"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".

When you say "beans", do you mean soy[a]beans?

When I shouted myself a trip around the US a few decades ago, I learnt
that soyabeans are typically "parasitic" on soil N in spite of their
ability to fix atmospheric N through their root nodule bacteria. In
other words, the yield of N in a crop was greater than the amount
fixed. I have to say I was a bit surprised by this outcome too,
because I had been well indoctrinated with the "legume/rhizobium
symbiosis" as an undergraduate.

This effect may well depend on the native N in the soil initially.
The more "naturally" available, the less efficient the RNB association
is likely to be. Certainly, the application of bag N typically
reduces the amount fixed (and is a common means of removing unwanted
legumes from domestic lawns here).

There are other issues too. Most obviously, growing any
harvestable crop will deplete soil nutrients. While legumes may
even be able to increase soil N levels in some circumstances, there
will still be a nett loss of things like P and K if they are harvested
and removed from the field. Indeed, most of the N will usually be
removed too if the crop is to have any value elsewhere.

Another aspect relating to your "makes a mess of the soil" is that
legume crops typically involve more cultivation and chemical
treatments than many broadacre gramineous crops. Their root system is
also inferior to the fibrous roots of grasses in preserving
soil structure (at least that's what I was taught many years ago .

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.

Yes. I believe soyabeans have been used this way for centuries. They
are very well adapted to the system, which may be one reason why they
are not so efficient as N fixers in conventional agriculture. Indeed,
their ability to tolerate wet soil is one reason they are now being
used as a green manure crop for rotation with sugar cane in the wet
tropical lowlands around here.

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?

Some can fix quite large amounts of N. But to take advantage of it,
you really need to plough the lot in. If you harvest for pulse, hay,
whatever, you'll most likely remove most of the N that was fixed.


Cheers, Phred.

--
LID