Bone meal used to be a quite decent 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 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 type of bone used and the manufacturing process: 0-12-0, 2-22-0, 4-10-0, etc. However, it always contains little or no nitrogen (first number), no potassium (third number) and 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?
Add to that fact the one that the type of phosphorus found in bone meal is pretty much insoluble except in very acid soils and thus largely unavailable to plants. What use comes of adding to your garden phosphorus plants can’t readily absorb.
Fertilizer companies still market bone meal though, in spite of its high percentage of nearly insoluble phosphorus, 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 other minerals used by plants for growth. Studies show no benefit whatsoever from the use of massive amounts of phosphorus.
The claim that bone meal stimulates bloom (another common claim) is just as preposterous. The phosphorus in bone meal that is supposed to encourage bloom is simply not available to plant roots.
Not Good for Beneficial Fungi
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. In soils acid enough that the phosphorus provided by bone meal is available, applying the product at the recommended rate prevents the mycorrhization from taking effect: the beneficial fungi spores remain dormant and simply don’t attach 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, that’s downright wasteful: the mycorrhizae will simply not take.
And finally, a last problem with bone meal: it tends to attract vermin. Humans may not notice it, but it has a bit of a dead animal smell that other animals do pick up. If you apply it, it’s not uncommon for animals (rats, dogs, squirrels, skunks, raccoons, etc.) to dig your plants up.
Pretty Much Useless
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 always be less than the first (nitrogen) or at least, nearly equal.
However, if you already have bone meal on hand, don’t simply toss it in the trash: it would even more environmentally friendly to find some way to use it up. You can still use it as a fertilizer, but at half the recommended dose. And only on established plants, not seedlings or transplants, so as not to harm mycorrhizal fungi associations. Even better: add it to your compost pile where it will be diluted to a safe level … and where microbes will aid in rendering the phosphorus more soluble.
Bone meal: inexpensive, but pretty much useless!