Take home messages

• Waterlogging reduces crop canopy cover, and affected areas should be reassessed as the lower yield potential is likely to reduce the nitrogen requirements.
• Yield potential of canola affected by waterlogging can be restored up to the flowering stage by surface application of urea if the soil is not flooded.
• Recently-applied urea-N remains relatively safe from denitrification under waterlogged conditions as most is in the ammonium form.
• After harvest, previously waterlogged areas will need to be assessed for salinity and low carryover nitrogen to proactively manage potential problems in the 2024 crop.

Background

In June 2023, the south-east of South Australia, including the Keith area, received record monthly rainfall, resulting in waterlogging in many crops and flooding of low-lying areas of paddocks. This paper draws together some illustrative experimental results from GRDC-supported studies in southern Victoria where waterlogging occurs more regularly to help guide growers and their advisors to minimise financial losses from waterlogging in 2023 and prepare for the following crop in 2024.

What happens during waterlogging?

Soil

High water tables cause the soil to become anaerobic as oxygen is consumed by microbes and plant roots. A study at Hamilton in 2017 showed that reduction-oxidation state at the 20cm depth went into the anaerobic range within a few days of a water table developing at that depth. In contrast, at 57cm, it took about two months before becoming anaerobic (Figure 1), reflecting the lower biological activity at 57cm.

Plant

Roots cannot use oxygen for respiration from the anaerobic soil layers. Uptake of plant nutrients ceases from these layers because this process requires energy. Some plants develop air-conducting tissue in their roots known as aerenchyma, which enables oxygen from the plant tops to support root respiration. Plants that develop aerenchyma include rice, wheat and faba beans, but not canola. Only the adventitious (nodal) roots of cereals develop aerenchyma, and the plants are particularly vulnerable to waterlogging as seedlings are dependent on their seminal (seed) roots. The aerenchyma of wheat only extends to about 10cm of soil depth, so the plant is entirely dependent on nutrient supplied from this shallow topsoil layer.

Nitrogen

Nitrogen in the nitrate form is at risk of loss through denitrification once a soil layer becomes anaerobic. However, under anaerobic conditions, N recently supplied as urea will mostly be in the ammonium form as there is insufficient oxygen to convert it to nitrate. It is therefore protected from denitrification, especially at low soil temperatures, and also, partially protected from leaching into deeper soil layers because the ammonium ion is adsorbed onto clays.

Figure 1. Redox and depth to the perched water table in a wheat crop at Hamilton in 2017. S = start of stem elongation, B = booting, A = anthesis, M = maturity. Water level ranged from near the surface (top) to 60cm below the surface (bottom).

Table 1: Wheat yield, protein, and ear density, and recovery of 15N on a poorly drained and well-drained area of a paddock at Glenthompson in 2017.

What can we learn about agronomic management from some recent experiments

Case study 1. Wheat tiller density was reduced so N requirements should be reassessed

In 2017, we monitored a well-drained and a poorly drained area of a paddock of wheat (cv LongReach Beaufort) in the Glenthompson area. Urea was top-dressed on 17 June (70kg/ha) and 12 September (120kg/ha), including small areas labelled with 15N to trace the fate of applied urea. Water tables were close to the surface from mid-July until late September in both areas of the paddock but were located deeper and drained more quickly in the well-drained site. This coincided with the period from tillering and the first half of the stem elongation phase.

Grain yield in the poorly drained area was 38% lower than in the well-drained area (Table 1). This was attributable to a 35% lower ear density because of waterlogging during the tillering and stem elongation phases period that had limited the number of reproductive tillers. Although urea was applied at the start of stem elongation, subsequent crop growth was unable to compensate for the low density of reproductive tillers. Only 16% of applied 15N was found in the plant at harvest compared with 33% in the well-drained area. For a crop where the density of reproductive tillers has been stunted by waterlogging, further N application will represent a poor return on investment. Most of the added 15N (55%) was not detected in either the plant or soil and is presumed to have been lost to denitrification.

Case study 2. Wheat tolerates flooding surprisingly well

Wheat (cv LongReach Beaufort) was sown in an experiment adjacent to the Bool Lagoon wetland in 2017. There was shallow flooding of the site resulting from backlogging of water from a natural wetland from late June until mid-November, apart from a brief period in late July when N fertiliser was applied as urea or sulfate of ammonia (SOA) (McCaskill et al. 2020). The highest-yielding treatment achieved a grain yield of 2.8t/ha, and there was no substantial yield response to in-crop nitrogen. There was a trend (P = 0.06) for lower yields where SOA had been applied, which was attributed to hydrogen sulfide forming under the flooded conditions. In an environment where wheat yields regularly achieve yields of 5–7t/ha in the absence of flooding, wheat was surprisingly tolerant of the extended period of shallow flooding.

Case study 3. Nitrification inhibitors unnecessary for urea applications from tillering onwards

A series of experiments near Hamilton in 2014 measured 15N recovery in wheat with and without a nitrification inhibitor (marketed as ENTEC®). ENTEC®-urea had an advantage over untreated urea with early application (GS 11), but not when applied during tillering (GS 25) (Figure 2) (Harris et al. 2016). In the early application, the applied nitrogen was stored in the soil for a two-month period and was at risk of losses to denitrification. By contrast, the tillering application was just ahead of the high plant demand, thereby minimising the period the nitrogen was stored in the soil. Early application of ENTEC-urea could be an option in paddocks where there are risks of poor trafficability in July and August, but in paddocks that can be spread reliably in late winter, high fertiliser recoveries should be achievable by spreading urea without ENTEC just ahead of plant demand.

Figure 2. Recovery of 15N-urea in soil and plant for urea and ENTEC®-urea applied to wheat at the 1-leaf stage (GS11) in mid-May compared to mid-tillering (GS25) in mid-July near Hamilton in 2014. Error bars show the 5% lsd. Source: Harris et al. (2016).

Case study 4. Waterlogged canola can yield well if nutrition is provided in the aerated surface layers

In 2017, we conducted an N x potassium (K) experiment on canola on raised beds at Hamilton (McCaskill et al. 2020). The season was typically wet with surface water in the furrows most of the time between the first application of urea on 8 August (GS1.05) and the second application on 16 September (GS3.5), but at no time did flooding exceed the height of the beds. We were not expecting a strong response to nitrogen because pre-sowing mineral N sampling indicated a store of 238kg/ha of nitrate-N in the top 60cm. However, there was a 6-fold increase in canola yield from 1t/ha with no application to 6t/ha with surface-applied urea (Figure 3). There was also a yield response to K, despite soil testing indicating adequate K for canola. We concluded that the crop was reliant on a shallow layer (top 5–10cm) of aerated soil for its entire nutrition during this period, and that deeper soil nitrate and K was inaccessible to the canola roots. However, where sufficient nutrition was supplied to the upper soil layers, high yields could be achieved.

Figure 3. Response of canola to top-dressed urea and potash (K) at Hamilton in 2016 on a site with 238kg/ha of nitrate-N in the top 60cm, and 160mg/kg of Colwell-extractable K in the top 10cm. Error bars indicate the 5% lsd. Source: McCaskill et al. (2020).

Case study 5. Faba beans are less affected by waterlogging at flowering than wheat

The impact of waterlogging on yield depends on crop and growth stage. An experiment conducted with wheat (cv LongReach Trojan) and faba beans (cv Bendoc) in Hamilton in 2021 compared tolerance to waterlogging occurring in the September–October period. Waterlogging was imposed through frequent irrigation for a short (17 days) or a long period (30 days) during flowering. Although waterlogging significantly reduced the biomass of both crops (Figure 4), we only measured significant yield differences in wheat (38%), but not faba bean (Table 2). The higher tolerance of faba beans to waterlogging at this stage of growth was attributed to its strong rhizobial association providing a nitrogen supply independent of the waterlogged soil.

Table 2: Grain yield of wheat and canola crops exposed to short and long periods of waterlogging during flowering. Differences significant at the 5% level are denoted by different letters.

Figure 4. Above ground biomass accumulation in wheat and faba bean crops, showing the period when irrigation was imposed to create short (17 days) and long (30 days) durations of waterlogging. Bars show the 5% lsd and asterisks when differences were significant.

Case study 6. Barley varieties differ in their waterlogging tolerance

In 2022, an experiment at Hamilton compared the waterlogging tolerance of several barley varieties. Waterlogging was imposed by frequent irrigation between mid-June and early August, after which some natural waterlogging occurred in both waterlogged and control areas. The highest-yielding variety under waterlogged conditions was Macquarie, a brewing variety bred in Tasmania, followed by Yerong, a feed variety (Figure 5). The most widely accepted malting variety RGT Planet performed poorly, whereas a variant bred as part of a GRDC project for improved waterlogging tolerance (P-52) had slightly higher yields. The lowest yield was from a breeding line PNA1, which has had no selection for waterlogging tolerance. Given the wide range in waterlogging tolerance in barley, an option for managing low areas of paddocks at the greatest risk of waterlogging is to plant more tolerant cereals such as Macquarie or Yerong barley, or a waterlogging-tolerant wheat such as LongReach Beaufort.

Figure 5. Grain yield of five lines of barley at Hamilton in 2022, where waterlogging was imposed through irrigation between mid-June and early August.

Managing carryover effects on the next year’s crop

The waterlogged areas in 2023 face two risks to production in 2024, salinity and low nitrogen, both of which may require separate soil testing. Salinity is caused by salts already in the soil rising through capillary movement to the surface, where the water is evaporated leaving salts on the surface. There is a risk that salts in the seedbed could limit germination of the 2024 crop. Since the ‘enemy’ is soil evaporation, the more water that can be used by plants through transpiration, the more salts stay in deeper layers where it has less effect on germinating crop seeds next year. Weeds in the winter crop will have a beneficial role in reducing salinity (until a knockdown is sprayed to prevent seed set), while any self-sown summer crop will also reduce soil evaporation. Affected areas should not be dry-sown and instead sown after the first rains of autumn 2024 flush the salts below the surface soil layer.

Waterlogged areas are also at risk of greater denitrification in spring 2023, resulting in potentially less carryover of soil nitrogen into 2024 (for example, McCaskill et al. 2021). Separate soil testing of such areas prior to sowing could be warranted, and if tests are lower than areas that were not waterlogged, variable application or a separate pass of the spreader along drainage lines may be necessary to reduce spatial variation of nitrogen in the paddock.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, and the authors would like to thank them for their continued support. Findings are drawn from a series of joint investment projects between GRDC and Agriculture Victoria Research, including DAV00125 and Victorian bilateral projects DAV00141, DAV00151 and VGIP2C. We thank Amanda Pearce of SARDI for conducting the experiment in Case Study 2.

References

Harris RH, Armstrong RD, Wallace AJ, Belyaeva ON (2016) Delaying nitrogen fertiliser application improves wheat ¹⁵N recovery from high rainfall cropping soils in south eastern Australia. Nutrient Cycling in Agroecosystems 106(1), 113-128.

McCaskill M, Dunsford K, Akpa S, Sheffield K, Crawford D, Norng S, Armstrong R (2021) Nitrogen dynamics in high-yielding wheat and canola crops. Proceedings of the 2021 Agronomy Australia Conference, 17–21 October, Toowoomba, Australia.

McCaskill MR, Riffkin P, Pearce A, Christy B, Norton R, Speirs A, Clough A, Midwood J, Merry A, Suraweera D, Partington D (2020) Soil-test critical values for wheat (Triticum aestivum) and canola (Brassica napus) in the high-rainfall cropping zone of southern Australia. Crop and Pasture Science 71 (12), 959-975.

Contact details

Malcolm McCaskill
Agriculture Victoria Research
915 Mt Napier Rd, Hamilton VIC 3300
0407 850 671
malcolm.mccaskill@agriculture.vic.gov.au