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Soilmaster Copyright 1996 -Reme Soilmaster(TM)
NITROGEN FIXATION:
How does it happen?
Sub clover produces a nodule on it - or an enlargement on the
root system. There can be one to dozens and dozens, and the nodule is formed
by the relationship of a bacteria that's in the soil called rhizobia, which
infects the plant root and an enlargement or nodule forms.
The nodule fixes nitrogen from the atmosphere and when the nodule dies,
the nitrogen is released into the soil as nitrate. (NO3-) This nitrate
is in the form which plants can use for growth.
And does the clover use nitrogen?
It is not commonly realised, but clovers also need nitrogen for growth, as well as grasses. But a side effect that can occur, is that the nitrogen, if it is not used by the plant, is very mobile in the soil and with rainfall, can move down the soil profile and cause acidification.
So the nitrate, although we need it for plant growth, is having a secondary, quite sinister effect on our soil, by causing soil acidification.
Can nitrogen come from other forms?
It certainly can. Nitrogen can be applied as bag nitrogen (chemical) and it is just as acidifying in that form.
There are other forms of nitrogen in plants that are very important
as sources of nitrogen. The blue green algae, which commonly occur
in our waterways, also can occur in soils. These tiny algae produce
nitrogen by fixing it from the air, just like our plants.
They fix up to 20 kgs per hectare per annum. By comparison, a
subclover plant can produce between 80 and 100 kgs of nitrogen per annum.
There is also another source of nitrogen and that's when we have an electrical storm there's nitrogen that can get fixed in the air and then get dissolved in rain and end up in the soil, but it's not very important.
And how important is fixation of nitrogen in Australian agriculture?
In Australia, 95% of our nitrogen needs are produced by plants. In other parts of the world a much lower percentage is produced.