Given the current situation with fertiliser supply, particularly nitrogen, wrapping your head around some basic nitrogen calculations before committing to rotations will be a useful exercise for all growers this year, not only due to the price but mainly due to availability. What exactly is nitrogen budgeting? Simply put, nitrogen budgeting is estimating yield potential, estimating crop nitrogen (N) demand, soil supply and calculating the difference!
Estimating Yield Potential – This early in the game there is too wide a range of yield possibilities to get an accurate estimate, so at this point using budgeted yield will be the simplest, with perhaps a slight adjustment for increased yield from current early seeding opportunities. Once the season progresses you can use your average water use efficiency (WUE) combined with effective growing season rainfall to date and expected effective growing season rainfall still to fall to calculate your rainfall yield potential WUE * (1/3 of Nov-March Rainfall + 2/3 of April to Sept rainfall). This is useful for predicting your likely average yields. Note that WUE does vary year to year and small changes will result in significant changes to yield predictions; think of the season to date for reasons why WUE might be above or below average. Once delving down to a paddock by paddock basis, start to think about variables that may give a better or worse yield in that area of land. Think soil type, organic carbon %, soil moisture, water holding capacity, summer rain, early break, frost risk, establishment, other nutrient supply, soil constraints, weed pressure, disease pressure, rotation including legume component and carryover stubble content etc. This can drive your decision on the yield you choose, once you have a yield in mind you can calculate crop N demand.
N demand is a function of yield potential and N required to grow 1t of grain. Generally, 40 units of N is required per tonne of wheat and barley and 80 for Canola (Laycock et al, 2022). Therefore, crop N demand = yield potential * N required per tonne of grain. That covers our demand side of things, the soil supply has a bit more to it and includes mineral N (inorganic N), in crop mineralisation (organic N, Stable Organic Nitrogen or SON) and rotational N (Residue Organic Nitrogen or RON) i.e. Soil N = Mineral N + In Crop Mineralisation + Rotational N.
Mineral N which is N available for plant uptake is the combination of Ammonium (NH4) and Nitrate (NO3) from your soil tests. This can be calculated simply by adding NH4 (mg/kg) and NO3 (mg/kg) together from your soil tests for each 10cm increment you have. If you only have topsoil tested it might be easier to assume no N below 10cm; however, it is most accurate to test as deep as plant roots normally go. Note that if your depth increments are larger than 10cm you will need to multiply your NH4 and N03 values by the increment/10cm. If you want to get into deeper detail, you can also use the formula below.
Mineral N (kg/ha) = (NO3 in mg/kg + NH4 in mg/kg) *bulk density (g/cm³) * depth increment (Hunt et al, 2024)
| Soil Type | Average Bulk Density (g/cm³) |
|---|---|
| Coarse Sand | 1.55 |
| Fine Sand | 1.3 |
| Light Sandy Clay Loam | 1.45 |
| Loam | 1.25 |
| Sandy Clay Loam | 1.45 |
| Clay Loam | 1.45 |
| Clay | 1.4 |
| Self-Mulching Clay | 1.25 |
To calculate in crop mineralisation, we simply take growing season rainfall and multiply it by organic carbon (OC) from the top 10cm soil test and 0.15 to make the following formula:
Crop Mineralisation = growing season rainfall * OC * 0.15 (Dunsford, 2015)
Research has found that this source of N can substantially reduce the need for fertiliser N inputs – often by up to 40 – 80 kg N/ha in WA (Peoples et al 2015). The following formulas are options if you want to delve deeper (note it is important to recognise the gravel component as in some soils this can artificially enhance OC% i.e. 2% OC with 50% gravel is likely to perform like 1% OC.
Note that soil test results usually used are paddock averages which means some sections of the paddocks will be more responsive to N and others less. Samples must be taken this season to be accurate.
Rotational N = Total fixed N (grain yield (kg) * ((0.0275/Harvest Index %) – 0.05) * 0.4
(White et al, 2008)
OR
Rotational N = Total fixed N (grain yield / Harvest Index %) * 25 (kg N /ha) – N removed by grain
(Armstrong et al, 2015)
The N removed by crop = yield * 61 (another useful rule of thumb is 3% of legume residue ie 4t equates to 120N. Remember approx. 50 units of N/t of grain removed with the lupin crop.
Harvest Index (HI) ranges from 20% to 40%, is the ratio of grain yield to dry matter. A normal HI would be 30%, whereas a bulky crop that did not pod well would be more like 20%, and short, well podded crop might be more like 40%.
All combined, this gives us a final formula of:
N required = Yield Potential * Crop N Requirement per tonne – (Mineral N + N Mineralisation – Rotational N)
Use the following as an example:
| Parameter | Value |
|---|---|
| Crop | Canola |
| Yield Potential | 2.3 |
| Soil Type | Loam |
| Crop N /t Required | 80 |
| NH4 (0–10 cm) | 10 |
| NO3 (0–10 cm) | 5 |
| NH4 (10–20 cm) | 2 |
| NO3 (10–20 cm) | 1 |
| Annual Rainfall | 270 |
| OC (0–10 cm) | 1.1 |
| Lupin Yield 2025 | 1.9 |
| Lupin HI | 30% |
N required = 2.3*80 – 1.25*(10+5+2+1) – (270*1.1*0.15) – 1900 *(((0.0275/0.3))-0.05) *0.4
=84.38 units of N required
=183kg/ha of urea
This is definitely not a simple process; there are a lot of variables at play that aren’t easily tangible, along with a host of factors impacting decisions. In the current state of play where nitrogen may need to be rationed my encouragement is to seek advice as we don’t want to hamstring the returns because of broadcast applications. Ultimately a lot of this comes down to gut feel and experience but hopefully now you should have the tools to make a basic N budget and set you on the right track to make more informed seeding decisions.
Reference:
Dunsford K, Armstrong R, Tang C, Sale P (2015) Estimating in-crop nitrogen mineralisation in dryland cropping systems of southern Australia. 17th Australian Agronomy Conference, ‘Building Productive, Diverse and Sustainable Landscapes’; Hobart: Australian Society of Agronomy; 4pp
Peoples M, Swan T, Goward L, Hunt J, Li G, Harris R, Ferrier D, Browne C, Craig S, Van Rees H, Mwendwa J, Pratt T, Turner F, Potter T, Glover A and Midwood J (2015). Legume effects on soil N dynamics. Comparison of crop responses to legume and fertiliser N. In: GRDC Grower Update 2015, Corowa and Finely, NSW.
Armstrong, E, Holding, D, Gov & Au 2015, Pulses Putting life into the farming system An insight from growers who are making pulses work, January.
Hunt, J 2024, Back to nitrogen basics – soil testing and nitrogen budgeting fundamentals – Grains Research and Development Corporation, Grains Research and Development Corporation.
Laycock, J 2022, Balancing risk and reward with high phosphorus and nitrogen input costs – Grains Research and Development Corporation, Grains Research and Development Corporation, viewed 8 April 2026, <https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2022/02/balancing-risk-and-reward-with-high-phosphorus-and-nitrogen-input-costs?utm_source=chatgpt.com>.
White, P, French, B & Mclarty, A 2008, Digital Library Digital Library Bulletins 4000 – Agriculture Producing lupins Producing lupins.
