Q. I have two quite large Argentine Giant cactus, each with many pups they've offset from their base. They are both located on a West facing patio, one is in a large pot and the other is planted in native soil. The one in the pot always turns yellow on its south side this time each year. The one in the ground does not. The potted one has been in that pot for about seven years, and I generally give it fertilizer a couple times a year. I think I've even tried a bit of iron, though I don't really think that is the problem.
A. The usual reason for this cactus becoming a yellowish in color is sun damage, particularly if it only on the side facing the sun. Other possibilities could be the buildup of salts in the soil, lack of nutrients, root damage and cold damage. But the bottom line is stress.
Because the cactus has a restricted root system and has no ability to take water from deeper or surrounding soils, the plant may be more stressed in a container than the one planted in the ground. That's how I would view it.
The yellowing is resulting from stress. I think the plant in the container will stress more easily than the one in the ground. This cactus will turn yellow (side facing the sun) in a stressful microclimate when planted in the ground.
You should consider all of these as possibilities but I think it is most likely sun damage or bleaching of the plant tissue by intense sunlight. They will do a little bit better without so much intense sunlight, particularly the one in the container.
All that being said, make sure that the soil has adequate drainage and you are not watering too often so that you can eliminate root damage because of the soil kept too wet.
Choose a good fertilizer for it. I like Cactus Juice as a fertilizer for cacti and succulents. I've had a lot of good reports from people using this fertilizer on them.
When you water it next time make sure you add enough water so that a good 20% of the water applied comes out the bottom holes or moves past the roots to keep the salts flushed.