To fix or not to fix: What Controls Free Living Nitrogen Fixation in Plants’ Root Zones?

by Dr Ash Martin  PhD BSc(For)Hons

The biological fixation of nitrogen from a gas in the air to a solid form in the soil is probably understood in its basics by many growers.  What’s less well known are the main factors that control it.  This edition’s science article on nitrogen fixation has some great information on how biological nitrogen fixation occurs and how it can be increased, and decreased, depending on soil factors such as carbon, structure, fertilisation and nutrition.  To that we would add agrochemicals, as many of the microbes involved in nitrogen fixation can be sensitive to herbicides and other pesticides.

Nitrogen fixation by free living nitrogen fixing bacteria (FLNFB) (as opposed to symbiotic nitrogen fixing bacteria, such as Rhizobium) (≈ 75 million tonnes/year worldwide) totals about half of the nitrogen made industrially by the Haber-Bosch method (e.g., urea) and about 15 times more than that fixed by lightening.  In Australian soils, the amount of nitrogen fixed by FLNFB has been measured at up to 25 kg/ha/year in dryland cereal soils, but can be up to 300 kg/ha/month or more under ideal conditions.  At a price of $230/t N under ideal conditions that can equate to around $70 N/ha/month free nitrogen, delivered in-soil!  That’s clearly a substantial benefit that’s well worth taking advantage of.

The best tool to help you measure and manage soil nitrogen fixation by FLNFB is N Wise.  N Wise gives you the complete picture of what’s happening with the interaction of soil and biology over time and comes with a nitrogen fixation ready reckoner and simple calculation tool to help you make the most of it.  When managed with nitrogen mineralisation (see separate article in this edition) it can provide a substantial benefit to economic and soil sustainability.

Click here for more information about N Wise.

For more information and some great diagrams about free living nitrogen fixation see this edition’s science article on nitrogen fixation:

To Fix or Not To Fix: Controls on Free-Living Nitrogen Fixation in the Rhizosphere.  Smecina et al. 2019.  Applied and Environmental Biology.  85(6): e02546-18  (Requires PDF viewer)

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