I am planning on starting a new garden. I'm a complete newbie to the process and feeling a little overwhelmed.

I actually have a few questions...

I think we are going to try the square foot method but I'm a little unsure about the 'Mel's Mix' soil. I've read that the mix can have issues retaining water and I live in Phoenix where it's very hot and arid. I'm not entirely sure what our soil is here but I think it's mostly decomposed granite. Should I plan to mix some in to hold the soil together or is there another type of soil I should add in to help?

I've also read that coconut coir can be used in place of peat moss. We have a russian tortoise whose reptarium substrate is made entirely of coconut coir. He eats nothing but leafy greens and we clean out his bio waste every few days. Once per year we replace his substrate so I'm wondering if we could use the old coir in our garden? Would it be safe? I don't think there's enough to completely replace the need for peat moss but we could probably reduce it?

Any comments are greatly appreciated.

Thanks!