Horticultural charcoal
Also known as Charcoal, Biochar, Activated charcoal
In short
Horticultural charcoal is small pieces of charcoal added to orchid and some houseplant mixes; biochar is charcoal made specifically as a soil amendment. It is porous and stable, and in research biochar has partly or fully replaced standard container substrates, but a charcoal layer at the bottom of a pot does not improve drainage.
How it looks
Black, lightweight, brittle chunks or granules that leave black marks on the fingers. Horticultural grades are usually pea- to thumb-sized. Biochar looks similar and is made by heating biomass without oxygen (pyrolysis).
Signs
Charcoal can hold fertiliser salts on its surface, so a crust or salt damage can develop if pots are not flushed. Biochar is often alkaline and can raise pH.
What favours it
According to the AOS, charcoal does not absorb water into itself but holds it on its surface, and it absorbs fertiliser residues. Biochar is porous, retains water and water-soluble nutrients and can raise cation-exchange capacity.
Prevention
- Mix charcoal through the medium rather than as a bottom layer; a layer of coarse material at the bottom raises the saturated zone (perched water table).
- Flush charcoal-containing pots with plain water now and then to remove salts.
- Watch pH when using biochar, which is often alkaline.
Treatment
Start with the first step and move down only if the problem persists.
- Leach pots with plenty of plain water if salt build-up is suspected.
Pros and cons
Pros
- Stable; does not decompose like bark or moss.
- Porous, adding air and holding some water and nutrients.
- Mississippi State researchers grew chrysanthemums, tomatoes, lettuce and basil in mixes of biochar with pine bark or potting mix, and in biochar alone.
- Biochar can store carbon in soil for a long time.
Cons
- Can accumulate salts that need leaching.
- Biochar quality varies widely with feedstock and production temperature, and it can raise pH or salinity if overused.
- Does not improve drainage when used as a bottom layer.
Look-alikes
- Lava rock
- Lava rock is heavy stone that does not mark the fingers.
- Carbonised rice husk
- Charred rice husk keeps the shape of small boat-shaped hulls.
Typical uses
Orchid mixes, some aroid and terrarium mixes, and as a partial substrate component for container crops.
Related
Sources
- Beginner’s Series — Part 2: Media Mania Revisited — American Orchid Society
- Biochar — Wikipedia
- Using Biochar as Container Substrate for Plant Growth — Mississippi Agricultural and Forestry Experiment Station
- Gardening myths — University of Missouri Integrated Pest Management
- Repotting Your Orchid — Clemson Home & Garden Information Center
Written in our own words from these sources. Check a product label before using any treatment.