Joel Williams says this season presents some wonderful opportunities to test and trial some new approaches towards optimising N inputs – valuable lessons can be learnt for future seasons ahead.
With fertiliser prices the way they are, I’m sure everyone is already thinking it is going to be an eyebrow raising season ahead. This year will likely bring out some of the best and worst examples of a soils potential to function with reduced inputs. Reducing inputs is typically a long term game, best adopted slowly, in stages, and, by combining multiple strategies into an integrated approach. There sure is no silver bullet or one size fits all. Don’t get me wrong, I know some have had great success with a cold turkey approach too, so it’s not impossible, but certainly a tad riskier. On the other side of the coin, this season presents some wonderful opportunities to test and trial some new approaches towards optimising N inputs – valuable lessons can be learnt for future seasons ahead. If you haven’t started thinking about a pathway toward N reductions, this year will be a great year to take your first steps.
By the time of print, this growing season will be very much upon us so for the purpose of this article, I just wanted to focus in on one of the key strategies that can be used in the more immediate short term – foliar N applications. It may be a shorter term measure, but foliar applications can be a valuable tool as part of a broader, longer term strategy to improve input efficiencies1,2 and transition from input dependency. There are many pieces of the puzzle that influence the success of a foliar N application (such as the formulation, application, crop and environmental conditions) but for this article I thought we could explore some of the nuance surrounding different forms of N.
I’m sure it will be no secret that my weapon of choice for foliar N is urea, but that said, I am not opposed to Urea-Ammonium-Sulphate (UAS), Ammonium sulphate (AS) or UreaAmmonium-Nitrate (UAN) – in that order of preference. Urea should really be the centrepiece of a foliar N approach with the other N-based inputs rounding out the options to mix in as auxiliary sources. This is because urea has multiple benefits over the other sources of inorganic-N3,4 including:
• Higher analysis so saves on transport and application costs per unit of N.
• There is a C atom embedded within the urea molecule which can be used for photosynthesis.
• It has a neutral charge so it passes through the foliage faster than ammonium or nitrate.
There are numerous studies that support the benefits of providing multiple sources of N for plant metabolism – it seems plants generally do better when supplied a mix of N sources5,6. The reason a diversity of N sources is desirable is that it has been shown to induce unique gene expression that ultimately leads to improved N metabolism overall3. Although nitrate is the least efficient form of N (as it drains the most metabolic energy to be converted into protein), I am not opposed to including small amounts with urea (for example, a Urea/UAN or Urea/AN combination). However, I would still lean more toward UAS or AS as auxiliary sources of N, as typically there is sufficient nitrate being cycled and supplied from soil organic matter (especially in more alkaline and higher organic matter soils) while the additional S from UAS/AS supports protein synthesis.Â
Similarly to this, it is additionally beneficial to include a mixture of organic and inorganic-N sources as well. Again, this relates to upregulating the genes that are involved with N metabolism and protein synthesis. A simple way to explain this would be that inorganic-N sources prime the genes involved in the early stages of N metabolism, while organic-N sources prime the genes involved in the latter stages. Therefore, by providing both N sources simultaneously, you are activating both the early and latter stage genes, which ultimately supports more efficient shuttling of inorganic-N sources along the full metabolic pathway into more complex proteins7–9. Organic-N options here include protein hydrolysates (such as fish or seed meal hydrolysates) as well as amino acid formulations.
My preference leans toward the use of protein hydrolysates as these inputs contain a combination of different organic-N sources (amino acids, peptides and proteins) and again, this diversity of organic-N sources is more beneficial than pure amino acids – especially for encouraging more root development10. Beyond this, there is a handful of very interesting papers that have been published just recently all highlighting the importance of organic-N forms in the soil and calling for a greater research agenda to explore their role in soil carbon dynamics and plant nutrition7,9,11,12. Perhaps this is an article topic for another time, but the take home message would be that application of composts, manures and legume companions/residues – which all provide organic-N – should be encouraged as much as possible within our production systems.
Let’s have a quick look at some practical examples of foliar recipes that could be used. I haven’t discussed the specifics in this particular article but like always, I would be including a C source (such as fulvic acid, molasses etc) and acidifying the spray mix with citric acid. Additionally, it’s typically a good idea to include a small dose of a multi-trace element package, as many of the traces also support N metabolism and improve nitrogen use efficiencies. So starting with the base ingredient of urea (at around 10-20 kg/ha), I would consider the follow additional synergists:
In summary, the take home message is that nitrogen is not an island and does not operate in isolation. Multiple forms of N supplied simultaneously can bring benefits to overall N metabolism and protein synthesis. On top of this, other nutrients (such as S, Fe, Mo, Ni, Mn, Mg) are also important as well as general synergists for spray tank/foliar performance. We did not discuss the latter two considerations in this article but readers will find information on this online. Taken together, a multi-ingredient tank mix of these various N forms and N synergists can support N utilisation and improve nitrogen use efficiencies – this will be an important factor to optimise in a season when fertiliser prices are as high as they are.Â
References
1. Comparing soil vs. Foliar nitrogen supply of the whole fertilizer dose in common wheat. (2021). doi.org/10.3390/agronomy11112138
2. Urea foliar application as a partial substitution of soil-applied nitrogen fertilization for some maize cultivars grown in newly cultivated soil. (2011). www.curresweb.com/mejar/mejar/2014/378-382.pdf
3. Stabilising amine urea in nitrogen fertiliser increases leaf chlorophyll content, tiller base diameter and root length of wheat plants. (2020). www.cabdirect.org/cabdirect/abstract/20210040856
4. Foliar urea fertilization of cereals: A review. (1992). doi.org/10.1007/ BF01048783
5. Plant Signaling & Behavior Molecular and physiological interactions of urea and nitrate uptake in plants. (2016). doi.org/10.1080/15592324 .2015.1076603.
6. Modulating tiller formation in cereal crops by the signalling function of fertilizer nitrogen forms. (2020). doi.org/10.1038/s41598-020- 77467-3
7. Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition. (2021). doi.org/10.1007/s11104-021- 04860-w
8. The carbon bonus of organic nitrogen enhances nitrogen use efficiency of plants. (2017). doi.org/10.1111/pce.12772
9. How do terrestrial plants access high molecular mass organic nitrogen, and why does it matter for soil organic matter stabilization? (2021). doi. org/10.1007/s11104-021-05022-8
10. Growth stimulatory effects and genome-wide transcriptional changes produced by protein hydrolysates in maize seedlings. (2017). doi.org/ 10.3389/fpls.2017.00433
11. Nitrogen Use Efficiency Definitions of Today and Tomorrow. (2021). doi.org/10.3389/fpls.2021.637108
12. A holistic framework integrating plant-microbe-mineral regulation of soil bioavailable nitrogen. (2021) doi.org/10.1007/s10533-021- 00793-9