Joshua trees are a desert dwelling plant. They need a fast draining soil. They have basically two sets of roots. One that takes up water and nutrients as do most plants. And the second set stores extra water in case it has to go through a drought. Almost like a bulb stores moisture or the trunk of a palm tree. That is why it needs such a large pot to establish those roots. Unless you let it sit in water or water too often you should not have a root rot problem. After your plant is established, you need to let it dry out a bit between watering. And a sandy cactus mix is the best soil for this one.
Drama Free Zone.
What every gardener loves the most, Begins and ends in rich compost. (Tina)