Optimising nitrogen decisions in waterlogged cropping systems

In Western Australia’s high rainfall zones, waterlogging is a frequent occurrence and an ongoing constraint to crop production. In the Albany and Esperance port zones, nitrogen management decisions are often influenced by the timing, duration and severity of waterlogging events. This project investigated nitrogen application strategies under waterlogged conditions to improve understanding of yield, grain quality and economic responses, and to inform nitrogen decision-making on affected soils.

This project funded by Grains Research and Development Corporation (GRDC) was conducted over two growing seasons across multiple sites to capture seasonal and site variability. Rather than aiming to define a single prescriptive nitrogen strategy, the trials evaluated how different nitrogen timing and rate strategies performed under contrasting waterlogging and rainfall conditions.

Background:

Previous research in Western Australia’s high rainfall zone has demonstrated that tactical nitrogen application, including split applications aligned with seasonal conditions, can result in higher yields compared with single applications at seeding or fixed growth stages. Waterlogging can affect root function, nutrient uptake and tiller development, altering crop nitrogen demand during the season. As a result, nitrogen requirements may vary depending on soil conditions and rainfall patterns.

Growers have identified a need for clearer guidance on nitrogen management in waterlogged paddocks, particularly where prolonged wet conditions delay or disrupt standard fertiliser programs. This project aimed to assess whether practical decision rules could be developed, or whether nitrogen responses remain highly dependent on site and seasonal conditions.

What was done:

Farm-scale trials were established in 2018 at Muradup and Frankland, with a further trial conducted at Frankland in 2019 following limited waterlogging at some sites in the first season. Nitrogen strategies included nil nitrogen, grower practice, single applications following waterlogging events, and split applications designed to align nitrogen supply with rainfall-limited yield potential and crop growth stage.

Measurements included rainfall, waterlogging duration, crop biomass, NDVI, grain yield, grain protein and a basic economic assessment of nitrogen inputs relative to returns. Trials were conducted in commercial paddocks to reflect typical farming conditions.

Observations:

At Muradup in 2018, where waterlogging occurred across part of the site, split nitrogen strategies resulted in higher grain yields than nil or single-application treatments, with an average increase of approximately 1 t/ha. In this instance, split applications at lower individual rates performed better than higher single applications, indicating that timing influenced crop response.

At Frankland in 2018, waterlogging was relatively mild and pre-season soil nitrogen levels were high. Under these conditions, grain yield differences between nitrogen strategies were not statistically significant. Higher nitrogen rates increased grain protein, but did not consistently improve yield, suggesting nitrogen supply was not a primary limitation at this site.

Results from the 2019 Frankland trial showed that all nitrogen treatments produced higher yields than the control. However, additional nitrogen applied later in the season did not increase yield once crops had reached their water-limited yield potential. While later nitrogen applications improved grain protein and grade, higher fertiliser costs reduced economic returns, and grower practice was the most profitable strategy in that season.

Implications for nitrogen management:

Across the sites and seasons evaluated, crop response to nitrogen was strongly influenced by soil nitrogen status, rainfall distribution, waterlogging severity and seasonal finish. Split nitrogen applications were most effective where waterlogging constrained early crop growth and where rainfall supported recovery. In seasons with limited waterlogging or adequate soil nitrogen, additional or later nitrogen applications provided limited yield benefit.

Later nitrogen applications consistently increased grain protein, but yield responses were uncommon once crops had reached their water-limited potential. In seasons with high grain prices, improvements in grade did not always offset the additional cost of fertiliser.

Key Learnings:

Nitrogen management in waterlogged systems requires consideration of seasonal conditions rather than reliance on fixed application strategies. Early nitrogen following rainfall was important for yield, while split applications provided benefits under specific conditions but did not guarantee improved profitability. Prolonged or severe waterlogging reduced the likelihood of a positive response to additional nitrogen.

The project highlighted the need for continued evaluation of nitrogen strategies under wetter seasonal conditions. Further work in seasons with more persistent waterlogging will improve confidence in nitrogen decision-making and refine guidance for growers in high rainfall environments.