There are a range of nitrogen tests available, all showing different things.

Ammonium and nitrate forms of N are plant available and together are termed ‘Mineral-N’. The amount of mineral-N can be determined in two ways. The first is by laboratory testing, which requires sending a chilled soil sample to a commercial lab. It will take a week or so to get results back from the lab. Alternatively, the nitrate component, which typically accounts for most of the mineral N present, can be assessed using the Nitrate Quick Test method (Figure 2).

The Nitrate Quick Test can be done by growers or consultants. This is a reasonably cost-effective way of checking N levels very quickly.

The rest of the tests need to be sent to commercial laboratories for analysis. If testing for nitrate, soil samples must be kept at about 4 degrees Celsius or soil microbes will continue to change the form of nitrogen in the soil. Do not freeze the samples. Make sure they get to the laboratory quickly, so do not send them on Friday or the day before public holidays.

Potentially Available N also called PAN or Anaerobically mineralisable N, is a test that can be used to determine how much N might be made available over the lifetime of a crop i.e., how much N might be converted from an organic form to a mineral form. This test should be done before the crop is planted, and is the test used by the Nutrient Management for Vegetable Crops guidelines to make nitrogen recommendations.

PAN has commonly been the test of choice for cropping, however the development of the potentially mineralisable N test (PMN) or Hot Water Extractable Organic N (HWEN) test which also measures what N might mineralise over time, has been found to be more accurate and will likely replace the PAN test as common practice.

Total N can also be tested, however this is mostly used for determining carbon to nitrogen ratios.