Let's get to the root of the problem before making any recommendations. Gently slip your cactus out of the pot and examine its roots. If they are dark brown or black and their skin slips off easily, suspect root rot. Trim off the diseased roots. Avoid the temptation to irrigate your entire collection at each watering. I expect that the smaller cacti growing in smaller pots would require more frequent watering, and, hence, have benefited from the attention. However, the larger cacti would prefer to be watered less often and only when the soil indicates a need. In the fall when shoot growth slows down, allow the soil surface to dry between waterings to reduce the chances for root rot to occur.

If the plant is simply pot-bound, the container will be filled with roots. Some may even be spiraling around the outside of the root-ball. Cut them with a sharp knife to prevent them from girdling the plant. Also, make shallow cuts on opposite sides of the root-ball and along the bottom to free the congested roots.

In either case, repot the cactus into a clean pot with well-drained media, such as two parts of a standard potting mix and one part each of peat moss and perlite or coarse, sharp sand. If you had to root-prune, monitor the plant closely over the next several weeks to help it produce new roots quickly and overcome transplant shock. Water thoroughly, but do not fertilize for at least a month. Thereafter, feed monthly with a balanced liquid houseplant fertilizer from early spring through early fall.

If after dislodging your cactus from the pot you find white, healthy roots dotting the walls of the root-ball, I would suspect that the leaf-yellowing was caused by a lack of fertility. If your cactus was growing in the same pot for several years without being fertilized or receiving an annual topdressing of compost or fresh media, the soil could have been exhausted of nutrients. Repotting the cactus and following the fertilization program as described above should improve the health and appearance of your plant.

When your cactus stops flowering, allow the top half-inch of soil to dry out before watering. Resume regular watering and fertilizing when new stem growth begins in the spring. For best growth, your cactus should receive bright, but not direct, sunlight. A location within 3 feet of a large window facing south, east, or west is ideal.