How much urea per hectare for wheat? A worked rule of thumb

The formula in four steps

1. Pick a realistic yield target in t/ha. GRDC uses “economic yield”, about 80% of the yield your water supply could support, rather than a best-case yield.
2. Multiply by 40 to get the crop’s nitrogen need in kg N/ha (soil plus fertiliser).
3. Subtract the soil’s mineral nitrogen from a pre-sowing soil test, and any N you have already put on (for example from MAP).
4. Divide what is left by 0.46 to turn kg of N into kg of urea.

A worked example (our assumptions, not your paddock)

These inputs are illustrations. Swap in your own yield target and soil test result, or use the nitrogen rate calculator.

The official worked example

GRDC’s “Back to nitrogen basics” paper runs a bigger example for a medium-rainfall South Australian wheat paddock: a 5.5 t/ha economic yield gives a crop N demand of 220 kg N/ha. Soil mineral N of 69 kg/ha plus 8 kg N/ha already applied as MAP gives a supply of 77, leaving a fertiliser requirement of 143 kg N/ha, or 311 kg/ha of urea. The full step-by-step is in our nitrogen budget walk-through.

What the simple formula leaves out

• Losses. Urea left on the surface can lose nitrogen to the air. GRDC’s national project found a median loss of 8% of applied N, with a range from 0 to 48%. If you want to allow for 10%, divide by 0.90 as well: 80 ÷ 0.90 ÷ 0.46 ≈ 193 kg/ha. That 10% step is our arithmetic for planning, not part of the GRDC formula. See how much N you really lose from urea.
• Soil test error. The paper warns that soil tests can easily be out by 20–40 kg/ha N or more, and that soil N below the sampled depth is not counted.
• Season. In wet springs, in-crop mineralisation can supply a lot of N; in dry seasons, yield falls and so does demand. The rule of thumb is a starting point, not a prediction.
• Timing and splitting. We have not verified timing guidance from a primary source for this article, so we do not give it here. Ask your agronomist.

Frequently asked

How many kg of N per tonne of wheat?

About 40 kg N per tonne per hectare of economic yield, supplied by the soil and fertiliser together. About 20 kg N per t/ha leaves the paddock in the grain.

How do I convert urea to kg of N?

Multiply kg of urea by 0.46. Going the other way, divide kg of N by 0.46. For example 100 kg of urea contains 46 kg of N.

Does fertiliser supply all the nitrogen a crop needs?

No. According to GRDC the soil usually supplies 55–70% of crop N and fertiliser 30–45% at most, so it is best seen as replacing N removed in grain rather than the main source.