No GMO Organisms

The Times has an article today on the search for supplies of commodities which aren't GMO.  Seems to be particularly inspired by Whole Foods decision to emphasize such products.  (I own some stock in Whole Foods.)  It cites a premium of $1.50 to $2 per bushel for grain, and discusses the difficulty in doing a dual-track supply system.   It also includes this, which I found astonishing:

And farmers cannot simply replace genetically engineered seeds with conventional ones, because soil in which genetically modified crops have been grown may not be immediately suitable for conventional crops.

“There’s a transition period required,” said Richard Kamolvathin, senior vice president at Verity Farms, which sells meats, grains and other products derived from conventional crops, as well as natural soil amendments. “You don’t just stop growing G.M.O. seed and then start growing non-G.M.O. seed.”

 Now I understand moving from conventional to organic production requires a 3-year phase in, but just switching seed?  I suppose there might be herbicide/pesticide residues, but how long do they last?