Need Help with My Avacado Tree

Hello Everyone,
I was wondering if anyone had any advice on a Avocado tree that I am growing. Right now it is about a year old and stands 4 feet or so high. In the last few months however I have been having problems with it's leaves turning brown. Information I have read from web sites suggest that it is getting to much water, but I only water it once a week if that.
In a effort to fix the problem, I moved it to a bigger pot (not such how big but at least 30 gallons or more) and changed out the oil from a Potting soil to something called Cactus soil, apparently this soil does not hold the water as much and is better for Avocado trees (Or so I read online)
Any help would be greatly appreciated
Bob Shafer
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