If any of your tree leaves or shrubs turn yellow before frost, it means they need iron. There is an excellent product on the market called Ironite. Pour this product around the base of your tree or shrub and water. We used this on our cottonwood trees, their leaves turned the darkest shade of green within a week or so.
Trees, like all plants, grow best when given appropriate nutrients. Fertilizers are a great way to ensure your trees get all the nutrients they need. This is a page about the basics of fertilizing trees.