Sometimes we are tempted to address the weak soil food web by adding synthetic chemicals to the soil (fertilizers). While these can provide the needed nutrients, it is hard for us to predict the amount needed, the timing of the need, and the possible negative effects. Fertilization tends to throw the soil food web farther out of balance, may lead to salt build up in the root zone (a bad thing!), and may add to pollution in waterways when excess fertilizer moves into surface and subsurface waterways. Use fertilizers very lightly with ornamental trees! An over-fertilized tree may actually attract more pests and have more problems than one on a “lean diet”.
Do not fertilize newly planted trees. Let them get established for a couple of years before specifically fertilizing them. Unless there is some reason to push growth at an unnatural rate, don’t fertilize them at all. Decomposing woodchip mulch will provide slow-release nutrients. Trees in lawns get enough fertilizer from turf applications, so don’t apply any more. Trees do not generally need rich soils.