Answer: If you look around the desert you'll see tons of cactus that get along without any care at all. They grow in full sun, in the native soil, and only get watered during the monsoon season. So, I'd eliminate the possibility of too much sun or lack of minerals in the soil as reasons for your cacti to die. That brings us to watering, and that's something you can control. The root systems on cacti are small compared to the aboveground parts. This is because the roots act like straws, drawing up available water for storage in the cactus. As it needs the water for cell growth, the cactus uses what it has in storage. Depending upon the size of the cactus, this water can last weeks or even months before it needs to be replenished. There are no hard and fast rules but a generally accepted practice is to water cactus deeply every 10-14 days during the months of June, July and August and once a month in September, October, March and April. These are the months of growth for your cactus and that's when they'll require water.
Sometimes you can tell by looking at a cactus if it needs watering. Those with ribs like an accordian will begin to shrink together when they need water. Watch your cactus carefully and I'll bet you'll be able to tell when they're thirsty.
Good luck with your cactus!
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