There are a few key points to consider before applying nitrogen (N) fertiliser:
- Nitrogen is the nutrient removed in the largest quantities in grain at harvest.
- Pulses in the system can provide some free N.
- Calculate a few target yields using one of the many tools available that vary yield with seasonal outlook.
- Consider doing year-to-year nutrient budget to track N removal.
- Do N test strips at sowing with high N and zero N rates.
Soil testing before sowing is beneficial to determine starting levels of N and other nutrients.They are especially useful after ‘unusual’ conditions such as dry fallows or following legume rotations. Also remember that interactions may occur with the other elements such as phosphorus (P) in shallow and deep applications.
The amount of N fertiliser needed depends on the crop’s likely demand for N (based on target yield and protein levels) and the available soil N (as indicated by initial soil tests). There are many budgeting calculators available through agronomists or re-sellers.
A fertiliser calculator called NitrogenARM incorporates potential climate and gross margin effects into calculations. It uses soil N supply (calculated from soil test results and likely mineralisation before sowing) and target yield to estimate N fertiliser requirements. Users can compare various target yields and associated gross margins for four-season types and two N fertiliser regimes.
Soil organic matter is an important source of mineral nitrogen. But the quantity of soil N has declined over time with cropping.
The most commonly used indicator of inherent soil fertility is the organic carbon (OC) percentage. Before farming commenced, some soils may have had 2 to 3 % OC. These levels have declined, but hopefully not below 1%.
The removal of 100 kg/ha of N through cropping can reduce soil OC by 0.1%. The decline in soil OC is inevitable unless the amount of N fertiliser applied balances the amount of N removed and other nutrients (such as P and S) are adequately supplied.
N can also be added to soil by including pulse crops in the cropping system. The N benefit from pulse crops grown the previous season can be calculated using the ‘rules of thumb’ described in the GRDC Communities – Crop Nutrition webinar Estimating nitrogen contribution from pulse crops.
Understanding nitrogen use efficiency
The amount of the applied N fertiliser that gets into the crop depends on:
- how much soil N was already present,
- whether it was a dry or normal finish to the growing season,
- if there was waterlogging, and
- how much stubble was incorporated into the soil.
- 40-70% will get into the plant (typically 50%)
- 20-50% will get into grain (typically 40%)
The rest is lost from the system (typically 15 – 20% from winter crops, 20 – 40% from summer crops) or left in the soil (0 – 20%, with the higher amount more likely in a dry season) The residual N can recycle for subsequent crops.