An iron supplement is effective for both trees with yellowing growth, and for
acid-loving trees such as Andromeda(pieris), Azalea, Camellia, Fir, Gardenia, Hawthorn, Hemlock, Holly,
Juniper, Oak, Pine, Rhododendron, Spruce and many more. If you have
a bonsai that prefers acid soil, check out the pH meters and/or combination
pH-moisture-light meter in the "Supplies" listing. If you have a
bonsai that has been yellowing, there's a good possibility that it's an iron
deficiency. In both cases, the following products may help.