Nutrient performance indicators (NPIs) assess how effective and efficient fertiliser applications have been. Last week we talked about using Partial Factor Productivity (PFP) and Partial Nutrient Balance (PNB) together. Those NPIs give you a good idea if your fertiliser investment was worthwhile, and if your paddock nutrient status is stable or going backwards.
Next you’ll want to know how big the effects on yield and nutrient balance are.
How important was the productivity gain?
If your PFP was low you can investigate further how beneficial the nutrient input was. Agronomic Efficiency (AE) measures how much productivity was gained by the use of this nutrient input. Using this NPI requires some field work, a ‘control’ area in the paddock that did not receive the input, but was otherwise treated the same.
AE = yield increase (yield with input t/ha – yield from control t/ha)
nutrient applied (kg/ha)
Well managed systems have AE values >25. Low AE values suggest the input was not very important in addressing a yield constraint. High AE values indicate a large effect on yield.
How quickly is nutrient status changing?
If you had a high PNB you’ll want to know how serious an issue it is. Nutrient Balance Intensity (NBI) is the NPI that tells you how much the nutrient balance is trending up or down for the paddock.
NBI = nutrient removed (kg/ha) – nutrient applied (kg/ha)
More accumulation is occurring in the paddock with values well below zero. Positive values could indicate soil fertility is declining. NBI >0.2 suggests nutrient status could be running down rapidly.
The best case scenario for nutrient performance
If NPIs are all coming up in the target ranges it tells you that your fertiliser use was:
- Effective – the crop responded to the nutrient input
- Efficient – the nutrient status of the paddock is stable, or building slowly
- Profitable – the yield benefit from the fertiliser input covered its cost.
The best set of numbers from NPIs would be to have:
- PFP in the target range (N 40–80, P 100–250 and K 75–200)
- PNB close to 0.9
- AE over 25
- NBI close to zero, indicating stability.
This isn’t going to happen every season in even the best-managed paddocks. The trend over many years is what’s important.