Golden Barrel Cactus Yellow at Base
I bought a golden barrel cactus last June from a botanical garden in Arizona. I left the cactus in its soil/plastic pot, watered it monthly by soaking it in some water with fertilizer and letting it drain, and kept it by a really sunny window.
The cactus seemed to be doing fine with no visible signs of distress. When I watered it at the end of March, I saw that the roots were growing out the bottom, so I repotted it. When I removed it from its pot, I saw that the base that was covered by the soil is yellow. Part of that yellow base is now sticking out of the soil in the new pot.
I'm wondering whether the yellowing a sign that there is something wrong with the cactus, or if it's just a result of that part of the cactus being covered by soil/rocks? If it's the latter, is it ok to have it exposed now?
Any advice would be appreciated. Thanks!