Knowing how much water to give a cactus plant can be difficult. A dehydrated cactus will often turn a purple shade, and become soft. Later it shows a wrinkly appearance and the branches may droop. Over-watered cacti will also become soft and droop and they may develop root rot. Planting in cactus potting soil ensures your plant has proper drainage and watering more frequently may help to resolve dehydration. Repotting a root-bound plant to a larger pot can also help. If your plant has been over-watered, allow the soil to dry out. Cactus plants only require water when the top 2 inches of soil have dried out and require only enough water to allow some drainage from the holes at the base of the pot. Once water begins to drain from these holes, the cactus has enough moisture.