I have had this cactus for a while now (about 4 or 5 years) and recently noticed browning near the base. The spines at the bottom are also a different color and pressed closer to the body of the cactus.
After all my internet research I still cannot figure out whether this is just regular corking or if there is something the matter. I don't regular too often (usually once every two-ish weeks) so I don't think that it's overwatered. It looks like there is still new growth at the top (little green spines) Any thoughts or recommendations would be greatly 
appreciated.
