I bone mealed ten azalea saplings and it took an average of 1.8 bone meal to grow it up, and ten oak saplings, it took an average of 5.8 bone meal. This means that if you have bone meal and oak or azalea saplings, it is easier to just use azalea saplings because it uses so much less bone meal.

Steps to reproduce

1, Bone meal several oak saplings

Notice they take a medium amount of bone meal to grow

2. Bone meal several azalea saplings

Notice they take very little bone meal to grow (usually growing with the 1st bone meal)