The Nitrogen Nutrition Index (NNI) is one way to help gauge in-season crop status. The NNI is the ratio of the amount of N in the crop to the critical N concentration (Nc) of the plant tops. By comparing crop N (from lab tests) to a dilution curve with Nc you can see if N is deficient, and if so, by how much. An NNI of 1 means the crop has an optimal N level. NNI <1 indicates N is deficient, while NNI > 1 means luxury levels.
In regions where late water supply is more reliable, the NNI is a simple way to gauge N status at different crop growth stages. It indicates if the fertiliser regime needs adjusting. But, the NNI needs adjustment for Australia’s dry climate. The problem is with the critical N concentration which varies with time and crop growth.
The critical nitrogen level
Nc reduces as biomass increases. This is captured by a N dilution curve.
Figure 1. Wheat N dilution curve capturing current varieties and environments in SA. Source: Victor Sadras, SARDI
Most N dilution curves come from temperate climates where yield and growth (biomass) are well correlated. Using these curves in dry areas over-estimates N deficiency. Plus – Nc is more variable in dry climates, so the curves do not translate very well.
An adapted NNI
Researchers in WA and SA have adapted the NNI for dry conditions. First, they created rainfall-scaled N dilution curves. They also found that Nc is less variable at stem elongation, even under varying levels of moisture stress.
Talk to your laboratory or agronomist about using the updated NNI (or any other method that is based on rainfall-scaled dilution curves). Sample whole tops from mid-tillering to stem elongation for nutrient tissue testing.
While NNI is a useful guide to N status during crop growth, it is only relevant at the time of testing. No tests can take into account future rainfall (or lack thereof), which will affect yield and drive N response.
This question was asked at the ‘Ask and Expert’ Panel in Adelaide, February 2019.