Nitrogen budget for wheat: a step-by-step worked example

Everything below is GRDC’s worked example from James Hunt’s “Back to nitrogen basics” paper (February 2024), rewritten with the arithmetic shown. The paper describes it as a simplified starting point for southern Australian farming systems, for a hypothetical medium-rainfall zone wheat paddock in South Australia that is sown on time with no other limits. Your region, rainfall and soil will differ.

Step 1: estimate how much water the crop will use

The paper counts 25% of the rain that falls in the summer fallow (November to March) and all of the growing-season rain (April to October). In the example it is late July, so April–July rain is measured and August–October uses long-term medians.

Step 2: turn water use into an economic yield

• Water-limited potential yield for wheat = (320 − 45) × 25 = 6,875 kg/ha.
• Economic yield = 80% of that = 5,500 kg/ha, or 5.5 t/ha.
• The paper gives similar equations for barley, (water use − 50) × 24, and canola, (water use − 80) × 15. It notes the method rests on many assumptions but is robust enough for N management.

Step 3: crop nitrogen demand

5.5 t/ha × 40 kg N per t/ha = 220 kg N/ha of supply needed from soil plus fertiliser. For canola the paper uses about 80 kg N per t/ha.

Step 4: turn the soil test into kg N/ha

The lab reports nitrate and ammonium in mg/kg for each depth. Mineral N (kg/ha) = (nitrate + ammonium) × bulk density × depth in decimetres. The example assumes a loam with a bulk density of 1.3.

Step 5: subtract supply, then convert to urea

• Supply = 69 kg N/ha in the soil + 8 kg N/ha already applied (80 kg/ha of MAP at 10% N) = 77.
• Fertiliser N requirement = 220 − 77 = 143 kg N/ha.
• Urea = 143 ÷ 0.46 ≈ 311 kg/ha.

Taking a soil test that the method can trust

• Sample in the month or so before sowing, and before any fertiliser N goes on.
• Most operators take 6–8 cores per production zone and bulk them; GRDC says that gives roughly an 80% chance of landing within 20 kg/ha N of the true mean. Mix bulked soil very well.
• Sample to at least 0.6 m, and ideally to 1.0 m; if you do not sample to rooting depth you underestimate supply. Segment cores into layers (at least 0–10 cm and 10–60 cm).
• Keep samples cool and get them to the lab quickly. If ammonium reads above about 2 mg/kg, the paper suggests the sample has been mineralising and you should use nitrate only.

Want to try your own numbers? Use the nitrogen rate and cost calculator, which uses this same demand-minus-supply logic with simple inputs, or read the shorter urea per hectare for wheat article.