Do a search here on "ortho". It's been discussed and rediscussed many times.

I think that this pretty much sums it up: "The conversion of polyphosphates to orthophosphates is fast. With broadcast applications to warm, moist soils, much of the polyphosphate will be converted to orthophosphate within two to three days after application."

So you see, it doesn't matter. Two to three days is way before root interception occurs.