JohnR wrote: Glyphosate becomes inactive on contact with soil.
It works on Japanese Knotweed to an extent.
However that claim about becoming inactive on contact with soil has always
irritated me. That sort of rapid reaction that is implied by the statement which comes from Monsanto's claim sounds more typical of an explosive than a weed killer.
The truth is more complicated. N-phosphonomethylglycine or Glyphosate
inhibits an enzyme called 5-Enolpyruvyl shikimate 3-phosphate synthase
It is involved in the synthesis of aromatic amino acids. It kills the plant by stopping
it making protein. Animals don't have this enzyme but there are plenty of soil bacteria etc that do. The malaria parasite has it for another example.
Glyphosate can be come adsorbed on clay particles but it can also come off again.
and if your soil is peat or sand this won't happen. I seem to remember around 10% might be present after 6 months.
Monsanto were involved in a court case about this in France a while ago and as a result no longer make this claim.
Monsanto guilty in 'false ad' row
Man in field of sugar beets in Colorado, US
Monsanto's weed-killer, Roundup, is the world's best-selling herbicide
France's highest court has ruled that US agrochemical giant Monsanto had not told the truth about the safety of its best-selling weed-killer, Roundup.
The court confirmed an earlier judgment that Monsanto had falsely advertised its herbicide as "biodegradable" and claimed it "left the soil clean".
for more of the story see this from the BBC.
http://news.bbc.co.uk/1/hi/8308903.stm