A bonemeal farm.
A bone meal farm can be created by connecting a farm of choice to a composter via a hopper or minecart with hopper, to produce bone meal.
The most efficient farm type to connect the bone meal farm with is a cactus farm, as this can generate a lot of bone meal per hour if the farm is big enough.
Most farms can be used to generate bone meal.
Examples
Composting spreader
The item of choice going into the farm can be funnelled into the chest on the top left (see picture). The items in the chest go into a minecart with hopper that travels over other hoppers into the composters. The bone meal emerges into the bottom chests.
Bonemeal gambling
Bonemeal gambling involves growing grass using bonemeal and shearing all the grass and flowers grown. When put into a composter, there is a 98% chance of regaining the original amount of bonemeal, and an 92% chance of regaining the original amount plus an additional layer.