Nitrogen fixation requires a lot of metabolic energy that the bean
plant could otherwise use to produce more plant tissue. Therefore,
using nitrogen fertilizers will often give a yield increase.

In an extreme case, consider beans grown in a nitrogen-free medium
such as sand but supplied with plenty of the other mineral nutrients
they require. They would survive with just nitrogen from biological
nitrogen fixation but the yield would be much less than if they
received all the nitrogen fertilizer they required.

Under natural conditions plants are often limited in growth by the
lack of nitrogen. They do not look deficient because plants slow their
growth to fit the supply of nitrogen.

Using nitrogen-fixing crops to increase the soil nitrogen content
usually involves allowing the crop to remain in the field so the fixed
nitrogen is not removed in the harvested part.






"BGGS" wrote in message ...
I was under the impression that nitrogen-fixing crops were a good thing
until a few weeks ago I heard two farmers discussing on radio how "growing
beans makes a mess of the soil and removes the nutrients".

I'd like to know how the two things can be true. I'm aware of the
root-nodules on bean plants so they certainly do fix their own N so why
would they not be a desirable crop ?

In Japanese agriculture of the Eddo period and possibly earlier, farmers
were required by quite rigid rules to grow rice in the middle of the field
and beans around the edges to shelter it and provide the soil with nutrients
so it must work.
I don't hear about crops being combined today outside of some rotation
systems even though this method makes run-off problems non-existent.
Is it perhaps because the natural fixing-qualities of bean plants is fairly
feeble compared to present-day fertilizers?
Thanks
BG.