Rules of thumb based on superseded agricultural systems can overestimate nitrogen use efficiency in southern cropping systems.

Key points

• Traditional rules of thumb estimate crop uptake of nitrogen fertiliser at 50 per cent
• Agriculture Victoria research suggests 34 per cent may be more appropriate for the southern region
• Accurate soil nitrogen measurements to depth will also improve nitrogen budgeting

Commonly used rules of thumb, which advisers and growers use to make fertiliser decisions, are based on a nitrogen fertiliser uptake of about 50 per cent of what is applied. Now Victorian research undertaken in growers’ paddocks puts that figure at just 34 per cent.

These rules are based on a measure of starter nitrogen from soil testing, an estimate of in-crop mineralisation and the crop utilisation figure of 50 per cent. They do not account for nitrogen losses.

Despite many advisers and growers relying on these rules of thumb and nitrogen budgets, a GRDC 2015 survey revealed most were concerned that major changes in soil fertility and cropping systems had compromised the rules’ ability to estimate fertiliser needs. Minor adjustments based on local experience and pre-sowing soil nitrogen testing were unlikely to address these misgivings.

Likewise, existing decision-support tools are based on data developed in an era when pasture legume-leys dominated cropping systems or on acid soils in southern New South Wales. These are vastly different to the continuous cropping, conservation cropping practices and alkaline soils that dominate cropping in the southern region.

Measuring uptake

With this in mind, Agriculture Victoria research supported by the Australian Government examined uptake and losses of applied nitrogen across low, medium and high-rainfall cropping zones and irrigated cropping systems in Victoria.

To measure nitrogen fertiliser uptake, nitrogen-15-labelled urea was top-dressed ahead of rain events to cereal crops – predominantly wheat – in line with grower practices. Experiments supported by the Australian Government’s ‘Action on the Ground’ initiative targeted nine paddocks each year from 2014–16. After harvest, nitrogen-15 was measured in the straw, grain and soil. Missing nitrogen was attributed to unaccounted losses, believed to be mainly denitrification, but possibly also volatilisation and leaching.

The average crop recovery of nitrogen was surprisingly consistent at 34 to 35 per cent across the different regions and cropping systems (Table 1). However, there was a great deal of variability between individual situations depending on seasonal conditions, with water supply the single-most-important determinant of nitrogen use efficiency.

Table 1: Mean crop recovery and losses (range in brackets) of nitrogen-15 labelled  fertiliser nitrogen from the crop/soil system within the season of application using pooled data from nine sites (2014–16). Nitrogen was applied as top-dressed urea during the vegetative/early tillering stage.

Source: Dr Ashley Wallace

In seasons with low growing-season rainfall, crop uptake was reduced but losses also tended to be reduced. Conversely, in wetter seasons, the amount of nitrogen fertiliser taken up by the crop increased but so did losses.

While seasonal variability will always be a challenge when estimating nitrogen fertiliser requirements, utilising a figure of 34 per cent crop recovery, rather than 50 per cent, will improve the usefulness of the rules of thumb in southern cropping systems. Accurately measuring nitrogen in the soil profile at sowing is another essential component to nitrogen budgeting.

Unaccounted for losses

An assessment of losses, which also included data collected as part of the Australian Government’s ‘Filling the Research Gap’ investment, showed that the extent of losses varied by environment and soil type (Table 1).

Significant losses can occur in low and medium-rainfall and irrigated cropping systems, not just in the high-rainfall zone as previously thought. However, the ability to mitigate these losses is hampered by uncertainty as to fate of fertiliser nitrogen and the dominant loss pathways.

The new national GRDC investment, led by the University of Queensland, is targeting a better understanding of nitrogen cycling and losses across Australian cropping systems. It is a first step towards preventing these losses and supporting growers to better manage nitrogen fertiliser.

More information: Professor Roger Armstrong, 0417 500 449, roger.armstrong@agriculture.vic.gov.au, GRDC Update paper: Nitrogen fertiliser use efficiency ‘rules of thumb’ - how reliable are they?