Zinc is actually relatively easy to get in the tree compared to all the other metals. John Kempf has often said that is why Zinc is the micro nutrient most often cited as helping plant growth. Has to do with the oxidation state being able to be taken up by plants unchelated, so people see results. I would suggest again neutral zinc would be a better fit for young trees as the rate of uptake is slower over time. If you can knock all the leaves of a mature tree with a spray of zinc sulfate a young tree won’t have much of a chance.