Fertilizing trees refers to the practice of supplementing soil nutrients with some of the 17 chemical elements required for growth and development.
Most soils have enough nutrients for most trees.
Trees adjust their growth and development rates to the level of nutrients, and will usually make out all right as long as the roots can continue to grow. In nature, trees get nutrients from air, organic matter, and soil minerals. Organic matter can be restored for urban trees by keeping a layer of composted leaves and twigs under the crown. The factsheet Micronutrients discusses this topic in detail. Lack of water and compacted soil have been shown to be much more limiting to the growth of urban trees than low nutrient levels in most cases.