In message , Franz Heymann
writes

"sw" wrote in message
...
Nick Maclaren wrote:

In article ,
PA wrote:
thanks every one for your input, I'm still in two minds though....
the bit I don't get is: if no residue is left in the soil, what about
when
the roots of the weeds start to break down wont they release chemicals,
which will be then taken up by whats ever growing?
the trouble is I've got about 450 square meters to clear and its taken
about
an hour to do one properly
and now I'm dreaming of roots

No, because it will be released into the soil, where it will be broken
down by bacteria. Apparently, some breakdown products disappear very
slowly from the anaerobic sludge at the bottom of ponds, but garden
soil has a very different (and much more vigorous) collection of
microorganisms.

I'd thought there was some research suggesting it doesn't break down in
soil quite as thoroughly/quickly as was originally believed? Or is that
more a concern where fields of GM-HT crops are resulting in higher
usage?

Surely if it does not break down pretty smartly, the plants surrounding the
targeted weed would die as well? I have never noticed an effect of this
kind.

Not necessarily, even some of the much more noxious and longer term
active weedkillers are physically adsorbed on clay soils and so rendered
inactive against plants. The same happens to glyphosate too (although it
isn't so important since to work it has to hit the green parts of the
plant).

It doesn't have to be broken down to prevent it working. It just has to
be made physically unavailable or kept locked up in the wrong place.

If you garden on pure peat and sand with no hint of clays you might have
real difficulties with weedkiller persistence (but not with glyphosate).

Regards,
--
Martin Brown