Very few people know the benefits of using charcoal and even fewer have a positive word to say.
Charcoal is by far one of the most important components in a Potting mix. Good quality, hardwood charcoal is rare and many nurseries have none for sale. It is “good business” to recommend something “that is better”. Bad quality charcoal, made from soft pinewood, timber off cuts or remains from a fireplace, are not a good alternative and should not be tried.
Charcoal is the product of slow burning natural wood in the absence of oxygen to prevent combustion. The burning occurs at temperatures of 400°C – 500°C (750°F – 930°F). Coal is not Charcoal and cannot be used in its place.
The motivation for using charcoal in agriculture for soil correction follows the discovery in the Amazon of "Terra Preta" or Dark Earth (Black Soil), the most fertile soil known to man. These “Black Soils" contain charcoal many times over that of the surrounding areas, a fact that explains the high quality of vegetation and soil fertility in the area.
Charcoal is added in to potting mixes for two main reasons:
a. to keep the "mix" sweet.
b. to filter impurities.
Laboratory tests show that charcoal possesses a remarkable ability to:
1. Keep nutrients in the soil and that way increase soil fertility.
2. Prevent decay and can endure in soil for thousands of years.
3. It is inert.
4. Allow the flow of air through the potting mix.
5. Retain moisture and help in good drainage.
6. Retains (adsorbs) excess nutrients (fertilizer), and release them later when needed.
7. Helps to increase the soil pH.
Charcoal has a very long “shelf life” in soil. That explains the high productivity of charcoal enriched soils. Charcoal, being absorbent, accumulates minerals salts (heavy metals) that can cause damage to the root system of the plant, causing the burning of the root tips.
Some "experts" recommend to growers using charcoal, to re-pot the plants on a regular basis in order to avoid root damage and plant poisoning; however, orchid growers use rain water or tab water (water suitable for human consumption) for watering, both types of water are free of any salts. Fertilizers used contain only traces, if any, of heavy metals or salts. Charcoal does not harm plants, on the contrary.
Charcoal has the tendency to bind toxins from the metabolism (breakdown) of the organic components of the potting mix, for example pine bark, resulting in doubling the life of some potting mixes. For example: Sphagnum moss has normally a life span of 6 months; however, with 10% charcoal added, the life span increases to over one year. Pine bark has a life span of about 12 months and again with charcoal added, the life span can increase to two years.
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Another opinion Discussion Orchworks
Hardwood charcoal has some qualities that work for the above. It is slow to decompose, helps drainage, provides some air circulation.
Most potting materials for orchids do not provide any nutrients. We add the nutrients in the water via the fertilizers we use and that’s how they are made available to the plants.
I am sure charcoal does bind some salts but hardwood charcoal does it less readily than activated charcoal. It also may also absorb some of the toxins from the breakdown of the organic material in the potting mix and those in the water. That is a good thing. I had seen one article, from a taxonomist, that some toxins are released from an orchids roots, which will be absorbed by the charcoal.
I don't think it needs to be used but there are many pros for it to be in a potting mix.
And worth a good read