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Be effective and efficient with N

Efficiency and effectiveness are different dimensions of nutrient use efficiency (NUE). The most efficient way to use fertilisers is not to use any, but that is not effective, as production is likely to be nutrient limited. IPNI’s Rob Norton gave a rundown on NUE terms and details, and how they might inform nitrogen fertiliser decisions, at the GRDC updates earlier this year.

Two aspects of NUE suitable for paddock and farm review are:

  • – Partial Factor Productivity (PFP) helps understand the grain yield benefit from the fertiliser – the effectiveness, and
  • – Partial Nutrient Balance (PNB) tells the source of the nutrients removed in the crop – the efficiency.

PFP and PNB must be considered together, and both need to show moderate values. A high PFP means you’d have done better applying the fertiliser at a higher rate. A very low PFP means excess fertiliser was used. A very high PNB (>100%) tells that soil reserves are being depleted, while a very low PNB indicates the nutrient applied is ending up somewhere other than the crop, such as being fixed in the soil or lost to the environment.

The 4 Rs for Nitrogen

    • Right time – Early N is generally used more efficiently, but if all application of N is up-front, the decision on rate can only be adjusted up, not down. Unless springs are good to very good, there will be little benefit from N applied later than booting. Depending on the season late N will mostly give a protein response.
    • Right rate – the actual yield response depends on the N rate meeting the gap between the target demand and expected supply (neither of which we know in advance). Analysis of prior PFP and PNB can help guide, alongside soil testing and agronomic modelling.
    • Right place – Foliar applied N has been proposed as the most efficient method to present N, but crops’ susceptibility to damage from urea toxicity and  leaf capacity to take up N dictates the upper level for effective N uptake at around 10-15 kg N/ha.
    • Right source – rate, timing and placement all interact so that efficiency options vary and no single source is a “silver bullet” to all situations. While there are logistical benefits to fluid N, top dressed granular urea is usually the cheapest source, so there would need to be compelling circumstances to justify moving to other sources.

Acknowledgements

Prepared by Stephanie Alt, NSW DPI with technical input from Robert Norton, International Plant Institute.

Image courtesy of the GRDC.

 

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