Thanks to Alex's comment elsewhere (which I only saw now), I discovered the Pische S'shuva, YD 294:13, who cites Parach Mate Aharon as saying the soil must last three years, and Shivas Tziyon as qualifying that that's only in eretz Yisrael: in chutz laaretz, he says, [where safek orla is permitted,] the soil must last "a few days".


The Pitchei Teshuva in Yoreh Deah 294 sk 13 quotes a number of opionions but rules that in Israel where we rule stringently about doubts relating to orla, there must be enough dirt to survive three whole years; in the Diaspora, however, where we rule leniently about such doubts, it only needs to be enough dirt to last a couple of days. h/t Alex

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