Charcoal is the gardener’s secret weapon. Its special properties work wonders on soil quality. This guide explains how to use charcoal in your potting mixture.

The difference between charcoal, activated charcoal, and horticultural charcoal

Charcoal, activated charcoal, and horticultural charcoal have the same chemical structure. They are the carbon left over when wood or other organic material such as coconut husks is heated to high temperatures at low oxygen levels. However, these three types of charcoal differ in their physical properties because they are made in slightly different ways.

Activated charcoal usually comes in a crushed form. It has a light, crumbly texture Adrien Olichon https://unsplash.com/photos/z-WbjZ3a6GU

Activated Charcoal and Horticultural Charcoal

You may have seen activated charcoal for sale both at the gardening store and the drugstore. Activated charcoal used in medicine is essentially the same as horticultural charcoal. The difference between activated charcoal used for medicinal or horticultural purposes and the charcoal used on the barbeque grill is that activated charcoal has been burnt at higher temperatures. Activated charcoal is made by heating the organic matter to between 1112 and 1650 degrees Fahrenheit while grill charcoal is made at between 572 and 716 degrees Fahrenheit.

Unlike normal charcoal, activated charcoal has materials added to it during the manufacturing process. This additional material could be zinc chloride solution mixed with peat or sawdust. The zinc is then removed by washing it with dilute acid and water. This makes activated charcoal more porous than normal grill charcoal. For finer pores, the charcoal is then oxidized with gases (a process called pyrolysis), usually steam or more rarely carbon dioxide. A teaspoon of activated charcoal can contain the surface area of a football field.

 Grill charcoal does not have the porosity of activated charcoal because of a different manufacturing process. Commercial grill charcoal will have added chemicals that are not good for your plants.  Kirsty TG https://unsplash.com/photos/OTsKXDu2O_g

Grill Charcoal

Do not use grill charcoal for your plants. Grill charcoal contains added chemicals that improve burning, like sodium nitrate or limestone, which can damage your plants. Either buy horticultural charcoal or use activated charcoal from the drugstore or health food store without additives.

Biochar

Biochar is a form of horticultural charcoal produced when wood or other organic matter is roasted (not burned) at 660 degrees Fahrenheit. The wood or organic matter does not combust because it is either placed into an airtight container in the oven, or the air inside ovens is replaced with an inert gas such as nitrogen.

The pores in biochar retain water and nutrients. They also provide habitats for micro-organisms that help plant roots take up  nutrients and more easily.

A pile of biocharOregon Department of Forestry commons.wikimedia.org/w/index.php?search=charcoal+soil+&title=Special:MediaSearch&go=Go&type=image

Biochar helps sandy soils retain moisture and nutrients while it increases air pockets in clay soils with poor drainage.

Biochar has become popular in Western horticultural practice over the last few decades but its use has a long history in Central America. Amazonian peoples applied what has been called ‘terra preta’ to the soil of their region since the pre-Columbian period. Archaeologists have discovered evidence that settlers added this form of charcoal to rainforest soil that would otherwise have been too acidic and nutrient-poor to support complex civilizations. Terra preta persists in the soil for centuries and contains more nutrients than surrounding uncultivated soils.

What are the benefits of using charcoal in your soil?

The high porosity of activated charcoal benefits plant soils because it offers more of the oxygen that plant roots need to stay alive.

Air pockets in the charcoal particles also decrease the risk of waterlogged soil by absorbing excess water. This is why activated charcoal is often placed at the bottom of terrarium jars, below the soil. It prevents soil saturation, fungal growth, and root rot in the humid terrarium micro-environment. Read about why waterlogged soils are dangerous for plants here.

Activated charcoal also helps prevent over-fertilization. A layer of charcoal at the bottom of your potting mix can soak up excess minerals from the soil if you accidentally add too much fertilizer.

Can I use non-horticultural activated charcoal for plants?

Activated charcoal from the drugstore is fine for gardening purposes if it does not have any additional chemicals.

You can use it as a potting material by mixing it into your soil. This aerates the soil by making it crumbly and the individual charcoal particles will have air pockets for added oxygenation. You can also place two inches of it at the bottom of your terrarium or plant pot to regulate soil moisture.

However, activated charcoal from the drugstore will not have the plant nutrients that manufacturers include in horticultural charcoal. If you want to add charcoal into your potting mixture without diluting the soil’s nutrient content, you can infuse the charcoal with nutrients yourself. Do this by steeping the nutrient-free charcoal in compost tea for 24 hours. Make the compost tea by filling a bucket with water and adding some organic material. Once steeped, leave the charcoal out to dry. Check out our organic plant fertilizer recipes to see what kinds of organic materials you should add to your compost tea.

One type of horticultural charcoal is Biochar, which is heated to even higher temperatures than activated charcoal. Add one pound of biochar for every two square feet of your garden.

Charcoal potting mixes for houseplants

Charcoal should be a soil supplement so use it in conjunction with other potting mediums.

Charcoal is so absorptive that eventually it leads to mineral build-up over time. You must replace the potting mixture with fresh material by keeping to a regular re-potting schedule.

Succulents

Charcoal mixes are ideal for creating succulent potting mixes. Mix two parts soil, one part perlite, and one part activated charcoal.

Tropical houseplants

For tropical houseplants that need richer soil loaded with organic material such as coir or bark. Add three parts bark, three parts coir, one part charcoal and one part worm castings.

Orchids

Use charcoal chips rather than charcoal powder for orchid mixes. You can get this from gardening stores or specialist orchid supply companies.

Add four parts medium-grade fir bark or medium grade coco husk chunks, 1 part medium charcoal chips, and 1 part horticultural perlite.

Charcoal layer

You can also place a 2-inch layer of activated charcoal at the bottom of your pots, below your normal potting mixture. This adds drainage and reduces the risk of fungus developing.

 Our Favorite Horticultural Charcoal

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