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4.6 Free nitrogen from the air

by Ross Ballard, SARDI.

Bacteria fix nitrogen

Legumes (clovers and Lucerne) ‘fix’ nitrogen from the air into a form which can be used by the plant.  This process is carried out by the bacteria, Rhizobia sp, which live in the nodules on the legume root. 

“Rules of thumb”

Nitrogen fixation

For each tonne of legume dry matter, 25kg of nitrogen is fixed.


A reasonable legume pasture may fix 125kg of nitrogen per hectare.


That is equivalent to 270kg of urea.


Different legumes fix and use different levels of nitrogen.


This is a symbiotic relationship, the plant receives nitrogen and the bacteria receive sugars.

Slow release fertiliser

Nitrogen fixed by a legume is organic nitrogen and acts like a slow release fertiliser.  It becomes available to the plant as the plant residues are decomposed by other soil organisms.  The process of converting organic nitrogen to the inorganic form which is available to plants is called mineralisation.  Grasses, cereals, or the legume may take up this inorganic nitrogen.


Graph 1: 

Preferred pH range for common pasture legumes


Soil texture and rainfall will also  impact on growth.

Horses for courses

Different legumes need different rhizobia.  Clover rhizobia will not nodulate lucerne.  Equally, lucerne rhizobia will not nodulate vetch.  Hence, if you sow a new legume in a paddock, inoculation should be practised. Ensure you use the correct inoculant strain.


“Rules of thumb”

Assessing nitrogen fixation


Plants are best assessed 8-10 weeks after germination.


There should be at least five red nodules close to the top of the root system.

Red nodules

Sometimes the nodules needs to be cut to see the red colouration.

White nodules

Numerous white nodules scattered over the roots indicate the symbiosis is not fixing nitrogen.

Old nodule

Active nodules (red) may turn green as their n-fixation activity declines with age or plant stress.  In subclover plants at flowering, it is common to observe that whilst the section of nodule close to the plant root is green, the nodule tip remains red.  This red area of the nodule is still fixing nitrogen.

Rhizobial inoculants are alive

Remember that rhizobial inoculants contain living bacteria.  They are fragile.  Do not expose them to excessive temperatures or freeze them.  Avoid mixing them with pesticides and fertilisers.  Sow seed as soon as possible after inoculation, into a moist seedbed. 

Rhizobia require warm moist soil conditions.  They must be the correct species for the legume so that nitrogen fixation occurs.

Factors that will limit nitrogen fixation:

  • Hot dry soil

  • Incompatibility of rhizobium and plant

  • Low pH

  • High levels of nitrogen fertilizer.

Impact of pH

Soil pH can be the major limitation to a good symbiosis (see Graph 1).

Nitrogen fixation by subclover may decline where soil pH falls below 5.  Whilst there are often still rhizobia in the soil, their ability to nodulate the clover may be reduced.  Liming to increase soil pH is the best solution.

Lucerne rhizobia are less tolerant of low pH.  They are rarely found in soils where the pH is less than 6.  Hence, it is absolutely essential that lucerne seed is inoculated and lime pelleted at sowing.  Liming to increase soil pH is also a good strategy.


Graph 2: 
Approximate relationship between amounts of nitrogen fixed and legume dry matter production

Based on Peoples et al, CSIRO


4.6 Free nitrogen from the air

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