Kristina

Could you possibly take few photos and post them? Whole plant, also the roots if you can.

These are the roots. I didn't want to dig it up too much to not damage anything. I'm afraid for them because of the lack of stay roots.

In the last picture you can see that at the base of the cactus there are a lot of folds and I'm wondering if this is normal.

I do notice it's sort of yellow, but I think this is because it's potted too deep so the sun never hits those parts (could that be a possibility?). I don't think it has a problem with watering simply because we don't water it too often, maybe once a month. My brother and dad spilled it a few times which is why the pot is not really filled with ground, hence the cactus simply collapsing too deep into the pot, I'd say. I read somewhere clay mixed with sand can be a good ground for it (I don't know what kind of ground is currently in the pot, the plant came in this ground and has not been replanted since). I'd re-pot it but I'm not sure if I should choose a bigger pot.

Cactus do well in small pots, and since this doesn't have many roots I would just use the same pot. You could get one a little bigger, but not too big. Clay and sand hold too much water for a cactus like this. Water should flow through the soil quickly and the soil should not stay wet very long. You could take the soil it is in and add about half perlite or rough gravel such as pumice or scoria. I would also try to get it more sun, as the new growth looks pale. If moving it to full sun, do it a little at a time so it doesn't sunburn. Good luck!

As above, NO to clay & sand (together or alone).

If existing soil is peat and very dry, it should be rewetted before using as it is hydrophobic. Such dry peat will absorb some water easier if it is warm, so it should be done when plant is not in it.

If you can't get pumice or scoria, grit (chicken grit) is very good and very inexpensive. Perlite is usually easy to find.

And yes, bottom could be discolored if it was buried in soil for a while.