Check the values for glyphosphate half-life. Longer than 3 weeks. I have used the stuff a lot in the past. Really looking for other options now.
http://npic.orst.edu/factsheets/archive/glyphotech.html#env
http://npic.orst.edu/factsheets/archive/glyphotech.html#env

and yes I think I'd be doing the same as you with that level of previous use. I'm the same - I dont' use anything at all - compost layer at the beginning of the spring and nettle tea type fertilisers. I hope my original post didn't suggest I was in any way fond of chemicals
- it is actually the opposite! The one exception is we have used round up on is our Japenese Knot weed infestation as part of our 3 pronged ongoing battle (3 years now and still not rid of it) but that is a whole other story!