Key Points
- Several different soil N tests are available with different purposes. Understanding these differences is important and the test required must be clearly identified for the laboratory
- Mineral N is available now for the plant to use. Take samples to a 60cm depth and use for calculating fertiliser N requirements. This method is needed for the Sirius wheat calculator
- The mineral N test does not account for the N that is mineralised during the growing season
- Mineralisable N is the amount that becomes available during the season. Fertiliser recommendations based on mineralisable N are only useful when there is a good relationship between the mineralisable N and the amount of N the soil will actually supply to the crop, as in the maize calculator
- The N index has potential for determining fertiliser N requirements, but it needs updating for current management practices and cultivars