Nutrient performance indicators (NPIs) are calculations you can do to assess how effective and efficient fertiliser applications have been. Underusing fertiliser limits crop yield and can draw down soil reserves. Unless building soil reserves, overuse is a waste of money, as excess nutrients can be lost and may cause environmental damage.
Two important Nutrient Performance Indicators
Partial Factor Productivity (PFP)
PFP measures how responsive the cropping system is to the nutrient input. This NPI calculates the ratio of grain produced to units of nutrient applied. This helps understand if the return on investment for the fertiliser cost was worthwhile.
PFP = grain produced (kg) / nutrient applied (kg)
Typical values for cereals, when soil nutrients are in recommended ranges, are:
- N 40–80
- P 100–250
- K 75–200
Lower values suggest over application of nutrients or less responsive soils.
Partial Nutrient Balance (PNB)
This indicator measures how much nutrient is being taken out of the system relative to how much is being applied. This PNB calculates the ratio of nutrient removed to nutrient added. PNB tells you if more nutrient is being removed than applied.
PNB = kg nutrient removed/ha / kg nutrient applied/ha
Values >1.0 indicate nutrient mining from the soil. PNB only indicates nutrients lost from harvest. Nutrient losses via other pathways are not included. An IPNI Crop Nutrient Removal Calculator can supply estimates for nutrient removal for different crops and straws.
Nutrient Performance Indicators are not report cards
Nutrient performance indicators are useful gauges rather than strict benchmarks:
- A low PNB might be appropriate if the soil requires nutrient pool build-up. Monitor for nutrient losses to the environment.
- A high PNB where residual fertility is low indicates soil degradation. A high PNB might be appropriate if available nutrient pools and residual fertility is high.
Rainfall and harvest strategies affect NPI values. It makes a difference if crops are cut for hay instead of harvesting grain. For example, a wheat crop that would have yielded 1t/ha grain will remove 2-3 times more N and up to 10 times more K if cut for hay.