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That would not be a mix I would use for potted citrus. There are many mixes which would suffice, but citrus generally prefer some amount of sand in the soil. I'll leave the exact recipes to those that do potted trees in colder climates.
Is that the same friend that thought you got a pommelo or other citrus from planting a lemon seed?
slightly acid, well draining, yet capable of good moisture retention.
I should say though that while I question this friend's thought s on seedling identification, if he grows citrus successfully for 30 years in the Czech Republic with that mixture , then who am I or who is anyone else, for that matter, to argue or to disagree.