Hi, I'm wondering if my cactus is rotting/melting? I watered it a week ago when I lived in Maryland and it was barely getting any sun. I just moved to Houston, Texas and it is 100+ degrees here. I put it on my apartment balcony and it gets direct sun. I have it pulled back a little, in a more shaded area, but nonetheless it is still very hot!! The soil is completely dry. Should I water it again? Is it too hot for it to be outside? It hasn't been acclimated to the weather, I kind of just put it outside. I really want to keep my cacti on the balcony year round. I've had him for 4 years and don't want him to die, please help My smaller one next to it seems to be doing ok so far, but the soil on that one isn't dried out completely yet.
I know it's a lot of trouble, but those plants should be exposed to their new situation very gradually. Expose them for and hour or two early and then put them inside by a window. Then after a day or two, increase the time outside. Continue that for a while until the plants are outdoors all day. Houston can be a tricky climate for cacti and succulents. The humidity keeps the soil from drying out properly, I think. I'd be really careful with water, especially until the plant has adjusted. Those columns store water, so they'll manage okay for a bit until you see how it goes. They are both attractive specimans so I hope they adjust well.
If they haven't been exposed directly to outside sun, then they will benefit from a gradual exposure to the sun. Just like people, plants can be sunburned. In plants, that can result in permanent disfigurement. In the case of cacti, they keep the same skin for years. Plants that grow new leaves all the time recover from the effects in time, but the scars left from too rapid an exposure to the sun can last the life of a cactus. They'll probably appreciate growing on the patio outside in time, just not all at once.
Donald has given you good advice. I would point out the sharp increases in exposure are more stressful for pretty much all plants (including your large spiny cactus) when accompanied by sharp increases in temperature. There is a sort of multiplier effect with serious heat and blazing overhead sun. So baby steps are good in your situation while the plant toughens up in its new environment. As Donald says it may be a bit of a hassle but it's not forever. When in doubt a position in mostly shade (especially during mid day) ought to be relatively safe, for now.
My smaller one next to it seems to be doing ok so far, but the soil on that one isn't dried out completely yet. 
