Nitrogen is different to other elements as it can be obtained from the atmosphere, changes its form/s quickly and is easily lost to the atmosphere. It moves down through the profile with water so is also easily lost by drainage (leaching) (Figure 9).

Nitrogen in soil exists in many forms, but only a few can be taken up by plants. Soil microbes are important in changing nitrogen forms, making it available or unavailable depending on conditions, especially the presence of oxygen in the soil.

The immediately plant available nitrogen includes both ammonia and nitrate, known together as mineral-N. This is determined by laboratory testing, or the nitrate component (typically most of it) can be assessed using the Nitrate Quick Test method.

There is also a supply of N from microbial mineralisation of soil organic matter. The amount varies according to temperature and moisture as well as soil type and paddock history. The amount likely to be released during the life of a crop can be estimated by lab tests, including anaerobic decomposition (anaerobically mineralisable nitrogen AMN) or potentially mineralisable nitrogen (PMN).

In Aotearoa New Zealand, nutrient recommendations for vegetable crops are based on 15 cm deep soil samples. But for nitrogen, deeper sampling of the whole root zone is useful. Sampling below the rootzone can show if leached nitrate is being lost from the system.

For a detailed explanation of nitrogen testing, see the Plant and Food Research “Guidelines for soil nitrogen testing and predicting soil nitrogen supply (2022)”