Yellow leaves on Fatsia japonica?

Please can you advise me on how to get my 6 year old Fatsia Japonica, which is planted oudoors in light shade, to shed it’s yellowing leaves and grow lush, shiny green ones as does another I have growing in a container.The soil is heavy, clay and alkaline in nature.Thid is an ongoing challange and feeding with either general fertiliser or ericacious ( ’twas worth a try!) feed has proved mediocre at best.