RoundUp is supposed to decay rapidly in the soil, so that there is no residual action, but if there is any clay content, the clay particles will adsorb it, and release it gradually over the next several seasons. I am still searching for the articles that detailed that process about 15 years ago, but I'm having to dig deep...Hmm..maybe in my back issues of Southwest Horticulture?...oh, well, I will get back to you! : /

I use the agricultural roundup which is s'posed to be 16 x stronger than that bought in shops for domestic use.
I use it on areas where I want to make a new border for herbasceous plants and sometimes veg. I have always sprayed the area, left it for about 6 weeks, cleared the area and then planted without any problem. I sometimes use it round and under shrubs. There has never been a problem with after effects. But then I believe the formulas changed a while back and that it is neutralised when it hits the soil. I have clay in my soil and there have never been any after effects that have appeared on any of my plants.

It might be worth sending a p.m. to Bamboo she had info. on the build up of Roundup in the soil

Thank you folks. How do I send a pm to Bamboo?

Easy, Steragram. Go to
http://www.growsonyou.com/bamboo/profile
and click on "Leave a private message."
I might do the same, maybe her info will jog my memory. My most recent experience has been with customers at the nursery I work for. They were drenching the foliage and soil with it through a watering can, rather than the frugal spraying of the weed foliage that is recommended.