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

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