058-Biochar Basics and More: Digging Deeper Into the Science of Soil

| Podcast, Prepare

This week, I spoke with Mark Highland of Organic Mechanics. Mark is passionate about soil and has quite the pedigree, having graduated from the Masters Program at Longwood Gardens. Well-versed in the basics of biochar, he’s also recently written Practical Organic Gardening – The no-nonsense guide to growing naturally. It’s a book packed with insight on many aspects of organic gardening.


Mark Highland & Brie Arthur

Mark Highland enjoying a light moment with fellow horticulturist and plant propagation expert, Brie Arthur.


Developing Peat Alternatives

Mark’s early academic studies focused on, among other things, identifying a replacement for peat moss as an ingredient in potting soil.

Peat has been used horticulturally for decades, but it isn’t an ideal ingredient. It does help improve soil water retention, however if allowed to dry completely, peat moss does more harm than good. If you’ve ever watered a dry container with peat moss and watched the water simply run off the surface, you realize how difficult it can be to rehydrate this stuff.

Most importantly, peat is a finite resource. It has been mined for generations and for a multitude of purposes, but it takes thousands of years for peat to develop in bogs. In fact, the United Kingdom has extracted all peat available to the horticultural industry. Their few remaining bogs are now protected as public parks. The peat industry in Ireland is aware that their days are numbered too, as Irish peat bogs are nearly deplenished.

Fortunately, Canada has millions of hectares of peat, but when you consider that bogs accumulate at a rate of approximately one millimeter per year – it’s easy to see that even Canadian peat is depleting quickly.


Seed Starting Mix comparison

The physical properties of coconut fiber (left) and peat moss (right) are very similar as this example of two popular seed starting mixes shows.


Why does this matter? Peat bogs are the best carbon sink we have on this planet. These remaining bogs capture and hold carbon dioxide, keeping the gas out of the atmosphere. They are an invaluable ecological resource.

Ok – If peat is unsustainable and has some functional drawbacks, what other alternatives are available for water retention and drainage properties in potting soil? That was a question Mark set out to answer.

The Compost and Coir Solution

It was believed for years that compost was too unstable for successful use in potting soil. Mark’s research found otherwise. While studying at Longwood Gardens, Mark’s intensive studies and growth trials determined that a compost and coir blend was an ideal and sustainable replacement for peat.

Compost and coir offer water-retentive benefits, and they improve drainage. As a bonus, these materials also provide nutrients and foster soil’s biology. More on soil biology in a moment.


Container Garden

Mark’s early academic studies focused on, among other things, identifying a replacement for peat moss as an ingredient in potting soil.


You’re probably familiar with compost but may not know coir. What is coir, exactly? Coir is the final byproduct in the coconut harvesting process. After the meat, oil and water have been extracted; the remaining coconut husk fiber and dust result in coir. The quality of the coir material you can buy has improved in recent years, and coir’s use horticulturally is on the rise.

Using compost and coir in your garden reduces strain on our planet’s peat bogs, and these ingredients feed and improve your soil in many more ways.

Respecting the Soil vs. Working the Soil

Soil is a living entity. Mark likes to refer to it as an Underground Safari. You can’t see them without a microscope, but there are billions of microbes moving around and eating each other down there. Fungi are eating nematodes, protozoa are eating bacteria, and so on.

In the process of all this voracious activity, organic materials are being broken down and nutrients are being released into the soil to be available for plants to take up through their roots.

These microbes, like all living creatures, need to breathe. Unfortunately, some common garden practices hinder the amount of oxygen held within the soil for these creatures. Tilling is a prime example.



In the “Underground Safari” of the soil food web, the worms are easy enough to spot but there are billions of microbes moving around down there too.


Gardeners have been tilling for generations. It’s a satisfying site to tear up the earth for a new bed and wind up with a workable, soft surface for planting. It’s true that plants may produce in tilled soil, but there is more to the story. The soil biology is harmed.

Microbes build millions of networks within the soil – highways for movement as well as oxygen and water retention. Picture the tunnels of an anthill on an epic scale. Microbial networks can take decades to build up efficiently. Tilling breaks up those networks, collapsing the air and water space.

Although freshly-tilled soil looks fluffy, it quickly compacts and reduces the ability of water to percolate deep into the surface. Have you ever heard of the “tiller pan layer”? This hard layer forms as a result of compaction after tilling. Under the hard layer of the surface, this second impermeable layer forms and creates a perched water table where water is trapped and unable to penetrate to deeper soil.

The tiller pan layer can prevent plant roots from growing more deeply too. By tilling, you have actually “worked” the soil into a less healthy state.

When the Underground Safari is allowed to do it’s dirty business without disruption, your soil becomes more fertile – and workable. So while working your soil may generate a crop, respecting your soil will generate a lifetime of fertility.

The Underground Safari serves a disease-fighting purpose too. Although plant roots look smooth to the naked eye, Mark compares their surface to a bus which has smashed through a brick wall. The root branches and hairs create cracks which are opportunities for pathogens to infiltrate the plant.

Microbes in the soil are drawn to plant roots as a carbon source. They’ll reach the roots before the pathogens arrive, and the microbes build a bacterial wall – blocking access to those pathogens. Pretty amazing, right? I want as many of those microbial good guys in my soil as possible.


Joe Lamp'l raised bed

I never cease to be amazed at how much is going on down there, under the surface, and how beautiful and rich it makes my soil.


Understanding Good Fertilization

How do fertilizers impact soil biology? Although fertilizers are thought of by most gardeners as good practice, there are drawbacks to be mindful of.

First things first: A plant doesn’t know the difference between nutrients created chemically or released through microbial activity. In either case, the plant will take up the nutrient. The difference lies beneath the surface.

Water-soluble, chemical, liquid fertilizers provide a flush of nutrients, but plants are only able to take up so much at a time. They will initially feast, but they will reach their limit. Any nutrients still remaining in the soil – because they are chemical in nature – do not become part of the natural microbial cycle. Instead, those chemicals often leach out during watering and enter waterways. Also, that plant feast can burn foliage and do more harm than good.

Chemical fertilizers are salt-based. While they add nutrients, they are also depositing a high salt content into the soil. Some microbes are able to sustain in a high salt environment, however other microbes are salt-sensitive and will be killed off. In other words – adding chemical fertilizer reduces the diversity of your soil biology and the fertility of your soil.


Garden bed

Amending with compost in all of your garden beds will foster the soil biology – resulting in healthier and more disease resistant plants.


Organic fertilizers, on the other hand, do become part of the microbial cycle. Microbes break the organic materials down, feed on the nutrients and the Underground Safari action releases the nutrients for plants to take up. Some organic or organic-based fertilizers such as fish emulsion provide a flush of nutrients.

Other organic nutrients and derivatives such as Milorganite® are released slowly, as the microbial activity does it’s work. In such cases, organic nutrients not immediately taken up by plant roots will remain in the soil biology for future use.

There are many types of organic nutrients available for the home gardener. In addition to those I’ve already named here, amendments such as worm castings, alfalfa meal, manure, and many more can be great options. Just do a bit of research to determine which fits the bill for your intended purpose. Also – check out the Perfect Soil Recipe section of my raised bed podcast series for a list of my favorite options. One material I included there is biochar.


Biochar growth comparison

Seen here is a comparison of plant growth in response to biochar amendment. Although planted at the same time and grown under the same conditions, the top row plants amended with biochar are leaps and bounds above the lower row plants grown without the benefit of biochar. (photo: Mark Highland)


Burning a New Path to Soil Health

Biochar is an organic amendment which is quickly rising in popularity. What is biochar? It’s carbon – formed through burning wood chips, waste from lumber processing, animal manure, or any other organic material. The material is burned in the absence of oxygen and at a high temperature – over 350 degrees centigrade. This process burns off everything flammable, leaving only the pure carbon.

This pure carbon material never breaks down – it lasts a lifetime. Mark calls it a super-charger for soil productivity.

Biochar isn’t a fertilizer. It helps fertilizers work better because it slows the flow of nutrients in the soil. This keeps the nutrients bound up in the soil and ready for plant roots to take up. Biochar also has excellent water retention properties.

There are good and poor quality biochars available, so purchase with caution. Many biochar products contain ash. The higher the content of ash, the more the biochar material will affect your soil pH. A good quality biochar will keep your soil within the neutral zone, while a biochar high in ash can significantly raise your pH.



Biochar is a product showing good promise as a long-term soil amendment. (photo: Mark Highland)


Biochar must be inoculated before you add it to your soil. It’s like an empty battery. Biochar needs to be charged with nutrients before it goes into your soil, otherwise it will suck up nutrients from your soil. By blending biochar with compost or an organic fertilizer for approximately a month before adding to your soil, it is able to absorb the nutrients from the fertilizer – essentially charging to full capacity – so that it is ready to go to work releasing those nutrients when you add the biochar blend to your soil.

This material and its effect on garden soil is still the subject of much study. It’s currently thought that adding approximately 5% (by volume) of biochar —other ingredients—-is the ideal ratio to see optimal benefits. Biochar is typically sold as a blend – with compost or other organic materials for inoculation, so pay attention to ratios.

It’s good to know how much is enough, because this stuff isn’t cheap. One cubic foot of biochar blend will run you $30-40 and provide just the right amount for a 3’ x 8’ garden bed (or 24 square-feet). The blend is lightly mixed in to the depth of a shovel blade. When you consider that single amendment it will last the lifetime of your garden, the cost doesn’t seem quite as daunting.

So, check out biochar. Try some experimenting in your own garden and compare results you get with and without biochar. Let me know what you find. I love to experiment at the GardenFarm™ and hearing about what other gardeners discover in their gardens.


Mark Highland front porch

Putting soil biology to work in your garden not only improves your results, it reduces your effort – leaving you more time to enjoy it all. Mark likes to do that from his front porch, seen here. (photo: Mark Highland)


I encourage you to take the time to listen to the podcast recording (if you haven’t already). Mark shares some other great analogies to explain the soil food web, and we cover a few finer points too.

Links & Resources

joegardener Episode 019: GardenFarm Audio Journal – First Day of Fall, 2017 

joegardener Blog: Backyard Composting: A Simple Recipe for Making Great Compost 

joegardener Blog: No-Till Gardening: If You Love Your Soil, Ditch the Tiller

Growing a Greener World® (recent Emmy award winner!)

Organic Mechanics

Practical Organic Gardening – The no-nonsense guide to growing naturally

Washington State University Extension Fact Sheet • FS147E Biochar

Longwood Gardens

Milorganite® – Our podcast episode sponsor and Brand Partner of

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

0 Responses to “058-Biochar Basics and More: Digging Deeper Into the Science of Soil”

  • AlexGardener says:

    Quick question, if we already have an established raised bed (as I’m sure most do at this time of the year), is it too late to add biochar to the beds?

  • Joe Lamp'l says:

    Hi Alex. It’s not too late. Biochar is a soil amendment and like all soil amendments, there’s never a bad time to add them. But there are better times, like in between seasons when the beds are empty.In an actively growing raised bed, you’ll just have to work it in lightly around the root zone of your plants. If you have mulch in the beds now, it would be best to remove the mulch temporarily to add the biochar, and then replace it. Hope that helps. Let us know what changes / improvements you see.Good luck!

  • Ric says:

    I’m interested in anything that improves my soil so I’ve read the above and the fact sheet from Washington State University. I hesitate to commit to biochar for two reasons. The fact sheet shows that biochar may reduce plant growth in soil that is rich in organic material if applied in excess. I use shredded leaves for mulch once my seedlings are transplanted to the beds and my soil is rich in organic matter. How is one to determine how much is “in excess”? Your article and the fact sheet both state that biochar is there for a lifetime. Once applied you can’t take it back short of replacing all of your soil. I hesitate to commit to using biochar until more studies show it’s worth the risk. I know sticking with shredded leaves and compost works. Love your podcasts, articles and website. I realize this article was written more than two years ago. Are there any more recent studies that support biochar use? Thank you.

  • Joe Lamp'l says:

    That’s a fair point, Ric and I can understand your apprehension. That old saying, “if it ain’t broke, don’t fix it”, certainly applies to many things, and for you, that may apply to your soil too. I don’t know what “too much” organic matter is for it to be a detriment to plant growth, but if you knew that number it sure would help. You can find out how much organic matter your soil has through a soil test where you specify that is one of the readings you want measured. Then you would know if there’s potential room for biochar. Organic matter content in the soil varies but based on what you’re already doing, I think you have a winning formula that I don’t think needs improving. Like you said (sort of), you can’t put the toothpaste back in the tube so undoing biochar in your soil would be pretty much the same concept. Great points and thanks for your thoughts, Ric!

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