A water-soluble fertilizer which includes iron compounds and which is intended for acidic soil-loving plants is best for gardenias.
Plants need water in order to take in fertilizer. Otherwise, the nutrients will remain in the soil and "burn" gardenia roots.
Gardenias can be prone to yellowing from not getting enough iron so the iron compounds can prevent or treat this condition (chlorosis).
Also, gardenias need soil pHs which range between the slightly acidic 5.0 and 6.0/6.5 on a soil analysis chart. Otherwise, they may not be able to take in certain necessary nutrients -- such as iron -- from soil.