I have a 30 year old cactus
that has grown to be 5 feet tall. It was knocked over and the top foot of the
plant broke off. I was hoping to start a new one from that piece but it keeps
rotting in the soil.
Re-cut the bottom of the
cactus you are trying to root. Let it sit overnight so the cut edge begins to
seal. Then try rooting it in vermiculite or sand. Once rooted you can move it
to a well-drained potting or cactus mix.