Remember sodium chlorate? Before 1940 that was about the only chemical option for killing weeds. But it is a very powerful oxidising agent, and therefore left dead vegetation, or indeed any organic matter, liable to burst into flames.
In the Thirties, New Zealand farmers famously suffered an epidemic of “exploding trousers”, owing to the widespread use of sodium chlorate to control ragwort.
From the Forties to the Sixties organic chemists were falling over themselves to devise new chemicals for killing weeds. Older gardeners will recall 2,4,5-T, simazine, atrazine, diuron, paraquat and others as practically ubiquitous.
But today the golden age of weedkillers is over. It gradually became apparent that these chemicals were very nasty indeed and approval for their use, including all those listed above, has gradually been withdrawn. Even good old sodium chlorate finally met its Waterloo in 2009...