How to extract phosphorus from bone meal
When observing orange and pomelo trees during their flowering period, I noticed clear signs of recovery and regeneration in both the soil and the trees. At this stage, the plants require a relatively high amount of phosphorus (P) to bloom and bear fruit. To support their growth and ensure successful fruiting, I extracted phosphorus from fish bone meal to fertilize the plants. Dried fish bones contain about 20% P (which is 45.8% PO4³⁻) and around 25% calcium in the form of hydroxyapatite, a substance that plants cannot absorb directly. I chose fish bone meal as a phosphorus source because it's a by-product of the fish processing industry, which is readily available in Vietnam.
There are two main methods to release P and Ca from animal bones:
Use microorganisms to decompose the bones slowly and release minerals gradually, or
Use acid to release minerals more quickly.
If you choose method 1, place the bone meal in a compost pile or ferment it in a barrel with water (hydrolysis), but this takes at least 3–6 months for the bones to break down and release the minerals. If you want to apply phosphorus to your plants immediately in an absorbable form, method 2 is recommended. Here's how to do it:
1. Transforming the Bone Meal
To extract phosphorus from bones, you first need to convert the bones into a form that reacts more easily with acid by heating the bone meal at 400–700°C in an oxygen-free environment (called pyrolysis). Under these conditions, all components except P and Ca vaporize into the atmosphere, leaving behind pure hydroxyapatite. This process also changes the structure of the bone, increasing surface area, making it more reactive with acid.
There are two relatively simple methods for this:
Bury the bone meal in a pile of rice husks and char it using traditional methods.
Place the bones in a metal (iron/stainless steel) container and burn them in a wood or charcoal furnace.
My family has a dedicated iron drum for making biochar that can reach 500–800°C, which is ideal for transforming bone meal. I use this biochar-making process to convert the bone meal. I place the bone meal into an old steel cookie tin, drill a hole in the lid for gases to escape, and put it in the furnace along with firewood. This method is also convenient for collecting the bone meal after burning.
Once ready, start the furnace to make biochar. When the char is ready, remove the bone tin and let it cool (I’ll write a separate article on how to make biochar). When the tin is cool, open the lid—you’ll see the bone meal has turned dark brown due to pyrolysis. In this batch, a few pieces in the center weren't fully transformed (still light-colored), as I didn’t maintain the heat long enough. To ensure full conversion, keep the furnace burning for about 1 hour at high temperature.
2. Reacting Bone Meal with Acid
Reacting the bone meal with acid will release the minerals from the pyrolyzed bones so plants can absorb them more easily. The result depends on the type of acid used. This article only discusses a common acid found in vinegar: acetic acid. When hydroxyapatite reacts with acetic acid, it dissolves and creates a solution containing calcium acetate and phosphoric acid. Both substances can be absorbed by plant roots and leaves and are vital for fruit development.
In this step, mix the converted bone meal with white vinegar at a 1:5 ratio in a glass or plastic container (avoid metal containers). The reaction begins immediately when the vinegar contacts the hydroxyapatite, releasing CO₂ bubbles. Store the container in a cool, ventilated place for 1–2 days and stir it twice a day. After that, you can use the solution as fertilizer.
3. Fertilizing the Plants
To fertilize, dilute the solution with water:
For root watering: 1:50 ratio with water
For foliar spray: 1:100 ratio (more diluted for young plants)
Apply once a month from fruit setting until harvest to supply phosphorus and calcium to the plants. Five kilograms of dried bone meal will yield about 20 liters of solution, which is enough for one hectare of fruit trees.
Note: This solution is relatively acidic (pH 3–5 depending on your method), so avoid overuse, especially for root applications. Start with small doses and increase gradually if needed.
Good luck and happy gardening!
