Bone meal used to be a very good fertilizer. That’s when it was made from cattle bones full of marrow that were ground whole into powder, giving a good quality fertilizer rich in nitrogen. But modern bone powder is no longer made in the same way. Now the marrow is extracted and then the bones are steamed to remove their nitrogen and most of their other minerals. The extracted products are used for purposes other than horticultural ones: for gelatin, glue, etc. What remains is reduced into powder and sold as an organic fertilizer under the name of bone meal.
Modern bone meal is quite a variable product and its NPK ratio changes according to the manufacturer: 0-12-0, 2-22-0, 4-10-0, etc. However, it always contains little or no nitrogen (first number), little or no potassium (third number), but a great deal of phosphorus (second number). However, the soil in most home gardens is already rich in phosphorus and it is almost never necessary to add more. Why then add a product to your plant’s soil that is already abundant?
This unused phosphorus ends up in our lakes, rivers and water tables where it is a major source of pollution. In recent years, several municipalities have banned the use of phosphorus-rich lawn fertilizers on their territory and fertilizer manufacturers were quick to adapt. All those 15-30-15 lawn fertilizers that they recommended so highly as recently as the early 2010s have now been replaced with low-phosphorous fertilizers. Indeed, many modern lawn fertilizers now proudly proclaim they contain no phosphorous whatsoever! That’s a radical change!
Fertilizer companies still market bone meal though, in spite of its high percentage of phosphorous, largely because it has such a long history that home gardeners trust it. Among others, they recommend adding bone meal to planting holes for seedlings and young plants, claiming bone meal “promotes rooting”. But the concept that a high percentage of phosphorus is necessary for rooting was disproved a long time ago. Yes, for good rooting, it takes some phosphorus in the soil, but also nitrogen, potassium, calcium and pretty much all the all other minerals used by plants for growth. Studies show no benefit whatsoever from the use of massive amounts of phosphorous. The era of “transplant fertilizers” (also called “plant starter fertilizers”), some with a NPK ratio as high as 10-52-10, should be long over, but since consumers haven’t yet assimilated that news, fertilizer manufacturers continue to promote them even though they know they are useless. And bone meal is one of the transplant fertilizers that don’t help transplants!
Another major downside to phosphorus-rich fertilizers like bone meal is that they negatively affect mycorrhizal symbiosis, the beneficial association between mycorrhizal fungi and the plants we grow. When the soil is too rich in phosphorus, beneficial fungi spores remain dormant and don’t fix themselves to the plant’s roots. So even if you decide to apply the bone meal to a plant, ideally you should reduce the dose recommended on the packaging by at least half. As to adding both mycorrhizae and bone meal together as you plant or sow, it’s downright wasteful: the mycorrhizae will simply not take.
And finally, a last problem with bone meal: it tends to attract vermin. If you apply it, it is not uncommon for animals (dogs, squirrels, skunks, raccoons, etc.) to dig your plants up.
I consider bone meal to be essentially useless as a fertilizer and it’s surprising to me that garden centers still recommend it so enthusiastically. If you want to enrich your soil, prefer compost or a good all-purpose organic fertilizer that is not too rich in phosphorus: the second number on the fertilizer label should be equal to or less than the first. However, if you already have bone meal on hand, don’t simply toss it in the trash: it would even more environmentally friendly to use in some way. You can still use it as a fertilizer, but at half the recommended dose. And on established plants, not seedlings or transplants, so as not to harm mycorrhizal fungi. Or add it to your compost pile where it will be diluted to a safe level.