Many Australian grain paddocks have a nitrogen (N) deficit. Analysis by the IPNI of 2016 data collected from grain growers across the southern region found few growers were in N surplus. On average, at a paddock level 1.14 kg N is removed for each kilogram of N fertiliser applied. These calculations accounted for fixed N from legumes, losses in product removal and stubble burning. N deficits compound ongoing declines in soil N reserves and soil organic matter.
Farmers are using increasing amounts of N fertiliser. Since 2010 N use has increased by around 50%. Average grain yields have increased by 30%, but grain export is not the only way N leaves the paddock.
Rundown of soil nitrogen
The long term store of N in the soil is in Soil Organic Matter (SOM). The N in harvested grain not sourced from fertiliser mostly comes from decomposing SOM. Stable SOM has a half-life of 30 years. Over the last century, with each generation of Australian farmers, half the SOM has been lost. We continue to use up SOM reserves faster than we replenish them. Including legume rotations in cropping slows the decline of SOM and soil N reserves.
Crops grown in low SOM paddocks have a greater reliance on fertiliser N. The crop’s N supply is less buffered by mineralisation, so timely N applications become even more important.
Using NPIs on the farm
Agronomists and growers can use the same Nutrient Performance Indicators (NPIs) that IPNI uses to look for regional or national trends in nutrient management. NPIs are calculations that use crop inputs and yield data. Using NPIs at the paddock scale helps you gauge how effective N applications have been, and understand paddock fertility trends.
The average NPI values from the 2016 IPNI analysis provide some insight into industry wide trends. However, the underpinning data set is relatively small, and data comes only from farmers with good farm records.
For individual paddocks, interpret NPI values alongside soil fertility indicators. Developing strategies for improving nutrient performance relies on understanding paddock conditions.
Partial Factor Productivity-Nitrogen (PFP-N)
PFP-N describes the grain yield benefit from fertiliser. It helps understand the effectiveness of N applications. The average PFP-N in cereal crops is 52 in Australia, 43 for the world. Southern Australian PFP-N values range from 46–105. On average, the farms assessed produced 77 kg grain for each kilogram of fertiliser-N they applied. If your PFP-N is outside the normal ranges consider:
- a high PFP-N means the crop yield responded strongly to N fertiliser. There might have been room to increase the fertiliser rate?
- a very low PFP-N means excess fertilizer was applied, or the crop was limited by other yield constraints.
Partial Nutrient Balance-Nitrogen (PNB-N)
PNB-N indicates whether a crop left the paddock with potentially higher, lower or stable N reserves. Values above 1 indicate soil N is running down. Look at PNB-N as a trend over years.
The goal is for average PNB-N to approach 0.9–1, so you can avoid running down soil fertility. Southern Australian PNB-N values ranged from 1.20–2.09. High PNB-N over multiple years indicates N rundown of the soil.