I need to add some phosphorus to tomato seedlings. Steamed bone meal is relatively insoluble, I have heard, so sprinkling it on the top of the flats would not help. Would soaking it in water and then watering the plants with it help any? What about adding fish emulsion to the soaking water, or vinegar? Would these things help leach phosphorus out of the bone meal?
The fastest way to get phosphorous to plants is with phosphates, as in trisodium phosphate cleaners* or the phosphoric acid in Coca-Cola. Adding vinegar to the bone meal might help out, since the acid may help to free up some of the calcium phosphate in the bone:
If you are going to do this, soak your steamed bone meal in the vinegar overnight first, as the reaction is slow to proceed.
But as long as your soil is fairly acidic, e.g., has lots of organic matter and humus in it, those humic acids will make the phosphorous in the bone meal available to the plants. Mycorrhizal fungi also do that, and they will even transport the phosphorous from where they find it to the plant root that is hungry for it.
*But this isn't a good idea, because the sodium part of the molecule isn't very plant-friendly.