Question
after my roommate knocked over my 3" red moon cactus, i replanted it in the same pot but used mostly soil with just a little bit of the original sand.  i also moved it from a north-facing window to a south-facing one that gets a lot of direct sunlight (i live in nyc).

i noticed today that it looks like the stem is buckling under the weight of the red top because the cactus is lop-sided. upon further inspection, i see that the stem a pukey green color and that it is mushy and soft pretty much throughout, with it being softest near the base, where the soil is.  

this makes me think that i overwatered it?  if that is the case, is it true that it's a goner?

i read on some site that i can either cut out the  rotted parts but this would mean cutting out the whole stem, which doesn't make sense.  i can also graft it but would my red moon be able to grow on another cactus and if so, what other cactus should i choose to graft it onto?

thanks,
ttn

Answer
Dear Thoa,

Actually your cactus is already grafted. If the root stock is rotted, there isn't much you can do. If there is any part of the root stock close to the top part (scoin), you can try cutting it off and letting it heal, then replanting. Sorry about the bad news. And yes, generally when a cactus dies from the bottom up it is because of over watering.

Generally the best root stock is Myrtillocactus geometrizans. Hope this helps.

Sincerely,

Greg