Iron and manganese are the most important for ornamental trees. Crop trees have special needs not discussed here. Different micronutrients have different functions. Many commonly affect specific cellular reactions such as chlorophyll synthesis, nitrogen fixation, and respiration. With extreme deficiency, the leaves become completely yellow.
Conifer needles turn yellow-green.
Also, manganese is now recognized to be very important. Do not plant susceptible species on soils with a high pH. Improve the drainage. In many cases, significant improvement can be made by simply reducing soil saturation. Acidify the soil. Best results have often come with a mix of a fast acidifier, a slow acidifier, and organic matter.