To get the best idea of what’s needed, FAR says that a soil mineral N test is a good start. Samples can be taken to any depth growers feel the roots will reach, from 10-12 points in the paddock. Keep samples from different depths and paddocks in separate bags, and keep them frozen until sending on to the lab for testing.

The results obtained from the soil mineral test will be mineral N (mg/kg). To convert this to mineral N (kg/ha) use the following equation: Mineral N (kg/ha) = Min N (mg/kg) x sampling depth (cm) x volumetric weight (g/mL) x 0.1 (Mineral N (mg/kg) and Volumetric weight (g/mL) will be provided with the soil test results). If volumetric weight is not provided, use 1.1 as the default value. 

Once the levels of N is already in the soil has been determined, it is then possible to determine how much then crop is likely to require, based on expected grain yield.

• Autumn sown irrigated feed wheat requires 26 kg N/t of grain produced (assuming 100% availability of soil available N, and not accounting for subsequent N mineralisation during the growing season);
• Autumn sown irrigated milling wheat requires on average 31 kg N/t of grain produced (assuming the same as with the feed wheat above);
• Autumn sown dryland feed and biscuit wheats require on average 33 kg N/t of grain produced;
• Autumn sown dryland milling wheat requires on average 38 kg N/t of grain produced.

Growers can then calculate how much extra N they will need to apply.