Orchid Cactus

About Horticultural Charcoal


In gardening newsgroups & bulletin boards, the value of charcoal in potting soils often arises. One person suggests the use of aquarium filter charcoal, & another suggests the use of crushed barbecue charcoal, both of which ideas may have certain problems. Yet another window-gardener is convinced charcoal has some purifying value mixed into potting soil, another thinks it's a nutrient. Other gardeners thought charcoal scattered in the garden might repel cats & surely would repel slugs. The majority of gardeners wrongly believe charcoal's presence in or under the soil of a potted plant prevents odors.

Packagers of gardening products are themselves responsible for some of this type of misinformation, as a lot of gardeners incorporate as factual even the most irresponsible advertising copy.

Barbecue charcoal has binders added that are reportedly sufficiently toxic to plants that barbecue ashes are not recommended for composts or garden soil enrichment. So I would presume it would have the same problems crushed for potting soils. Horticultural charcoal is a cheap grade of natural charcoal, but there is no reason to go for the highest grade "aquarium" filtering carbon which costs ten times as much with no added benefit.

Filtering charcoal is not the same thing as horticulatural charcoal. Much of what is sold for aquariums is not a wood charcoal at all, but is made by a completely different process from bituminous coal, peat, lignite, hardwood, or animal bone, followed by further processing & chemical washes. It retains a great deal more water than does horticultural charcoal, & less oxygen. Unlike filtering carbon, horticultural charcoal is untreated & unprocessed, hence much more a "natural" product.

Of the more-or-less inert moisture-retaining potting soil ingredients, perlite, vermiculite, peat moss, grated bark, & charcoal, serve the same functions. Any of these, or mixtures thereof, can be excellent addition(s) to potting soils especially for ferns, african violets, pitcher plants, & others that are particularly sensitive to dryness.

The inert quality of these additions makes any of them especially good for pitcher plants or orchids, as opposed to ferns or African violets which also require a rich soil. Charcoal is sometimes used as a dominant component in home-made potting mediums for orchids & epiphytes, but reports are confused, some hobby growers claiming a great deal of charcoal harmed their plants, others swearing they had good outcomes. In tried-&-true recipes for orchid potting medium, charcoal is frequently a minor component, & there is more certainly nothing wrong with it in such mixes. However, consumers should understand that the numerous claims of amazing additional values beyond that of the bark or peat or perlite are baseless.

Any claims that charcoal has some benefit above oxygen- & moisture-retention in the soil are unfounded. Its one real value is for its porosity. If a gardener were planning to use it for any other reason, such other uses should be carefully re-assessed.

Any claims of value above that of sphagnum, bark, or perlite are either vendor bullshit or gardener mythology. It does NOT lower the possibility of odor-causing bacteria. It does NOT "absorb odors" or "improve drainage" when put in the bottom of pots before adding soil. It does NOT "retain Nitrogen for future use by your potted plants." It does NOT purify water by mixing it with soil. It is NOT a soil enrichment per se (though cheap charcoal may be as little as half carbon & the other half will be organic & break down in time; activated charcoal will not have much if any organic component).

Furthermore, it is NOT a good source of potash unless first burned, then it loses its porosity value. It does NOT "ward off plant diseases, parasites, insects, & slugs." It does NOT have a special purifying quality when layered into non-draining terrariums. It is NOT an anti-fungal agent equivalent to sulfer or copper. As a surface-spread it does NOT inhibit algae growth; indeed the opposite may be true: if it is an activated carbon, it will have some phosphate content from having been washed in phosphoric acid; this will leech from the product & increase algae growth. All these claims are made for charcoal without foundation.

Activated carbon charcoal sold in aquarium stores has an "activation" lifespan (in aquaria filters) of as little as 15 minutes & is by & large a "traditional product for rubes!" although for its brief period of activation it can indeed remove many impurities from air or water. It is rather expensive to adapt for potting soils when no "activation" value actually exists when it is mixed with soils. In aquariums that 15 minutes of "activation" requires water to be cycled through the filtering charcoal repeatedly to have its short period of effectiveness. You can't just add it to the gravel & expect some benefit, nor can it be added to potting soils with any serious expectation that it can filter out odors just sitting there. Bacteria live very happily in a soil mixture with charcoal, & whether it stinks or not depends on type of bacteria & overall condition & degree of sterilization of the soil.

During its brief activitation life, water filtered through activated carbon can be "softened" toward the acidic side of the pH scale. In hydroponic systems, it has a short-lived value as a water softener without requiring chemical softening. When adding chemicals to soften water, this can be harmful to certain sensitive plants & epiphytes due to an accumulation of chemical salts no less hazardous than the impurities in contaminated or hard water, so lowering the pH naturally with charcoal is significantly better. But this usefulness is not part of its value in soil mixtures; bark & peat are better natural acidifers in the potting medium per se.

Bear in mind that horticultural charcoal is not the same as filtering or activated carbon for hydroponic systems or aquariums or air filters. Activated or filtering carbon is made at very high temperatures then chemically processed, but horticultural charcoal is made at normal burning temperatures without further processing. This makes the claims that horticultural charcoal has filtering properties all the more manufacturer- & vendor-generated lie or exaggeration. Neither activated nor horticultural charcoal affects odors when mixed with soils, but horticultural wouldn't have a filtering capacity in any context whatsoever.

The "activated" type of charcoal is more porous & would retain more water but less oxygen. If water were cycled through it repeatedly for about 15 minutes (completely separate from any soil mixture), it could indeed absorb accumulative salts that may have dissolved in recycling water systems from fertilizers or pesticides or alkalinizing minerals. It has no added effect of this kind mixed right into the soil. Indeed, it would have the opposite effect of keeping the salts from washing through with normal watering.

Recently the Consumers Union issued a report on air filters that use activated carbon, & found that the majority of such filters on the market do not filter anything whatsoever from the air! This is again because the activation period of charcoal/carbon is quite short, & any good it might do it will do immediately, then no more.

A side-issue would be what to do with "spent" activated charcoal when one is changing an aquarium filter. This will no longer be quite as inert because it will be infused with minute particles from pond or aquarium water. It seems to me a waste to just toss this in the garbage, & it should cause no problem adding it to compost or garden in small amounts. Filtering charcoal from some sources might be contaminated by harmful salts & chemicals after use, but from aquariums the contaminant will be nitrates & phosphates which fertilize a garden. The charcoal itself will still have moisture-retentive value & encourage beneficial nitrogen-generating microorganisms. The charcoal itself does not break down over time as peat or shavings would, so one would not want to be adding endless amounts, no more than one would add endless amounts of lava-rock crumbs or pea-gravel, rendering soils decreasingly loamy.

The factors that might place charcoal higher than other porous potting soil conditioners are these: 1) It is "natural" & even woodland soils are exposed to charcoal after burn-through. 2) It is "black" & might look nice mixed with black pea-gravel or very black loam or rich black compost, whereas some artificial conditioners make soils black & white pokadot. 3) For the few sorts of plants which are often transplanted from pot to pot so that they always have a fresh potting medium, charcoal & other coarse ingredients shake out of the roots very easily to be easily discarded & completely replaced. And 4) in the future (but not presently) various grades of charcoal might be manufactured from metropolitan green-waste, with environmental benefits of green-waste recycling. So far this is being done experimentally to produce high grade filtering-charcoal only, because the process is too expensive to make cheap charcoal. At present, the charcoals on the market use more energy & generate more pollution than is true of bark particles or ground lava rock, so that it cannot be regarded as the most environmentally friendly of all possible choices, though in & of itself intert.

In general charcoal should be thought of only as a substitute for perlite with identical value, not more not less. Like perlite it also functions to retain oxygen in the soil, so might be marginally better than vermiculite, which holds more water in the soil but less oxygen than either perlite or charcoal. As a porous potting soil ingredient charcoal needs to be mixed in thoroughly, & recommendations to use it as a surface layer or in the bottom of pots adds no actual benefit.

If one has made the assessments & is pretty certain it'll be useful, do a google search with the term "horticultural charcoal" & that will bring up dozens of vendors, or call around to the local garden centers, bigger ones should have it, or any that also sell orchids or pitcher plants should have it.

See also these related articles:
Deadly Polymers In Your Garden
Fireplace Ashes for the Garden


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