David Hare-Scott wrote:
....
finds nearest soapbox
Plants are autotrophs, that is they don't eat, they take in fairly
substances (air, water, minerals etc) and photosynthesise more complex
substances using sunlight energy. Those complex substances may be food for
organisms that do eat (heterotrophs) like cows and us. The inputs we are
interested in, NPK and other elements, are often loosely called "plant food"
which can be confusing in comparison with animal nutrients such as protein,
carbohydrates etc as the two are not similar classes of substances nor do
they have the same role in metabolism. Gardening terminology is also loose
in talking about inputs as elements when to a chemist none of them are
present in the form of elements but as compounds and molecules.

This leads us to the case of N (nitrogen) as a plant input which is what I
think you were asking about. Although it is four fifths of air plants
cannot absorb N directly as nitrogen gas is a molecule of two atoms (N2) and
that molecule is extremely stable and chemically inaccessible to the plant.
So plants need some help to absorb N. This can be from microbes that fix
nitrogen, such microbes can take in N2 from the air and produce useable N
compounds. Often such are symbiotic with plants as in legumes. Plants can
also get N as compounds as part of synthetic fertilisers, manures and
composts, and from rain during electrical storms. This is why the N
component of manures in its NPK value is of interest not the protein
content.
descends soapbox

If this was not the point of your question ask and I will try again.

i would be interested in a good list of studies
done on actual nitrogen uptake from soil using
tagged sources (radio isotopes?).

so far in my readings i have come across one
study mentioned (which i didn't follow up on)
that said very little of applied nitrogen from
chemical fertilizers actually is taken up by
plants, but that it must act somehow by freeing
other nitrogen in the soil/organisms that plants
can take up. this was consistent for both
the first and second year after application...

so i'm curious if anyone else has gotten into
this topic beyond the surface?


songbird