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. A number of other species show reduced vigor and health. Do not plant susceptible species on soils with a high pH. Acidify the soil. Best results have often come with a mix of a fast acidifier, a slow acidifier, and organic matter. Repeat as necessary .