Gary350 here. My computer crashed and i don't remember my password i used up all my guesses so I used a different email and user name to get on here again. Hope I can figure out how to reset my password for Gary350 again.
Where do you live? Soil in the pictures looks a lot like the soil i had in my Arizona yard. If you live out west, AZ, NM, CA, UT, NV you will not have much trouble growing this cactus. Actually this is not a true cactus.
Your cactus is hard to grow. It took me about 2 years to learn how to grow those. Your cactus is very sun sensitive if exposed to too much sun the cactus will turn yellow and die if not moved to a location with less sun or give the plants shade. I had best results growing it on the south and north side of the house where it got limited sun, full sun from about 9:30 am to 12 noon or shade all day outside with lots of indirect sun light. This cactus is also very water sensitive and does not like to grow in pots where the soil changes from dry to wet to dry to wet to dry to wet.
If you really want to grow that cactus in pots you need to dig a hole big enough to put the pot in so when you push all the dirt back into the hole and pot all you see is the rim of the red clay pot showing on the soil surface.
That black spot on your cactus will spread like cancer. Soil in your picture looks a lot like the soil in my Arizona Yard. Where do you live?
Plant your cactus is sandy soil remove all the large rocks. Plant on the north side of the house where the moisture in the soil stays stable all the time. If that black rotted place gets worse you will need to cut the cactus off about 1/2" to 1" both sides of the rot. Worse things to do is not to cut enough of the rot off a tiny amount will spread like cancer so make sure you cut all the rot off. The cactus plant that you have growing in the soil with roots will scab over at the cut and grow 1 or 2 or 3 new tops where it was cut in about 3 months.
Save the cut piece. Be sure to cut off the rotted place. Let the cactus stay outside under the patio roof or some place where it stays dry for the cut to scab over for 1 week. Once the cutting scabs over it is ready to plant. Very interesting thing about all cactus you can plant the cutting right side up, upside down, or laying on its side, it does not care, it will still grow.
This cactus is hard to root and slow to root. It will not root in Arizona summer when it is 114 degrees. It is much easier to root in 75 degree weather. With NO rooting powder there is a 10% chance it will root in 3 to 4 months. If you use rooting powder there is a 90% chance it will root in 3 months. I learned to make my cactus multiply fast to plant the cuttings laying on their side cover them complete over with soil so they are about 1/4" below the surface. I used a small paint brush to wipe the entire cutting with rooting powder then cover it with soil. In about 3 months you will see maybe 5 to 7 new tops starting to grow. This cactus will grow about 12" tall in 3 months when temperatures are in the 70 to 80 degree range spring, winter and fall. In about 1 year the new growth will be maybe 20 to 24 inches tall, if you want to make more cactus plants cut them all off near the soil and plant them all on their side. 1 cactus cutting can turn into 5 to 7 cactus in 1 year, and those can turn into 30 to 50 cactus the 2nd year. If you cut them again and plant them on their side you could have about 350 cactus the 3rd year.
If you don't want to make more cactus plants from cutting you can expect your plant to grow 5 feet tall in about 2 years if you live out west where it can do all its growing in 70 degree weather, fall ,winter and spring. It will not grow very much in summer in hot weather.
Even though most people call it a cactus it is not a true cactus. I don't remember what it is? I never could get that type cactus to grow in pots it does not like soil conditions that go from wet to dry to wet to dry to wet to dry. It likes a very stable soil that stays slightly moist all the time the only way I could ever got those soil conditions was to grow it in the yard. .