Is Bone Meal Good for the Garden?

Bone meal, or bonemeal, is a common organic soil amendment and many consider it to be a good fertilizer. It is a “must add” when planting spring bulbs and it’s claimed to help plants establish after being moved by improving root growth. Plant not flowering? Add bone meal to make more flowers.

This sounds like a great product every gardener should use, but are the claims real or just myths?

What is Bone Meal?

In the early part of the 1800s agricultural soil was being heavily cropped and yields were dropping. To increase nutrients “some farmers collected bones (horse and human) from major battlefields like Waterloo and Austerlitz to crush them and fertilize the soil.” Through most of the 19th century commercial trade in bone meal was seen as an essential part of agriculture.

Originally bone meal was made by crushing complete bones containing marrow, tendons, and bits of meat and fat. These extra bits added important nitrogen to the product but modern day bone meal is steamed at high temperatures to remove most of the nitrogen so that it can be used to make other products like gelatin.

Towards the end of the 19 th century it became obvious that bone meal was not making a significant difference in yield and agriculture switched to using synthetic superphosphate which was made by digesting bones with acids.

The NPK of agricultural bone meal varies with nitrogen being between 0 and 4% and phosphorus being 8-15%. It also contains about 12% calcium, 0.3% potassium, 0.4% sulfur, 0.5% magnesium as well as some heavy metals (zinc, copper, lead, cadmium, nickel, mercury and chromium).

Is Bone Meal a Good Fertilizer?

Fertilizer is used to replace the nutrients missing from soil. If you need potassium, this is a terrible fertilizer because it contains very little potassium. The nutrient most often in low supply is nitrogen and bone meal is a poor source of that. Most soil has enough calcium, but if your soil is deficient, bone meal is a good calcium source.

Bone meal does have a fairly high level of phosphorus (P), but again, most garden soil has enough P.

It is only a good fertilizer if your soil needs calcium or phosphorus.

Phosphorus in Soil

Many gardening sources talk about phosphorus being good for plants and discuss ways of adding it to soil but almost none discuss what happens when phosphorus is added to soil and that is crucial to understanding the use of fertilizers.

For a complete review of this topic see my book Soil Science for Gardeners.

The free phosphorus in soil exists in three different forms.

• Soluble P is dissolved in the soil solution and is available in very small quantities.
• Labile P is held loosely by soil particles.
• Stable P makes up the majority of free phosphorus and is held strongly by soil particles.

Plants can access the soluble and labile P through their root system, but they mostly use the soluble form. As they use up soluble P, some of the labile P is converted to soluble P so that there is always some available to plants.

Bone meal is mostly non-soluble phosphorus and adding it to soil increases the amount of labile and stable P. It has limited effect on soluble P.

Movement of Phosphorus in Soil

If you sprinkle some phosphorus on the surface of soil, how quickly does it reach plant roots?

The nitrogen in fertilizer moves through soil very quickly and reaches roots in a matter of hours or days, depending on water flow. Potassium also moves quickly in soil but its slower then nitrogen.

Phosphorus moves at glacial speeds in the order of a few mm (a fraction of an inch) a year. Adding soluble phosphorus fertilizer to the surface of soil will take years to reach roots. Adding bone meal this way is even slower. Unless you are planning for 10 years down the road, adding it on the surface is a waste of time.

Bone Meal and Soil pH

Bone meal added to alkaline soil remains in an insoluble form (stable P), that will not benefit plants.

In acidic soil, bone meal is slowly converted to soluble phosphorus. If the soil is deficient in P this is a benefit to plants but if the level of P is high, then this soluble P is quickly converted to labile P and then to stable P, where it is of no benefit to plants.

Very acidic soil can have a different problem. At pH values below 6, more iron and aluminum are available and they combine with phosphorus to form insoluble phosphate compounds, thereby reducing the P available to plants.

Bone Meal and Mycorrhizal Fungi

Soil contains a limited amount of soluble P and plants have difficulty getting enough of it. To solve this problem, most plants form a symbiotic relationship with mycorrhizal fungi. Fungi are better at extracting phosphorus out of soil and they transport that to plant roots in exchange for sugars.

What happens when you add too much bone meal to soil? Nothing happens in alkaline or neutral soil because the phosphorus doesn’t become soluble. In acidic soil, bone meal can increase phosphorus to high levels. Now plants have easy access to P and so they stop making connections with fungi. The fungi loose their carbon source (sugar) and they die off. This is one reason why it makes no sense adding commercial mycorrhizal fungi to fertilized soil.

Does Soil Need More Phosphorus?

Phosphorus is routinely added to agricultural soil to increase yield. This has convinced gardeners that they need to add phosphorus to their soil, but that is rarely the case, unless the soil is very sandy, and in particular acidic sandy soil. Most other garden soil does not benefit from added phosphorus.

The only way to know if you have a phosphorus deficiency is to have the soil tested.

Does Bone Meal Increase Root Growth?

You see this claim all the time in gardening circles but it’s a myth.

Plants need all the essential nutrients to grow leaves, stems, flowers, fruits and even roots. One nutrient is not more important than another, but they are used in different amounts. If a nutrient like phosphorus is too low, all parts of the plant will suffer. In this case, adding phosphorus will result in more growth – in all parts of the plant, not just the roots.

At higher levels of phosphorus and adequate levels of everything else, roots actually grow less because they are getting the nutrients they need.

Bone meal should not be added to soil in the hopes of growing more roots.

Is Bone Meal a Bloom Booster?

It is a common myth that fertilizer containing a high level of P will increase the number of blooms. Many such “Bloom Booster” products, including bone meal, are marketed for more blooms but they just don’t work for the same reason it does not promote more roots.

Do Spring Bulbs Need Bone Meal?

It is common to add bone meal into the planting hole for bulbs. Is this needed? Is it a good idea?

By now you know that most garden soil has enough phosphorus and calcium, in which case bone meal adds no value. If these nutrients are needed, a synthetic phosphorus, like superphosphate will release P much faster. Calcium is better added in other forms such as lime.

Another reason for not using bone meal is that it attracts animals (rats, dogs, squirrels, skunks, raccoons, etc.) which will dig up spring bulbs to get at the bone meal. I have never added bone meal to bulbs and I have never had problems with rodents digging them up, but to be honest I have not been able to find any scientific studies to support the claim that animals dig up bulbs treated with bone meal. Several people have reported that their dog loves eating the stuff.

Best Way to Use Bone Meal in the Garden

Simple – don’t use it.

Most soil does not need more phosphorus or calcium. In neutral or alkaline soil it will not increase the amount of plant available P. If you have very sandy soil and need phosphorus or calcium, there are better options.

Excess phosphorus causes environmental pollution problems when it seeps into rivers and lakes.

Bone meal does not produce more blooms or better root growth.

It is surprising to me that gardeners still buy this product! I am sure sales will drop as soon as I press “publish”!