Making Bonemeal Fertilizer

Tim Motis in Ethiopia asked us about grinding animal bones to make bonemeal fertilizer for increasing phosphorus levels in poor soils. Bonemeal fertilizer is produced commercially and at one time was much more widely used. Motis is interested in village-level production. The following is extracted from information sent to us after we asked VITA (Volunteers in Technical Assistance) for help and from the Food and Agriculture Organization book Animal By-Products: processing and utilization.

VITA sent an excerpt from a book published in 1947 called Commercial Fertilizers, their sources and use. Author Gilbeart Collins states that bones were used as fertilizer in England as early as 1653. “Their value as a fertilizer appears to have been recognized in England much earlier than in any other country…. During the latter part of the 19th century deposits of bone were sought, particularly by the English, and collected from all parts of the world for use as fertilizer.” Even battlefields and catacombs were used. In early USA history, “large quantities of buffalo bones were collected from the western plains for making fertilizer.” “As virulent anthrax organisms have sometimes been discovered on old bones … many countries require a certificate of sterilization before bone may be imported.”

Processed bones may have been cooked, steamed, or treated with acid, or just been exposed to the elements for some time (desert bone). Any of these make grinding easier. Equipment for grinding can range from simple mortar-andpestle pounding to animal-powered grinding wheels to modern hammer or roller mills.

Green [untreated] bones are sometimes ground and sold as ‘raw bonemeal.’ “The fatty materials found in raw bonemeal tend to delay the decomposition of the material when it is added to the soil. [Raw bonemeal] contains 2-4% nitrogen and 22-25% phosphate.” The raw bone contains elastic materials which make the grinding process considerably more difficult, though the protein they contain adds a bit of nitrogen to the final product.

Most commercial bonemeal is steamed. Bones are boiled or steamed at high pressure to remove the gelatinous material (used commercially to make gelatin and glue). Thus treated, they can be ground finer, making the phosphates more readily available. Bonemeal is superior to mineral phosphates in its crop-producing powers. Its effectiveness is increased by the modest nitrogen content and the various micronutrients it contains. The calcium salts (lime) also present tend to reduce soil acidity.

“Bones are sometimes heated in a closed retort…. The residual charcoal is known as bone-black [and is] used to clarify sugar. It contains 30-35% phosphate and 10% carbon.”

So is it practical to make bonemeal at the farm or community level? Possibly. The FAO publication Animal By-Products: processing and utilization says that “a crude but effective method is to burn the bones and to use the meal so obtained either as a mineral livestock-feed supplement or as a phosphate fertilizer.” Both dry and fresh bones can be used though the process goes faster with older, dry bones.

“If the bones are only required for soil dressing, they can be piled directly over firewood or any other combustible material and fired. The charcoal and bones are collected together and poured into sacks.” “To obtain a clean product [as opposed the charcoal/bone mixture] is to erect some form of large grill from old piping, (or perhaps from old car springs or similar material), pile the bones on top and make a fire underneath.” The bars should be spaced close enough to prevent small bones from falling through, and should not be piled too high. They recommend a pile about one foot high (30.5 cm). The whole process will take from half to one hour. The bones are ready to be taken from the fire once they have become spongy and brittle.“

A variation on this method is "trench-firing”. A fire is built in a trench a minimum of 2 feet (30 cm) deep. The grid is laid across a shelf dug some 6 inches (15 cm) below ground level along the trench and the bones piled on top of the grid. “The advantages of this simple method are that large logs may be used for firing and that the heat is concentrated so that the required temperature is reached more quickly.”

The firing process achieves three aims: “(1) it sterilizes the bones; (2) it burns off all the fat, blood vessels, marrow etc.; (3) the 'calcined’ bones are so soft that they can be pounded easily with a pestle and mortar….” It can also be done with little equipment.

“The average analysis of several samples of bonemeal obtained in this way was as follows:

1. (dry bones) 15.5% phosphorus (equivilant to 35.5% P₂O₅), and 30.5% calcium (equivilant to 42.8% CaO).
2. (fresh bones with meat first stripped away) 15.2% phosphorus and 31.0% calcium.”

“The meal is equal to the best quality steamed bonemeal,” which is often unobtainable locally or imported at high prices even though bones may be freely obtainable.

Because older, dry bones have already lost a lot of water and organic substances, they do not lose as much weight upon burning and the yield is higher. One hundred pounds of dry bones should yield about 66 pounds of bonemeal. Fresh bones may yield about 33 pounds.

A junior-high student in the Ft. Myers area recently came to ECHO asking for an idea for a science fair project. We suggested he make some bonemeal and do a trial. He grew four containers of radishes one without phosphorus, one with phosphorus supplied by triple-super-phosphate, one with commercial bonemeal, and one with bonemeal he and his father made with a barbeque grill and mortar and pestle. The radishes grown with his preparation produced the best of the four.

It is easy to see why one might want to add bone-meal to soil as a fertilizer, but why feed it to animals? Many tropical and subtropical soils are “highly deficient in phosphorus. Pastures grown on such soil are low in phosphates, especially when the fully mature plants start to dry out. Animals grazed on such land have a low blood phosphorus level.” “The deterioration of livestock manifests itself by unthriftiness, lack of production, reduced fertility, poor calves, lack of resistance to parasitic infestation, losses in meat and milk. Because the appetite decreases proportionately to the decrease of phosphorus in the blood, the animal’s intake of protein is reduced.” Unfortunately, such losses of production are often attributed to droughts and diseases and rarely to phosphorus deficiency, which can easily be remedied by supplementation of phosphorus!

In extreme cases, called pica, “Animals suffering from lack of phosphorus have a depraved appetite. A craving to eat bones leads them to ingest putrid material which often contains toxins produced by the botulinus bacteria. In such cases, animals usually succumb to paralysis…. Healthy animals very rarely touch decomposed matter or bones. Two or three ounces of moistened bonemeal, spoon dosed, is sufficient to remedy phosphorus deficiency. It may also be given in troughs, as a lick in brick form, or mixed with salt and trace elements.”

You can also make your own cattle lick to overcome mineral deficiencies. “Bonemeal can be fed alone to cattle, but it is better to enrich it by addition of other trace elements which may be lacking in your particular area. In Kenya, very good results have been obtained from the following formula: 66 pounds of bonemeal, 33 pounds of red oxide salt (containing iron), 6 ounces of copper sulphate, 1/15 ounce of potassium or sodium iodide, and 1.5 ounces of cobalt nitrate or cobalt sulphate or cobalt chloride…. In countries where other trace deficiencies occur, different trace elements should be used.”

“The weighing of the trace element fraction and the initial mixing of such a small percentage is impractical in the field. Hence the trace elements for 100 pounds of mix should be weighed previously, thoroughly mixed with 1 pound of bonemeal and sealed in a small package. Then to each 66 pounds of bonemeal and 33 pounds of red oxide salt, there is added one such pack and the whole is mixed together….”