Can you compost bones? The answer is conditionally yes, but it requires specific conditions and considerable effort far beyond tossing fruit and vegetable scraps into a bin. Composting is the controlled biological decomposition of organic materials by microorganisms, resulting in a dark, nutrient-rich soil amendment. However, the general consensus for most backyard, low-heat composting systems is to avoid adding bones, especially larger ones. This reluctance stems from the bone’s dense structure and the difficulty of achieving the high heat and specific microbial activity needed for successful decomposition in a home setting.
Practical Difficulties in Standard Composting
The primary reason bones are discouraged in standard home compost piles is the practical problems they create before decomposition occurs. Bones, particularly those with residual tissue, fat, or marrow, emit odors that attract unwelcome scavengers. Raccoons, rats, dogs, and other wildlife are easily lured by these smells, risking the compost pile being dug up and scattered. This disturbance compromises the composting process and can turn a garden feature into a nuisance.
The slow rate of breakdown also contributes to the odor problem. If organic residues like fat or meat remain on the bone, they undergo anaerobic putrefaction within the cool, oxygen-deprived sections of a compost pile. This process generates foul-smelling compounds like hydrogen sulfide (rotten-egg odor) and butyric acid (rancid smell). Even without scavenging, bones can remain intact for months or years in a cold pile, acting as a persistent source of these undesirable smells.
The Chemical Composition of Bone and Decomposition Rate
The scientific reason behind the bone’s resistance to decomposition lies in its unique dual composition. Bone is a composite material made up of an organic matrix and a dense mineral component. The organic part is primarily collagen, a tough, fibrous protein that provides flexibility. The mineral component is a crystalline form of calcium phosphate known as hydroxyapatite.
Hydroxyapatite is an extremely stable, hard mineral that forms the majority of bone mass. It is not easily broken down by the typical microbes found in a cool compost environment. The dense structure of this mineral matrix requires specific conditions, often involving high heat or strong acids, to dissolve and release nutrients like phosphorus and calcium. Compared to soft vegetable scraps, the mineralized bone structure offers minimal surface area for microbial colonization. This density means that even in an active compost pile, a bone can take six months to over two years to fully decompose, far longer than the few months needed for most other organic waste.
Pre-Processing Methods for Successful Bone Composting
For those determined to incorporate bones into their compost, pre-processing is mandatory to overcome the challenges of density and decomposition time. The first step involves maximizing the surface area of the bone material. This is achieved by crushing or pulverizing the bones into small fragments or a fine powder before adding them to the pile. Using a hammer, a heavy-duty grinder, or a mortar and pestle can significantly accelerate the breakdown process by exposing the interior structure to microbial action.
Another preparatory method is using heat to soften the bone and remove all residual organic matter, which prevents odor and pest attraction. Boiling the bones for up to ten hours, or using a pressure cooker for about three hours, helps dissolve fat and tissue. After cleaning, some composters recommend drying the bones thoroughly, or baking them, to make them brittle and easier to crush.
Hot Composting Requirement
The pre-processed bone fragments must be added to a hot composting system that consistently maintains temperatures above 135°F (57°C). This sustained heat is necessary to facilitate the breakdown of the dense organic and mineral components, a condition rarely met in a passive backyard bin.
Alternative Uses for Bone Material
If the intensive pre-processing and high-heat requirements for composting bones are too much effort, several alternative methods exist for utilizing the material’s nutrient content. One of the most common and effective uses is creating bone meal, a powerful organic fertilizer. Bone meal is bone material that has been thoroughly cleaned, dried, and ground into a fine powder.
This powder is rich in phosphorus, which supports strong root growth and flower development, and calcium, which is important for overall plant structure. Bone meal acts as a slow-release fertilizer, gradually delivering these minerals to the soil over several months. Rather than adding whole bones to the compost, the pulverized material can be worked directly into the soil when planting bulbs or vegetables, or mixed sparingly into potting soil for containers. For those who prefer to avoid processing, small, clean bones can be buried deep in the garden soil, where they will eventually decompose and release their nutrients over a longer timeframe.