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Help, I think I might have potted my avocado seedling too early?! It was growing great whilst in a glass jar of water, had some nice roots and 5 leaves on the top, then about 3 weeks ago I planted the seedling and pinched off the top few leaves as recommended by an online tutorial. Now, the seedling seems to have stopped growing. Have I planted it too early? If so, is there anything I can do to save it and encourage growth?

It's fine. Since it was growing in water with "water roots" now it needs to grow some "air/soil roots" before it can take off. Be sure to keep the potting mix moist but not soggy -- the pot should have a drainage hole and the potting mix should be well draining (avocado actually hates wet soil and can easily get root rot fungus).

I see that you left the top of the seed exposed, so your planting depth is good.

Just from the picture, this area looks too dark. Avocado is best grown on a south or west facing window sill.

Ditto applestar's advice. Everything looks fine except for the light factor. Avocado needs quite a bit more light than your plant is getting. Needs to be on an unobstructed east, west, or south-facing windowsill.