Something Amaranth-related is growing in my citrus pot.

I have the habit of reusing old potting soil when I fail my sowings.
So this may be a vegetable, but it could also be a weed.
It has started growing in my Citrus digitata pot, and I suspect it could be Amaranth-related... But it grows so fast compared to the Amaranth-related plants I grow that I find it fishy.
Would anyone know what it is. It's more than 4 feet high.

Thanks.

Try looking at this thread. Your picture looks like either pigweed or one of the weed amaranths. There is an excellent post with pictures of the various bloomouts of the different weedy amaranths.

https://permies.com/t/68756/Red-Pigweed-Palmer-Amaranth-massively

Thanks, will do.