Fertilising trees refers to the practice of adding supplemental nutrients (chemical elements) required for normal growth and development.
You can’t ” feed” a tree, since trees produce their own sugar via photosynthesis, for which they use nutrients as part of the process.
A reasonably fertile soil will have enough nutrients to satisfy the requirements of growth on most established trees. Trees adjust their growth and development rates to the level of nutrients present in the soil, and will usually grow vigourously as long as the roots can continue to grow.
In nature, trees get nutrients from air, recycled organic matter, beneficial microbes, and soil minerals. In urban settings, the recycling of organic matter is often reduced, beneficial microbes may be minimal, and some minerals can be unavailable be- cause of the soil pH.
Lack of water and organic matter, and soil compaction often limit growth of urban trees much more than nutrient levels.
Infertile soils are most common in new building areas, both commercial and residential, where the ‘natural’ soil has been altered. Altered soils display poor physical and chemical properties for tree and shrub growth. Soil modifications, where required, along with proper fertilisation may improve nutrient uptake of plants.
Fertilising a tree can improve growth; however, if fertiliser is not applied wisely, it may adversely affect the tree. Mature trees making satisfactory growth may not require fertilisation.
Adding fertiliser, especially nitrogen, around stressed or root damaged trees can be harmful unless you have determined that the stress is due to nutrient deficiency.
Harm can occur when a tree in decline with low energy reserves attempts to incorporate nitrogen into cell components.
Since this process requires energy expenditure, reserves can be lowered further. This could lead to a further decline in health as trees with low energy reserves have reduced ability to deal with the effects of injury and pests.

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