What is Biochar and how do you use it?
Welcome to our biochar article discussing what biochar is and how to use it as a soil improver to enhance plant, vegetable and crop growth.
Biochar is an expanding subject area - we updated the blog in October 2020. For easy navigation, please use the table of green links to jump down the generic biochar topics, the brown links to jump to specific SoilFixer biochar information or one of the black hyperlinks for a non-soil related biochar article.
Biochar is a made from plant (bio) materials that have been heated in the absence of oxygen - a process called ‘pyrolysis’ or 'charring'. Charring wood to make charcoal fuel and using horticultural charcoal have long histories. The term biochar has been adopted to specify a material designed to improve soil and hence plant growth. But, biochar also describes production using sustainable methods and resources, ie a material that helps mitigate climate change.
There are four aspects to quality biochar:
- It should improve plant and crop growth
- It should be made from a sustainable plant resource
- It needs to stable in soil (ie last hundreds of years)
- The production life-cycle needs to be climate positive - ie the process is carbon-neutral or carbon-negative (reduces CO2)
With any "new and exciting" product, there is the risk of hype and hijacking of the term. 'Biochar' is being used to describe many types of carbon residue from many different types of processes. We will talk later about the important differences in biochars, for now, be aware: not all biochars are the same and it is well documented that not all biochars enhance plant growth.
When the right type of biochar is added to soil and compost, it can dramatically improve:
- Water retention
- Nutrient supply to roots
- Nutrient losses due to 'run-of'
- Support for microbes and AMF (mycorrhizal fungi)
- Support the formation and stability of colloidal humus
- Improve soil aggregation (ie tilth)
There are further environmental benefits when using biochar made from sustainable raw materials processed in well-engineered equipment designed to be non-polluting and heat efficient:
- Sustainable use of resources
- Carbon capture - locking carbon in your soil to offset climate change
- Reduced NOx and methane soil emissions
Biochar can be added to all soils and most growing media
- Raised beds
- General gardens
- Compost (see below)
The most powerful biochar benefits involve interactions with the plant root zone. It follows the best place for biochar is in the root zone.
Our recommendation is to dig it when planting (eg seed drills, tree holes or when cultivating the soil. This is not always possible (nor desirable if following no-till regime!). Biochar benefits have been shown when used as a top dressing or when lightly hoed into top few cm of the soil.
Raw biochar needs to absorb nutrients and develop microbial root zone relationships - this can take 3-6 months. This inoculation ‘lag’ period can be solved by 'activation' (see below), or customers can buy SF60 Biochar Super Compost ready to use.
We recommend (along with most of the biochar scientific community) that you 'activate/inoculate' (soak, charge, activate, enrich) your biochar before adding it to the soil. The simplest method is to add biochar to your composting process. You can also soak the biochar in a liquid fertiliser before adding to the soil.
How much biochar should I apply to the soil?
|Minimum||0.25 kg/m2||2.5 t/ha||1%|
|Ideal range||1.0 Kg/m2||10 t/ha||5%|
|Ideal range||2.5 Kg/m2||25 t/ha||10%|
|Maximum||5 Kg/m2||50 t/ha||20%|
*The % volume ratio varies with depth of soil mix. For more detail on application rate calculations by area, volume and weight and tips on how to make your budget go further, please visit the application ratio blog. We also have a free Excel calculator if you need to work out accurate application ratios.
You can make biochar go further and maximise budget by focusing on only adding into seeds drills and planting holes then repeating application each year.
Biochar will be in your soil for hundreds of years. You can add a bit more each year as funds allow.
The short answer is it acts like a sponge absorbing nutrients and water. Beneficial soil microbes attach to the biochar and transfer nutrients to the plant roots. In exchange, the roots transfer sugars (made via photosynthesis) to the microbes.
In more depth....
Over the past 10-15 years, thousands of academic reports have been published on biochar. We now have various summaries ('meta') analyses of these papers (Jefferies et al, 20011, 2014). These summaries indicate biochar sometimes works incredibly well (+100% increase in yield), often delivers no benefits (0%) and sometimes creates negative impacts (-20%). With hundreds of different types of biochar from different organic sources (eg wood, palm shells, coco, straw, miscanthus, sewage sludge, the list goes on) combined with many different process conditions (low & high-temperature pyrolysis, fast & slow, torrefaction, gasification, etc), excuse the pun, but things have got ‘muddied’. A huge amount of time has (and still) goes into looking at the properties of biochar to determine which production method and source material will give a positive result in all soils.
A slight detour....
SoilFixer got involved in biochar about 8 years ago. Our technical background is in compost and composting methods. (TC invented the Hotbin composer and drove the uptake of hot composting of food and garden waste.). We pride ourselves in understanding how compost and soil work and bringing to market products that make a real difference. We had a research project looking at the possibility of making “the world’s best compost". We understood fertile soils and the role of soil organic matter (SOM) and in particular 'humus' in soil fertility. It was logical (to us), as humus is key to soil fertility, then adding humus to soil would be beneficial. We starting to look at how much humus was in different composts. We 'pulled apart’ different composts and found the humus level varied significantly - from as little as 1% to as much as 20%. (You can read more about the humus journey here). We reasoned, if we could establish a recipe to make more humus in compost, this would be an exceptional ‘compost’ and soil improver. We looked at a great many recipes and additives. On our journey, we came across articles related to highly fertile Amazonian Dark Earths with high levels of soil organic matter (humus) and charcoal. The world of biochar as a compost additive opened up. We tried many types of biochar/charcoal/carbon mixed with other things. We also tested a range of composting conditions. The compost activator (humification agent) we now sell is a result of this – it increases the humus (colloidal humus) levels from around 5% (dm basis) in typical composts to 40% (dm). We now make our own “biochar super compost" and still use the original project code name: SF60.
How does biochar work? We believe our knowledge of compost and humus allowed us to ‘join the dots’ and 'spot the missing link' on how to make biochar work.
Many biochar reports indicate biochar acts like a sponge holding water and plant nutrients. We have noted on our blog how to make a container growing media these properties are about the same as clay, but are very low compared to humus (eg WHC x2.5 in biochar & clay vs x10 in humus, CEC 100 in clay & biochar, 800 in humus). In our opinion, the chemical properties of biochar are not central to success. We believe the physical properties and in particular, the presence of specific pore sizes (channels) in the biochar that support microbes and retain humus are the keys to success.
** Beneficial soil microbes and humus are keys to better plant health and growth **
Only specific starting materials (notably hardwoods) have these pores, and only certain types of processing retain them in the final biochar - see examples in images below.
SoilFixer ONLY supplies biochar (charcoal) made from woods using a low-temperature (350-550C), slow pyrolysis (3-6 hours) retort kiln. We use this same biochar to make SF60 (biochar super compost) - a blend of biochar, colloidal humus and compost particles.
Chemical effects: biochar has beneficial chemical effects based on the surface chemistry of the carbon atoms and the surface area of the carbon. These properties are determined by the raw material, time and temperature used to make the biochar.
Many different raw materials (eg straw, coco, softwoods, hardwoods) are used to make biochars. Many of these 'explode' into tiny fragments when pyrolysed - they convert into amorphous carbon (tiny fragments of flat sheets of atoms (see photo). This amorphous carbon has a huge chemical surface area (ie micro pores <2 nm) but has no mesopores (ie pores of 2-50 nm size) and no macropores (50-100 nm size). (See photo below). This is crucial when we look at beneficial soil microbes.
Let us cross-reference carbon surface area: millions of tonnes of activated carbon pellets are made each year - the quantity dwarfs biochar. Activated carbon is used because it has a high surface area of carbon - it can hold nutrients (metal ions) just like biochar. There is no published data that "activated carbons" added to soil to give any benefit. We infer from this that 'high surface area chemical benefits' of biochar are not paramount to biochars impact on soil. (Ignoring the fossil fuel aspect of activated carbon, if carbon surface area was needed, surely we would be adding the cheaper and readily available activated carbon to soil!). We can also match a number of papers where biochar has not worked to material and processes that result in 'amorphous' biochars.
Particle size effects: biochars have physical interactions with oil aggregates based on the size of particles. This can impact water flow (irrigation) and soil particle aggregation. Large particles will increase water flow and decrease aggregation. Fine particles will as a generalisation will be opposite. On balance fine granules (0-2mm) will be more beneficial in sandy soils, and medium granules (0-8mm) will be better for heavy clay soils. Granules above 8mm will have negative effects on soil tilth. The granule size also affects handling - granules are easier to handle and spread. Biochar powders and fine granules tend to be more susceptible to wind blow. (When using in the garden, we suggest a quick spray of water (via water-mist-sprayer) into the tub of granules).
Biological effects: biochars also have a biological impact on soil microbes in the root zone. The exact mechanism is still under debate. Our theory is as follows: microbes are small (5-100 nm), small enough to live in the mesopores(2-50 nm) and macro (>50 nm) pores formed in some biochars. The solid biochar pore walls act as a defence against being eaten by predators. The surface chemical structure of the biochar adsorbs nutrients. Normally once these sites are full of nutrients, the adsorption ceases. The microbes take up these trapped nutrients - releasing the sites to take up more. The microbes form symbiotic relationships with root hairs. They exchange nutrients (NPKs etc) for sugars. This symbiotic relationship is more viable when microbes have a safe home in biochar pores. We conclude this symbiotic biological impact from biochars is more important than the chemical or physical impact of biochar. (see photo below comparing amorphous powder with fine granules).
For the very best results, we conclude biochar should have the right pores present AND accessible to microbes for microbes to inhabit. The amorphous powder does not have the right pores. Large granules (eg 25 mm) have lots of pores (channels), but only those on the outer surface are accessible to microbes).
SoilFixer believes the most advantageous size of biochars is small granules (0.1-2mm) with accessible pores of the right size. Hence we sieve and grind to produce a specific 0-2 mm grade of biochar.
A quick read through the IBI/EBC certification documents will confirm that a vast range of plant organic matter can be converted to biochar. The list covers wood, coco, plan shells, straw, miscanthus, etc.
We understand the technical basis for the definition of biochar. We know different biochars have "worked". However, we feel (for reasons discussed above), the underlying reasons why biochar works dictate that biochar from wood processed in a specific way are 'guaranteed' to work.
There are numerous types of commercial equipment for making biochar. The costs can carry from £20K for a simple retort kiln up to £10m for and advanced energy-from-waste gasification system linked to the electricity grid. Each type uses slightly different variables, for example:
Low temperature (300-500C)
High temperature (400-900C)
Continuous or batch
Torrefaction / Pyrolysis / Gasification
Earth mounds to metal ring kilns, to retorts (metal, brick, square, oblong, Adam, Missouri)
Soilfixer biochar is made in an advanced retort kiln linked with an energy (heat) recovery unit. It meets the EBC certification on sustainability.
In Europe, there are only a few dedicated biochar production facilities. Most biochar either comes from 'fines' (small bits) leftover from BBQ charcoal production or is high-char ash residues from energy from waste plants.
Remember to look out for these things when buying biochar:
Use of sustainable resources: when biochar is made from wood (see why we exclusively use wood) then you need to ensure it is from a managed woodland/forest source. (Some, but not all, imported BBQ/biochar is still made from slash and burn deforestation).
The manufacturing process should be climate-neutral (or carbon-negative). It is not sufficient to for the supplier to state they make biochar from managed woodland. Pyrolysis converts a lot of the input to heat, carbon dioxide, methane and other non-desirable VOCs. The carbon-efficiency (C-sink potential) of the conversion process is essential to know. The EBC certification body has mow mandated biochar from ring-kilns is not C-neutral and does not meet the criteria for certification. This means a number of UK artisan woodland charcoal makers who use ring kilns have biochar fines that would fail to meet the biochar certification criteria. This is a challenging subject. On the one side, 'fines' (potential biochar product) is sometimes left unused in woodlands. Biochar is a beneficial outlet for what is often a "waste". We have yet to meet a 'rich' charcoal maker - most have strong environmental ethics and spend many hours for very little money looking after coppice woodland. Some UK charcoal makers have moved to cleaner more efficient retorts. There is a significant cost barrier against change (a ring kiln costs £1,000 an entry-level retort costs £20,000). We hope more will convert in the near future and we urge all charcoal makers to review the C-sink paper published by Ithaka/EBC on the carbon-neutral production
However, both BB charcoal and certified biochar are made in "advanced retorts" (virtually no air pollution due to retort of gasses) and these retorts are now configured for heat (energy) recovery. Some of these advanced retorts have already passed the EBC certification scheme.
There are loads of internet posts and blogs that proudly state charcoal cannot be biochar and vice versa. Even some noted biochar experts argue about this topic. It is all tied up in the semantics, incorrect assumptions and lack of journalism to cross-check facts.
From our growth tests and technical knowledge without hesitation: biochar that works is derived from wood that has been through a low temperature, slow pyrolysis process. This is exactly how BBQ charcoal (the product) is made, and hence, in our opinion, the best performing biochar (product) is “wood charcoal”.
However, this statement is entirely consistent with a statement that making charcoal does not make biochar. This is because the method of production (ie the type of kiln or process) very often DOES NOT meet the wider biochar goal for sustainability and a reduction in climate change (ie biochar is a true C-sink). The European biochar certificate excludes ALL ring kiln charcoal from its certification – not because it is not good biochar but because ring kilns create pollution and waste heat.
We can make a similar observation around the many excellent gasification processes. They use sustainable wood and create sustainable energy very efficiently. They often produce a high carbon ash residue that will pass all the biochar certification tests. BUT - very few of these 'biochars' have (or IOHO, ever will) give positive biochar growth tests beyond a 'liming' impact. We always say: ask for real plants growth tests based on the exact biochar you are buying not on based on 'published' generic published data.
The main driver for making biochar at home or on the allotment is usually the cost of commercial biochar. Before starting to make it at home, a few things to consider: how much time it will take, the cost of equipment, the environmental impact, the quality you can make, how you will grind it to right particle size, how much dust and dirt and odour you are prepared to put up with. (Biochar dust gets everywhere, it is a nightmare to clean of your skin (think coal miner type), impossible to get out if trodden into carpets, very tough on work clothes and washing machines. There are three main DIY methods
- Earth mounds, ie cover a wood fire with soil
- Kon-tiki kiln (flame-curtain)
- Metal (Oil drum) type retorts
There are hundreds of articles and YouTube videos covering these options. We recommend the Kon-tiki flame curtain because it is easy to load and achieve a 'clean-burn'. If you have the time and patience to build a retort great – but you will need welding skills and a fair amount of technical skill to get it to work cleanly.
Note: the Environmental Agency and Local Government Pollution and Nuisance control positions on making biochar at home not absolutely clear. You are not ‘burning/combusting/having a fire’ – you are attempting to pyrolyze material. In our view, if you create lots of smoke often – you are going to get neighbour complaints. If you release ‘dirty’ or ‘smelly’ smoke these are giveaways you are polluting the air with particulates and volatile organics that have a significantly greater impact on greenhouse gasses (climate change) than CO2.
There are many excellent teams working on biochar such as the European and British Biochar organisations and Ithaka Institute. They have many links to scientific papers.
- International Biochar Initiative (IBI)
- Ithaka Institute led by Hans Peter Schmidt
- European Biochar Certification org (EBC)
- Cornell University (led by Joseph Lehmann)
Is the biochar certified according to EBC or IBI certification? Yes
Has the biochar been proven to increase plant growth in UK soils Yes -see our biochar test results. (We acknowledge farm-scale tests are pending - watch this space)
SoilFixer grinds and sieves biochar to get the right ratio of particles. The 0-8 mm granules are graded to work well in most soils. The 0-2mm granules offer a little bit more water holding capacity which is useful in sandy soils. We do not recommend fine (amorphous) biochar powder as they do not have the macro-pores needed by soil microbes. We also advise care when using large granules (>8mm sieve) because the volume of surface pores available to the soil microbes in very low and large particles negatively impact soil aggregation (ie tilth).
Yes! Too many 2-8 mm particles will negatively affect aggregation and water holding and retention. Water will drain away too easily with too many large particles. We recommend a maximum 20% biochar (by volume) for heavy clayey soils, 10% for existing loams, and 5% for sandy soils.
Biochar does help with water retention in sandy soils, but it is expensive. There are lower costs ways to create water retention and tilth - such as adding colloidal humus (see SF60).
The biochar industry is young with many research projects still underway looking at many different biochar formulations. Working with SoilFixer, we can offer a range of biochars:
- Inoculate (charge) biochar with various items such as nitrates, urine, NKPs, trace minerals, rock dusk, AMF, specific root fungi (eg RootGrow™)
- Mixtures of biochar with PAS100 compost
- Various crushed and sieved grades from 0-25mm (eg 2-8mm, <2mm)
- Supply the biochar fresh and dry, or aged and wet
Returning visitors might spot we have amended our descriptions on how we make biochar. We are in an ongoing search to find the right equipment and input materials that give us sustainability, clean burn and a lower-cost product (that performs brilliantly!). Solving these issues will help drive adoption and hence make a difference.
This means less labour intensive systems and lower-cost raw materials.
Our R&D Kon-tiki kiln was great for years 1-2. It was clean burn and produced good biochar from local coppice woodlands. However, it had no heat recovery and was very labour intensive. Great for 10t year, but never going to hit 100 tpa never mind +1000 tpa!
We have tried an inhouse design of a 'giant' kon-tiki. This worked with expensive 'logs' but not with the lower costs wood resources we need to move to. Amending and configuring our own design would be as expensive as off-the-shelf purchase. We are currently looking at a number of advanced retorts. Until resolved, the higher volume of sales has led us to work with a fully EBC certified partner who makes to our needs using their advanced retorts.
Thank you for reading about biochar. This is one of five articles on the uses of biochar - follow the links below:
|Biochar for gardeners - some surprising uses (link)|
|Biochar uses around the home (link)|
|Biochar for industrial use (link)|
|Biochar for water and gas filtration (link)|
|Biochar for farming and agriculture (link)