Cut the cactus straight across about an inch above the brown where it's green and fleshy and let the bottom dry and callus over. Without being planted in a pot the cactus will grow more roots .When you see the roots growing you can replant the cactus but don't water it for a couple of weeks to let the roots grow more.
One of my columnar cacti, which is like yours, I keep with the top sliced off . It develops little cacti all round the rim of the cut and when they're about an inch across I cut them off with a sharp knife let the bottoms dry for a few days and then plant them dry until they've rooted. I slice off the top below where the little ones have grown and another lot grow round the new rim.
You could restart your cactus using the same method I use for my baby cacti it takes longer to dry and callus over being larger but it will work.
I've had this same cactus for a few years now and it's done well. I believe I've treated it well, but for like a year now it's been turning brown at the base. It's not soft and mushy, it's very hard (the brown part). I recently re-potted the cactus because I believed it was getting a little big for it's old pot. I did extensive research on what to get to make some cactus potting soil, but did not have most of the ingredients necessary. So I used what I had, and I'd say my mixture of soil is about 50% premium potting soil, 25% small gravel, 25% sand, give or take. I figured that would be good and enough soil and sand to let water travel freely through the pot. Perhaps I'm overreacting, but I feel like my cactus is dying from the re-potting. Does anyone have any ideas, suggestions, anything at all to help me?
