Biochar – Does it Really Work in the Garden?

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Robert Pavlis

Biochar is a special kind of charcoal that seems to have many benefits as a soil amendment. It holds water, acts like a fertilizer and grows bigger plants. While doing all this it is also eco-friendly and sequesters carbon in the soil for many thousands of years. Sounds like a win-win-win.

What is biochar? Are the claimed benefits real? Should gardeners be using this product to amend their soil? Let’s check it out.

Different samples of Biochar, photo from UC Davis Biochar Database
Different samples of Biochar, photo from UC Davis Biochar Database

What is Biochar?

Everyone agrees it is charcoal made from pyrolyzed plant-based organic matter including such things as manure. The term biochar is quite new and was coined by Peter Read in 2005.

Several sources including the UC Davis biochar database and Wikipedia say it is charcoal that is primarily used for soil amendment and not for heating. That is interesting, but does not really describe what it is.

Microbe Science for Gardeners Book, by Robert Pavlis

It is created in a low, or no, oxygen environment so that it maintains its fine grained structure.

The International Biochar Initiative says it’s produced at low temperatures and then goes on to define this as <700°C. The UK Biochar Research Center says it should be made above 250°C in a zero oxygen environment. “In general higher pyrolysis temperatures mean a smaller amount of char, but containing a greater proportion of highly stable carbon”. Others say it needs high temperatures, over 500°C.

Biochar vs Charcoal

We have all seen charcoal and maybe even used it in a barbecue (real charcoal, not the briquettes). How does this differ from the new biochar?

Biochar is a form of carbon, slide from Kurt Spokas, The Science Behind the Hype
Biochar is a form of carbon, slide from Kurt Spokas, The Science Behind the Hype

After much searching all I can determine is that the end use determines the name of the product. Some sources hint at the fact that the manufacturing processes are slightly different to produce products with different characteristics, but nobody gives details.

I did find a report that says Biochar is not a new product, just a new name for material that is used to amend soil. In fact it says biochar covers a wide range of products including char, charcoal and soot.

This certainly would explain why nobody seems to be able to describe the difference between charcoal and biochar. It is charcoal when used for burning, and biochar when used to amend the soil.

Biochar Varies a Lot

Biochar is a variable product and its chemical and physical characteristics depend very much on the input ingredients, the process used to pyrolize it ( temperature and length of heating) and any chemical treatment after production.

This makes it difficult to make any definitive statements about its use.

Claims for Biochar

The claims made for biochar include:

  • increase yields
  • increase fertilizer efficiency
  • remove pollutants and pesticides
  • mitigate climate change
  • increase soil moisture
  • increase soil pH
  • increase soil microbe populations
  • increase cation exchange of soil

Most of the studies that have led to these claims have been done in the lab and very few field tests exist.

Both charcoal and biochar absorb chemicals. They do have negatively charged sites which increases the CEC (cation exchange capacity) but if they absorb pesticides or nutrients then this can be detrimental to plants since it makes these products less effective. We also know that activated charcoal, which is used to scrub chemicals, gets saturated and needs to be replaced. I would expect that Biochar in soil will also become less effective over time, but nobody mentions this. There are no long term studies.

Biochar does increase the pH of soil and holds more water, neither of which are benefits in alkaline clay soil. It does increase microbe populations.

One review of current studies reported, ” In biochar studies reviewed, half reported an increase in plant yield after adding black carbon or biochar, while 20% noted decreases in plant yield, and 30% reported no difference in plant yield from the addition of biochar.”

Field studies are mixed on the yield issue with some showing an increase and others showing a decrease. Some studies show positive results when used with fertilize, or on acidic, low fertility, soils.

A four year study looking at growing vegetables in a simulated back yard garden, at three different sites, found mixed results; some increases in yield and some decreases in yield.

It is important to keep in mind that the characteristics of Biochar vary a lot and soil plays a key role in determining how it might affect yield. It is therefore not surprising to see mixed yield results.

It is clearly not the cure-all bullet some claim it to be.

Charging Biochar

Many sources recommend that you charge biochar before using it. This can be done by mixing the biochar with compost, or just adding it right into a compost pile and composting the whole thing. You can also soak it in manure tea or just add some synthetic fertilizer to the biochar.

I am not surprised charged biochar works better. All of the active negative sites on the biochar will now be saturated with nutrients. When it is added to soil, it can add nutrients to the soil and make them available to plants. On the flip side, un-charged biochar is like a magnet. When added to soil it sucks nutrients away from plants, at least for a short period of time. This may explain why some studies show a decrease in plant productivity when the soil is amended with biochar.

Are the reported increased yields due to the added fertilizer?

Does Biochar Sequester Carbon

Even if biochar does not increase plant growth, it is believed to be a really good solution to our CO2 problem. If plant material is left to compost, it produces CO2. If it can be turned into biochar, and then added to soil, it will essentially bury our CO2 problem.

The efficiency of this all depends on how long biochar remains in its charcoal form. People have suggested it is very stable and will last thousands of years but there is almost no science to prove this. A report from the Earth Island Institute says, “Field trails proved rare; only five such studies were found, which between them tested biochar on 11 different combinations of soil and vegetation. In only three cases did biochar result in any additional carbon sequestration. In most cases, there was either no measurable difference in soil carbon, or even a reduction in soil carbon. These results from short-term studies —none spanned more than four years — fly in the face of repeated claims that biochar will sequester carbon in soils for tens, hundreds or even thousands of years.”

A recent peer-reviewed study revealed that 72% of the carbon was lost in the first 20-30 years. “There is little basis for confidence that charcoal will retain carbon in soils“.

There are also environmental costs for collecting the raw material, creating the biochar and redistributing it back to soil. Maybe the whole process is not ecofriendly?

Organic Matter or Biochar

One of the big benefits of biochar is its ability to hold onto positively charged nutrient cations (calcium, potassium, magnesium, etc). Clay and organic matter do the same thing. The organic matter turns into humus which sequesters carbon long term, but maybe not as long as charcoal.

One has to wonder if it might be better to just add the organic matter to soil and let nature take care of things?

Biochar as a Byproduct

Most producers of commercial biochar are more interested in producing biofuels and biochar is a waste product for this industry.  Using it for soil amendment in this case may be eco-friendly.

Can You make Biochar at Home?

Some sources say you can’t make it at home because you can’t get things hot enough and control oxygen well enough. It is clear that simply burning things in a fire or in a pit won’t make good biochar, and if it is not made properly it can release harmful gases such as methane and carbon monoxide.

Others have designed special DIY stoves that seem to make reasonable biochar, although the process is not tightly controlled. Campfires can reach 1,100 °C, so a properly built system could reach the temperatures needed. Stoves built with two chambers, one for burning to make heat and one for making the charcoal in low oxygen conditions might work. With the right system you can make biochar at home.

Since there is no clear definition of what biochar is, I guess it is hard to say you can’t make it at home. But keep two things in mind:

  1. Just because you make some black charcoal does not mean it will have any of the properties reported for biochar. Its properties depend on how it is made.
  2. If it is not made properly you will be creating more pollution than you save.

It really is better not to make your own.

Biochar in a Potting Mix

Biochar has been shown to inhibit seed germination so it should not be used in a seedling mix.

It can be used to replace some of the peat moss in soilless mixes where the increase in pH counterbalances the acidic nature of peat moss.

Should Gardeners Use Biochar?

Biochar has been shown to have some benefits in the garden and it might be a good amendment for soilless potting mixes, but here is the problem.

Biochar is not one product. It can be made from many different input ingredients, in many different ways. The result of each process is a different product with its own special characteristics. There are no standards so you can’t pick up a bag and say, “this is the one that produces good results in my soil type”.

You are left with buying a bag of something and hoping it works.

You have the same issue with buying compost – who knows what is really in the bag?

But there is one big difference between compost and biochar. Unlike biochar, compost decomposes in a few years. A bad batch of compost is a short term problem. Biochar may not last a thousand years, but it certainly lasts a lifetime. What do you do if you add a bad batch to your soil?

Many studies have shown that too much biochar is harmful to plants and soil. You can’t correct this problem without removing the soil.

It may be more beneficial in acidic soil where the increase in pH is desirable. It has also been shown useful in reducing compaction in lawns and poorly drained soil.

If you want to try it, start small and use some test areas. Keep the amounts small. Document the product you use so you can buy the same product again.

I think I will sit on the sidelines and stay away from it, for now. Compost seems to offer most of the same benefits without the downside.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

38 thoughts on “Biochar – Does it Really Work in the Garden?”

  1. Couldn’t find a definition for biochar either. I think the term “biochar” is a marketing gimmick to trick people into believing it’s a special product and not just charcoal thus limiting supply and increasing its price. A meta study shows it increases yield with or without added fertilizer, consistent with anecdotal evidence in comments below:
    https://www.css.cornell.edu/faculty/lehmann/publ/SoilUseManage%2036,%202-18%202020%20Ye.pdf
    and
    https://www.css.cornell.edu/faculty/lehmann/publ/SoilUseManage%2036,%202-18%202020%20Ye%20supporting%20online.pdf
    Unless one is okay with a production dip the first year it’s best to prime it first by marrying it for a few weeks with manure/fertilizer/compost/worm castings/etc. before application.

    Reply
    • “Compared with the use of IF (inorganic fertilizer) only, the addition of biochar along
      with IF caused a 15% (CI: 11%–19%) increase in yield, indicating that biochar was
      as effective as fertilizers in increasing crop yields when added in combination. The
      use of biochar alone did not increase crop yield regardless of the control considered.”

      Using only biochar does not increase yield.

      Adding both fertilizer and biochar increases the yield by 15%.

      They go on to say, “Whereas in the short term, liming may have partly contributed to the beneficial effect
      of biochar (>90% was plant-derived) when added along with IF”

      So maybe the increase in yield is not even as big as 15%.

      It is hard to understand how effective biochar is in any given situation. Or how gardeners should use it.

      Reply

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