I'm sure someone will have a different opinion, but I don't think you need to fertilize them at all. Grow them in their native soil; that's what they're best adapted to. They grow fairly quickly, faster growth in trees can make them top heavy and prone to breakage and uprooting in high winds with more top growth than root growth.

HTH, good luck!

Those tree pots aren't big enough to hold the root system of a 2-3 foot mesquite seedling. They would be root-bound and unlikely to survive. Plant them out as soon as the roots are nicely holding the soil in the pots together, or move them up to a larger (and therefore harder to plant) pot.

The 6-inch mesquite sprouts I pull in the back yard have a 12-inch tap root.

Yes, just use native soil. They will be stronger if you do that.

Cool. Thank you Lazygardens for the heads up on the tap root. I knew they grew quickly (that's why I bought the tree pots), but I wasn't sure how quickly.