Biochar is basically charcoal produced at a temperature above 660 degrees Fahrenheit with little or no oxygen. This process is all important to making good biochar, because the combination of high heat and low oxygen preserves the capillary structure of the original material. The result is a porous and highly stable form of carbon.
The material smoldered to produce biochar is organic matter from agricultural and forestry wastes. Examples of suitable organic materials include:
- Husks, shells, and pits.
- Cotton gin waste.
- Animal manure.
The most obvious difference between charcoal, activated charcoal, and biochar is the intended use of the product. Charcoal is fuel, activated charcoal is a filter, and biochar is a soil amendment. However, the methods used to produce each are different, resulting in a slightly different structure.
Some Historical Background
The peoples of the Amazon produced biochar before the arrival of Columbus by burning agricultural waste in pits or trenches and then covering the fire with soil to cause it to smolder. Earthworms would subsequently incorporate the biochar into the soil, resulting in particularly dark, fertile soils known as Terra Preta, or “dark soil.”
Biochar is a superb soil amendment in either fields or gardens. It can improve the nutrient-retaining properties of soil tremendously, thus preventing nutrient depletion and contamination of water sources.
Likewise, biochar can be added to the compost pile to prevent nutrient leaching and improve the quality of the final product. This amendment promotes microbial activity, which can in turn speed up the composting process.
In Australia, experiments have been conducted with using biochar and molasses as cattle fodder to improve digestion, thus reducing odor and increasing animal productivity.
Biochar can also be used as a substitute for coal in energy production.
Biochar has many benefits when added to garden soil:
- Loosens up heavy clay.
- Absorbs toxins such as heavy metals, preventing plants from taking them up.
- Aerates the soil.
- Provides habitat for beneficial microorganisms.
- Improves soil conductivity.
- Raises soil pH.
- Retains moisture.
- Retains water-soluble nutrients, thus reducing leaching.
Manufacture and Application
Biochar can be made in soil-filled pits as done by ancient peoples, but for greater control it is often manufactured with the aid of a special kiln, retort, stove, or reactor. Unfortunately, reactor machines can be quite expensive, while kilns must be replaced from time to time. Many people opt to smolder their own “biochar” in various closed containers instead, but it is important to note that this will result in a product more like activated charcoal. (Commercially produced biochar can be purchased, saving you a great deal of trouble.)
Before use, biochar is often inoculated to add nutrients and establish a community of microorganisms. One way this can be achieved is by soaking it in compost tea. Alternatively, you could mix it with manure or worm castings, or just use it in your compost pile and apply it that way.
To add biochar to the soil for the first time, spread up to half an inch on top of the soil and dig or till it in. Use lighter applications in the future, perhaps spreading a thin layer on top of the soil when fertilizing, mulching, or transplanting. You can also use biochar to cover seeds to the desired depth when planting.
Biochar obtained from low-quality sources, such as land contaminated with heavy metals, may contain toxins. Also, some materials (e.g., leaves) do not make good biochar, turning instead into a low-nutrient ash.
Inoculating or “charging” biochar before application is strongly recommended, because otherwise it will absorb nutrients from the surrounding soil, making them unavailable to plants.
Do not apply biochar to acid-loving plants, as it is far too alkaline.