Which of the following is NOT a method for applying fertilization to trees?

Crown thinning is primarily a pruning technique aimed at improving the structure and health of a tree rather than a method of fertilization. It involves selectively removing branches to reduce density and allow more light to penetrate the canopy, which can encourage growth and improve air circulation. Although this may have indirect benefits for the tree's overall health, it does not provide nutrients directly to the soil or the tree, which is the main purpose of fertilization methods.

On the other hand, drilling holes, foliar application, and injection are all recognized techniques for delivering nutrients to trees. Drilling holes can facilitate the direct application of fertilizer into the root zone, foliar application involves spraying nutrients directly onto the leaves for absorption, and injection allows for targeted nutrient delivery into the tree's vascular system. Each of these methods supplies essential nutrients to support tree growth and health, distinguishing them from crown thinning, which does not fulfill that role.