Plants contain nitrogen and although you can supply it in a fertiliser,
in
the wild it must be absorbed from the air


It's absorbed from the soil in the form of nitrates, nitrites and salts
of ammonia, mainly.

peterlsutton writes
You have got to be right Jaques. As soon as I wrote the above, I realised
that if plants could utilise nitrogen from the atmosphere you would never
have to give them a nitrogen fertiliser


IIRC trees breathe in Carbon Dioxide CO2, and take up water H20 from the
ground. As the tree gets bigger it tends to dry out the central part,
which becomes a pure hydrocarbon (which is why there is little potash),
and the liquids are sucked up just under the bark through the Xylem
through 'transpiration' which is a bit like evaporation but done through
'pores' in the surface of the leaves. The power of this is so great it
can suck huge volumes per day up to the top leaves.

They use the hydrogen H in the water and the carbon C in the CO2 to make
a hydrocarbon structure H-C and release the waste products, the oxygen
O2, into the air. (When you burn the wood this process is reversed, you
burn oxygen and produce CO2 and water vapour).

Which is just as well as we breathe in the oxygen and breathe out carbon
dioxide.

Not too sure about the nitrogen cycle but I know some plants such as
beans are known to 'fix' nitrogen in the soil from the atmosphere and
there are bacteria which do the same in the compost heap. Much of the
rest comes from animal waste products in the form of urine /urea
/ammonia which is high in nitrogen and fish / bone / blood products are
also high in Nitrogen and Phosphorus. Animal manures have Phosphorus
besides Nitrogen and Potassium as well as trace elements.

Bear in mind its not just direct feeding that is important if you are
being organic, you want to also feed the soil and improve its structure,
so all sorts of other things are important too.


No doubt someone will correct my mistakes and add to our bank of shared
knowledge .....
--
David