A- "Keeping it Simple!"
Generally speaking compost and mulch will provide the tree with everything it needs.
Apply compost in March June, September.
A March/April application of addition organic fertilizer in the first 2 years OR if the trees are deficient in nutrients is a good idea
Look for good quality compost and/or a balanced fertilizer with micro-nutrients (see below for more). Because fertilizers vary in strength, you need to read the Label to find out how much to feed each tree.
Trees need to be well watered before fertilizing, so water them the day before or early on the day you're going to fertilize. The dripline is where the majority of the feeder roots are sucking up nutrients (not directly around the trunk).
First, push aside any mulch that is around the dripline of the tree. You want the compost / fertilizer on the soil. Second, spread the fertilizer in a circular band just within the radius where the branches and leaves end, not near or touching the trunk. Third, cultivate the fertilizer into the top few inches of soil (scratch it in with a toothed tool). Finally, water the entire area, to soak fertilizer into the soil and down to the roots. Replace the Mulch.
You've done it !
B- "More in Depth"
Just as we do best with a balanced diet of; Fat, Protein, and Carbohydrate, the Trees do best with a balanced diet of N (Nitrogen), P (Phosphorous), and K (Potassium). That's the N-P-K, and the numbers give the percent in the bag (or box) by weight. Just as we need Vitamins and Minerals the Trees need Macro and Micro Nutrients, such as; Iron, Zinc, Sulfur, Calcium etc. Many California soils are deficient in available Iron and Zinc
If you've got some yellow leaves, or leaves with yellow areas (chlorosis) you would do well to add some Micro-Nutrients. Particularly if using a general fertilizer of just N-P-K. If you're in a store where sales people are always, "busy" look for an Organic mixture with balanced NPK like 5-5-5.
C- "But I'm 'Certified' Organic!"
Compost does the trick here. Or if your trees need a boost make sure your fertilizer is OMRI approved.