Yes, you can cut sections - probably 4-6' sections would be easiest to prop / reroot. Cut with a hygenic knife / saw, set the sections aside to callous for a few days. Once the ends are dry, prop them up in pots with the cut end just into a thinnish layer of soil (and this soil should be porous). Heavily mist / fine spray the cuttings planted end every few days. If you can place the cuttings' pots on warm cement so much the better - the cut end will root faster. Repot / plant into the ground when the cuttings are rooted (should be in about 2 months, if that).

I have many San Pedro, some much larger and in much smaller pots so I dont think planting it in the ground is the issue. Suggest you use what we call in the UK John Innes type compost. This has sand and is low in humus content. It drains quickly which is key with cacti. As for the brown scar I suspect others are correct when they say is is sunburn. I have it on some of my plants alos- I think we sometimes overestimates cacti love of sunshine. As for the rock in your garden- these plant typically thrive in denuded soil but by the look of the soil you have used I would suggest repotting with sharp sand mixed in for added drainage. Of course a little cacti fertiliser that is low in nitogen would help. Good luck! As for cutting the tip off and starting again- that way you can avoid the scarring and the base will srout new clean shoot also. Just remember to let the section you cut off scar (callous) for a week or two before you place it in soil- refrain from watering the cutting until roots appear. I could send you a link to my site but understand that it is forbidden on here- there is information on soil mixes there. Good Luck!

I have seen this on some of my plants. I am convinced it is a fungus (although it could be a virus or possibly even normal although unsightly for some species). It will manifest when the plant is under stress, ill, or sometimes during dormancy. Keep the roots healthy. Do not over water. Soil should dry out completely between waterings. Give it lots of light. Be careful of too much direct sunlight as this can also cause burning and stress. Keep an eye for pests (and root mealies). Keep it mostly dry in winter in a cool place. It should do fine. Waz

I have a number of rooted, seed grown san pedros, about 6-8" or more, all decent looking specimens. I'm going to guess about 5 years old. They are just beginning to take off, rapid growth! Interested in trading for what have you. These are somewhat "rare" to an extent so hopefully you have something similar? Looking particularly for hardy edibly plants (fruiting or perennial veggies), hardy water lilies, MAYBE rooted Brugmansias, rooted figs, etc, things along those lines? Thanks!