Neither, its not bothered, but I will point out that Castor Oil Plant is actually Ricinus, and is a houseplant or half hardy annual outside; Fatsia japonica, despite often being referred to as castor oil plant, is not related to Ricinus in any way, and is hardy outdoors. If your plant is a Fatsia, and its outside, just give it a topdressing of something like Growmore granules, raked or turned into the soil at the base. If it's in a pot outside, something like Miracle Gro general purpose every 4 weeks until June, but not after.

Fatsia japonica is a tough plant that doesn't normally need any special feeding at all. Just keep it watered if you are worried about it. Or a general fertiliser if you feel you must feed it but , as I say, I don't think it is necessary.

Bamboo

The very point I was going to make. Fatsia japonica is false castor oil plant but it seems to have lost the 'false' a lot of the time.

Thanks for all your answers - very helpful.