Yeah if thats the case, cut off the rotten end, let it dry out for a couple of weeks until the wound has gone woody.
Probably not worth planting horizontally unless you are putting it into the ground, as you most likely wont have a pot wide enough. Either way, I am pretty sure that with logs, you just lay it horizontally and bury 75% of it in a soil mix as I will list below.
Since its a mid-section, you could almost cut it in half too and plant both, as the only way this is going to grow is from new pups at the tip end. The shaft isn't going to grow in height any more. Double your plants! You could try one as a log and one vertically.
If you plant vertically you can do the following:
Buy
- cacti soil mix
- perlite
- propagation sand
(some people use only sand for getting roots to grow, but I use this combo)
Mix these together, almost 1:1:1 for good drainage, even let the mix dry a bit if the soil is too damp - you dont want to cause rot.
Plant your cutting, making sure the correct end is up - if you are unsure we can help, just take a closer photo of the spines. I plant them a little deeper than usual.
If you are in the southern hemisphere and its warm, leave this outside in a shady but bright area for a month or two. A light watering every couple of weeks. I'm not sure what to do in Northern Hemisphere winter. A good spot is where the ground gets warm. I put mine on the brick floor against a wall - it gets nice and hot.
Never check for roots by taking out the cutting as you will break any young roots.
Being a mid-section, your cutting will end up growing a pup or two from the tip end, more if you are lucky.
You can tell when it's strongly rooted when you can pick the whole pot up off the ground by hanging onto the cactus only.
Sorry if this is TMI - im bored!