Soil Nitrogen Tests Demystified
They say a picture paints a thousand words, so with that in mind this article aims to demystify the different soil tests that are in use for measuring soil nitrogen (N). It all comes down to the different forms of nitrogen in soil, with up to 98% bound in organic compounds and clay minerals (very small amount for this latter fraction) – shown pictorially in Figure 1 below.
What is important to understand is that plants cannot take up the organic forms of nitrogen. The process of mineralisation that converts organic N forms into the mineral forms of N, being ammonium-N (NH4-N) and nitrate-N (NO3-N), provides the N that can be taken up by plants. This process is driven by soil microbes, and is variable depending on soil temperature and moisture. At any one time, the mineral forms of N are usually only about 1-2% of the total soil N pool. Note that the mineral N is also utilised by the microbes for their own use, in a process called immobilisation. The plant-available mineral N left in the soil is the excess mineralised N not required by these microbes. High levels of mineral N (NO3-N) are undesirable, as this can easily leach to waterways if not utilised by plants.
Fig 1: Simplified view of the soil nitrogen pool
The Mineral N soil test measures just that – the amount of NO3-N and NH4-N at the time of sample collection. Samples need to be kept chilled and sent to the laboratory quickly, to avoid any soil changes while the samples are in transit. This test reports results as mg N/kg of soil and a conversion to kg N/ha needs sample depth and bulk density factors.
The Total N soil test measures just that – i.e. the whole soil nitrogen pool. It is useful when related to total carbon as the C:N ratio, as this drives net mineralisation. Total N measured in pasture soils was field-calibrated a few year ago, and the test result can be used to aid N fertiliser decisions.
The next two soil N tests is where it gets a bit more confusing. For cropping farmers, it is very important to get the N fertiliser decisions correct so that yield targets are met, input cost is managed and that the environment is not compromised (by excess N). The question is: how much N will the soil organic pool provide via mineralisation over the growing season?
For many years, the Potentially Available N test has been used to estimate this. The test utilises a 7-day anaerobic incubation when run conventionally, although Hill Laboratories have developed a quicker and more cost-effective NIR calibration for this test. This test is reported as Anaerobic Mineralisable N (AMN) with mg/kg units, as well as a calculation to Potentially Available N (AN) as kg/ha (assumes a 15cm sample depth). This AMN/AN test has not been extensively field-calibrated and is known to have relatively high uncertainty.
More recently, a new test called Potentially Mineralisable N (PMN) has been developed and field-calibrated for cropping soils. It uses a Hot Water Extractable Organic Nitrogen test (HWEON) and the result from that test is calculated to PMN using a calibration factor. The lab reports the PMN as mg/kg, but this can be converted to kg N/ha using sample depth and bulk density factors.
For both of these “Mineralisable N” tests, the actual amount of N released is dependent on soil moisture and temperature at different times through the growing season. As lab tests, they will generally overestimate what actually occurs in the paddock. As part of the SFF project that worked on the PMN test development, a further refinement for estimating the amount of N that will most probably mineralise in the paddock has also been published (Plant & Food Research document “Fact Sheet: Guidelines for Soil Nitrogen Testing and Predicting Soil Nitrogen Supply”). This uses climate factors to more precisely calculate the likely amount of mineralised N.
A series of Technical Notes on soil nitrogen tests as well as all of the main soil tests offered at Hill Laboratories can be found in the Client Resources section of our website www.hill-laboratories.com.