Good potting mix for Citrus + fertilizer?
I got 2 Citrus trees (Eureka Lemon and Persian Lime) potted in a 15 gallon pot each. Have had them both for 4-5 years, Lemon is about 4 foot while the Lime is a bushy 2 1/2 feet. Winter of '12-'13 I lightly pruned both and they responded well to that. I have no problems with the lime. However, the lemon runs into problems every now and then. One year in June it got yellow and dropped all its flowers and fruits. I started giving it Epsom salt and Azomite and it seemed to fix the problem. But it has this yellowing problem from time to time, but no fruit drop. Right now some of its leaves are starting to yellow. Some of the leaves have yellow veins, while others are just yellow. The Lime has no issues.
I think what this comes down to is the potting soil. I potted the lemon at a time I knew very little about Citrus and the mix I used was mostly wood chips with some compost I bought at a nursery. So I want to change out the potting mix. So my question is: What would be a good mix for citrus?
Materials I have on hand:
Would azomite be sufficient enough for providing trace minerals to Citrus? I was looking at some 10% chelated iron, but azomite has like 67 trace minerals and about 1.67% iron in it. Also, anyone care to share their fertilizing regimen.