栀子花最适合使用含磷、含钾的肥料，因为磷和钾对于花的生长和开花具有重要作用。可以选择使用有机肥料如磷酸二铵、磷酸三铵或磷酸二钾等，也可以选择配合肥料中含有磷和钾元素的化学肥料。英文回答是：Phosphorus and potassium-rich fertilizers are most suitable for gardenias, as these nutrients are important for their growth and flowering. Organic fertilizers like diammonium phosphate, ammonium phosphate, or dipotassium phosphate can be used, or chemical fertilizers that contain phosphorus and potassium can also be selected.