I'm still trying to get a rubber sapling ... any clue ? As I don't have any slime balls yet.
I use bones > soup, or actually to bonemeal the melon seeds I got. Chopped fruit salad is just as good. I also got a nice wheat farm going as well.
keep sifting dirt, or add milk to witch water (IIRC)
once the melon stem is grown you can't use more bonemeal on it