Ask Mr. Smarty Plants

Barbara Medford answered a previous Mr. Smarty Plants question about fertilzing native trees in Texas and here is her response ... Our vote would be "none." One of the big reasons for using plants native to North America and to the area in which they are being grown is that those natives are already acclimated by millions of years to the soils, climate, temperatures and rainfall in their area. We always recommend that the soil be checked, especially in terms of drainage, as most trees do not do well with wet feet, and too much moisture without sufficient drainage can cause rots and fungi to move in. We do like to see compost or other organic matter worked into the soil in the area where a tree is going to be planted, and not just the hole. As those trees get bigger and the roots move farther out, good soil with the amendments will help them to continue to grow.

There are experts who recommend no amendments to the soil at all, because when the roots of the new tree begin to get out beyond their original hole, the real world, or the dirt thereof, could be a terrible shock. This is personal choice, but we like to give little trees a boost, partucularly in the alkaline clay soils of Central Texas.

Here is another take on that from a previous Mr. Smarty Plants answer: "A standard fertilizer should be fine. One landscaper I interviewed advised an 8-2-4 compost-based fertilizer, meaning 8% nitrogen, 2% phosphorus, and 4% potassium. Others weren't so specific as to proportions, saying only "standard" or "regular."
All my informants said that if the tree is a naturally occurring specimen that has survived on its own without human intervention and still looks good, nothing may be needed. However, if the tree is now surrounded by new construction that may have damaged its roots, a fertilizer with significant phosphorus for root growth will help it adjust. Significant phosphorus will also be needed if your tree is a newly planted purchase that you want to encourage to spread its roots beyond its root ball.
All said that surface applications are best. Spread the fertilizer from near, but not on, the trunk, to a foot past the extent of the leaves. A couple of friends of mine have maintained beautiful live oaks for two decades with little more than compost applied two or three inches thick from near the base of the tree to just past the dripline. A deep, slow, soaking watering just after you fertilize will insure that the nutrients get to the roots. Doing it just before a good rain is even better. Whatever you use should be applied once or twice a year. The most important time to fertilize is in early spring, just before new foliage appears, to help fuel the new growth.  A second application in early summer can help fortify the tree during the harsh Texas heat. So, take your pick; the point in both of these answers is that Nature is already doing a good job, you can give Her a little boost if you wish.