (Mininera, Vic) experienced severe waterlogging in winter. Results found as the rate of banded urea increased, grain protein decreased, however yield remained the same. This site was top-dressed in season to the same total nitrogen, for example, where less nitrogen was deep banded at sowing, more was applied as top-dressed urea in-season. These results were somewhat surprising as it was predicted there would have been a higher uptake of nitrogen where increased rates of urea were banded. This is compared with surface applied treatments where it was expected higher losses to denitrification would have been observed. It would be interesting to determine how persistent waterlogging may impact the banded urea in the soil.
• In 2020, where starting soil nitrogen was extremely low at only 39 kg N/ha (0-60cm) and additional in-season nitrogen was applied, grain yield decreased as the rate of deep banded urea increased. This again indicated a higher nitrogen use efficiency from surface applied urea, compared with deep banded. Grain protein however did not show any significant differences.
• In 2020, at Willaura where starting soil nitrogen was higher at 129 kg N/ha (0-60cm), an application of 100 kg N/ha deep banded at sowing alone produced a yield equivalent to all top-dressed treatments. From a practical perspective, in situations where in-season trafficability may be an issue this result indicates that under the right conditions, where there is sufficient starting soil nitrogen and moisture, there could be potential to use the technique.

Nitrogen Use Efficiency

Similar to grain yield and protein findings, mixed nitrogen use efficiency results were also observed across the trials. Where no additional nitrogen was applied in-season in 2018, fertiliser recovery decreased as rates of deep banded nitrogen increased (figure 2). The opposite however was observed in 2020 where the trend saw an increase in nitrogen use efficiency with higher rates of deep banded nitrogen, particularly at the Willaura site. The low levels of fertiliser recovery from Inverleigh and Bradvale in 2018 were probably a result of the high levels of soil nitrogen in the top 0-60cm (208 and 362 kg N/ha respectively) and high grain yields and protein levels were achieved in the absence of additional fertiliser.

Figure 2. Fertiliser recovery, indicating nitrogen use efficiency, from deep banding nitrogen alone in 2018 and 2020.

Where treatments received additional in-season nitrogen applications, fertiliser recovery declined with increased rates of deep banded nitrogen (figure 3). These results indicate that in 2020 top-dressed nitrogen had a higher fertiliser recovery than deep banded urea.

Figure 3. Fertiliser recovering, indicating nitrogen use efficiency, where deep banded treatments were combined with a top-dressing application in-season in 2020.

Conclusion

Results so far do not consistently support an improvement in nitrogen use efficiency using this technique. However, the practice of deep banding nitrogen at sowing could still have a place in the HRZ, to be used as an additional management tool. The technique is not designed to replace in-season applications, but potentially used to mitigate risks of sub-optimal timings of in-season fertiliser applications. Economics would need to be applied on a farm or paddock basis to determine if the technique is a viable option to consider, particularly in areas known to have trafficability and practical limitations in-season.

Acknowledgements

This research has been made possible through funding from the National Landcare Program as a part of the Smart Farms Small Grants program and follows on from work completed by Southern Farming Systems from 2018-2019 through a grant from DELWP as a part of the Virtual Centre for Climate Change Innovations. The author would like to thank them for their continued support.

Useful resources

Fertiliser toxicity Fact Sheet

References

Norton R, Desbiolles J (2011) Fertiliser toxicity fact sheet. GRDC Fact Sheets.

Rochette P, Angers D.A, Chantigny M.H, Gasser M-O, MacDonald J.D, Pelster D.E, Bertrand N (2013) ‘Ammonia volatalization and nitrogen retention: How deep to incorporate urea?’ Journal of Environmental Quality 42(6), 1635-42.

Wetselaar R, Passioura JB, Singh BR (1972) ‘Consequences of banding nitrogen fertilizers in soil’. Plant and Soil 36, 159-175.

Contact details

Ashley Amourgis
Southern Farming Systems
23 High Street, Inverleigh VIC 3321
0439 005 071
aamourgis@sfs.org.au
@aamourgis_sfs