Q. I am a renter, 2-1/4 years at present location. Landlord sprayed roundup before I moved in, so I've done container gardening from day one (and got him to quit spraying). What is your opinion/guess on how long, if any, to let the ground lie fallow before raising food in it?
A. According to Monsanto (the maker of Roundup)in a document they published called Glysophate Half-life in Soil (link to PDF file) the half-life of glysophate is around 32 days. This can vary quite a bit due to soil conditions. Microbial activity can play a big factor. The more activity the faster the glysophate degradation. Soils with very little microbial activity have been found to retain measurable amounts of the chemical for 3 years after application.
A half-life is the amount of time it takes something to break down 50%. If the half-life is 32 days, then half of that amount will reduce in another 32 days. Essentially in 64 days you would be down to a quarter of the chemical. It would continue to break down over time. An example of a possible half-life break down would works as follows:
|1 Day||32 Days||64 Days||96 Days||128 Days|
To rectify a situation where glysophate has been used, I would recommend adding organic matter to the soil in the form of compost, mulch, or even a cover crop tilled into the ground. The more organic matter you add the better the situation is for microbes in the soil and the faster the glysophate will break down.
Once sprayed in an area it may be a couple months before anything can be planted again in the same location. This depends on environmental factors related to the soil and weather conditions.
After 2 1/4 years of no more use of glysophate at your current location the amount of glysophate in your soil should be almost completely gone based on the estimated half-life time. As a general guideline I would avoid Roundup products as much as possible and resort to natural or organic methods of weed removal.