New Soil PMN test for Cropping Soils | Hill Laboratories - NZ


New Soil PMN test for Cropping Soils

Date: 3 Aug 2021
Author: Hill Laboratories

We are pleased to announce that a new soil Nitrogen test specific for cropping soils is now on offer from Hill Laboratories. The new test, called Potentially Mineralisable Nitrogen (PMN), has been developed and calibrated over the last two years under a Sustainable Farming Fund (SFF) project, with trials covering several geographies and growing systems. The aim of the project was to establish a soil test to better evaluate the amount of nitrogen mineralised from soil organic matter, to enable more accurate N fertiliser decisions for crops. PMN is calculated from the new Hot Water Extractable Organic N (HWEON) test and both tests will be reported as a weight:weight concentration in the soil (i.e. mg/kg). A further calculation through to kg N/ha will be described in the test report, where sample depth and soil bulk density values will be needed as factors.

About 98 % of the total soil N exists as a component of organic matter called the organic nitrogen fraction, which includes humus and soil organisms. In its organic forms, soil nitrogen is insoluble and is unavailable to plants.

Soluble (or immediately plant available) nitrogen comprises only 2–3 % of the total soil nitrogen and exists as ammonium (NH4+) or nitrate (NO3-) ions, called the mineral nitrogen fraction. Nitrogen is converted from organic to mineral forms, ammonium-N and nitrate-N, through the decomposition of organic soil compounds. This process is referred to as mineralisation.

Mineral nitrogen is constantly being ‘taken up’ by plants and organisms (as NO3-N and NH4-N) and converted to protein, or is lost by leaching (as NO3-N). As plants and organisms die and their organic mass is subject to decomposition, nitrogen is eventually released as ammonium and subsequently converted to nitrate in a continuous process referred to as the ‘Nitrogen Cycle’.

As for any test that attempts to measure a “mineralisable” component, the PMN test expresses a potential amount of nitrogen that may be mineralised (and therefore available to crops) under ideal growing conditions. Temperature and moisture during the growing season will affect what proportion of this “mineralisable potential” actually occurs.

The new PMN (& HWEON) test currently takes 2-3 days in the lab and can be requested by selecting HWEON on the analysis request form or by selecting this in the mobile sample submission app (for enabled customers).

The new PMN test is correlated with the more familiar AN/AMN (Potentially Available N/Anaerobic Mineralisable N) test, but is thought likely to be more accurate in predicting the mineralisable N fraction of cropping soils, given the recent field calibration. Hill Laboratories will keep both tests on offer at this stage, as the AN/AMN test is now measured by NIRS (rather than a 7-day incubation) and is therefore very fast and affordable. We may however review this situation in the future.

Summary of Soil N test for Crops:

  • Mineral N [MinN or MinNCh] = the mineral N (nitrate-N and ammonium-N) in the soil at any point in time. A soil moisture test is also included in the Mineral N profile. This test is carried out in both our Hamilton and Christchurch labs, as samples should be kept chilled and delivered to the lab soon after sampling. TAT 3-4 days
  • Potentially Available N [AN from AMN] = the existing test that has been used to measure the potential amount of nitrogen that may be mineralised from a soil over the growing season, given ideal temperature and moisture conditions. TAT 3-5 days (unless the NIRS result does not qualify in which case the 7-day incubation would then be carried out)
  • Potentially Mineralisable N [PMN from HWEON] – the new test to measure the potential amount of nitrogen that may be mineralised from a crop soil over the growing season, given ideal temperature and moisture conditions. TAT 5-7 days