Biochar - what is it and how to use it
What is biochar and how is it made?
Biochar is a form of charcoal used to improve compost and top soil. It is created by heating wood in the absence of oxygen - the process is called ‘pyrolysis’. (This differs to a normal biomass boiler that ‘combusts wood’ – ie burns the wood with oxygen).
Biochar is a form of horticultural charcoal. However, the modern day biochar is formulated to a specific quality standard (eg UK-BBF, EU and IBC quality mandates). These standards not only require the biochar meets a specific chemical analysis, they often dictate that it is made in from sustainable resources using an environmentally 'clean' processes. Modern biochar retort kilns reduce the harmful emissions over traditional charcoal ‘ring’ kilns.
Why use biochar
Add biochar to soil and compost to improve:
- Water retention
- Nutrient supply to root
- Decrease nutrient run of
- Support microbes and AMF (mycorrhizal fungi)
- Support the formation and stability of colloidal humus
- Improve soil tilth
Environmental benefits include:
- Carbon capture by locking carbon in soil for hundreds of years – offsetting your carbon foot print
- Reduced NOx and methane soil emissions
Where to use biochar
- Raised beds
- General gardens
- Add to compost* (see below)
How to apply biochar
The most powerful biochar benefits involve interactions with the 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 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 soil.
How much biochar should I apply to soil?
It depends on your soil tilth, aggregation, nutrients, pH, texture (sand, clay, humus), and what you are trying to achieve! The range is 0.5-5 Kg/m2 (5-50 t/ha), around 1-10% by volume.Heavy clay soils tend to benefit from higher application rates.
Annual plants – biochar should be sprinkled onto the beds/containers then gently mixed with your soil/compost. (It is recommended the biochar used has been charged by passing through the composting process).
Perennial plants – 500g of char should be sprinkled into the planting hole alongside a mycorrhizal inoculant (eg Root Grow) before backfilling the hole with soil.
Which grade (size) of biochar should I use?
We do not recommend adding amorphous powder or large (>8mm) granules of biochar.
Our logic (supported by our testing) is as follows:
Biochar has three impacts on soil and microbes: chemical, physical and biological.
Chemical effects: biochar has beneficial chemical effects based on the surface chemistry of the sheets of carbon and the surface area of the carbon. These properties are determined by the raw material, time and temperature used to make the biochar. The actual granule size has a minuscule impact on the surface area - the active high surface comes from the sheets of carbon - see photo.
Many different raw materials (eg straw, coco, softwoods, hardwoods) are used to make biochars. Many will pyrolyse 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 meso (2-50 nm) or macro (50-100 nm) pores. (See photo below) Activated carbon pellets also have high surface area but low mesopores and macropores. We can find no published papers or evidence that "activated carbons" which have been manufactured for many years have any benefit when added to soil. We infer from this that 'high surface area chemical benefits' are not paramount to biochars impact on soil. (If it was- we'd all be adding activated carbon to soil!)
There is a large and growing body of evidence that some biochars work in some circumstances - (ref Jeffery, biochar metadata analysis). The huge variety of biochars used in testing means it is proving difficult to establish exactly which biochars work.
Physical effects: biochars have physical interactions 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. Hence on balance fine granules will be more beneficial in sandy soils, and medium granules will be better for heavy clay soils. 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 the root zone and soil microbes. 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. Small fragments (<5nm) and amorphous powder do not have the right pores. Large granules (eg 25 mm) have many pores - but most are inaccessible to microbes). We believe 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. We believe we are unique at this point in time in preparing this grade.
We aim to combine the chemical functions of biochar with the biological functions. We only use the powder in our compost humification agent. We recommend the powder/fines for soil use, but we recognise many customers use and prefer the granules.
Can I add too much biochar?
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. Please note – there are case studies illustrating biochar improves sandy soils. Biochar is a better growth media than sand as it holds water and nutrients better than sand. Adding biochar to sand often gives a better growth result. But - you would achieve an even better result by adding some sticky, water retentive colloidal humus to your sandy soil rather than just biochar.
Our biochar is made in the UK using wood from managed coppice woodland here in the North East of England. We supplement this wood with occasional deliveries of wood from tree surgeons (arboricultural arisings).
Our biochar will be tested according to the British Biochar Foundation (BBF) Quality Mandate. This is similar to the European Biochar Certificate. Why do we package by volume (litres) not weight (Kg)Biochar can vary in weight enormously due to water absorption. It is, therefore, more accurate to use volume.
Our goal is to supply the best value for money biochar in the UK. We offer a price match promise: if you buy biochar online from SoilFixer and find the same product delivered to a UK address at lower price we refund the difference or discount your next purchase to the difference. (Our promise is based on comparing the price per litre of each product. The price per Kg varies enormously due to biochar absorbing water and different supplies opting to supply material dry or damp).
How to use biochar - activation / charging it!
We recommend (along with most of the biochar scientific community) that you 'inoculate' (soak, charge, activate) your biochar with nutrients before adding it to soil.
If you prefer to buy activated biochar - please review our SF60 super compost product - it more than just activated biochar.
How do I activate / charge up my biochar ready for use?
We recommend you add it into your compost. Should I add my biochar direct to soil, mix with compost or add it into the composting process? From our test results, we believe it is better to add it into the compost heap. Biochar appears to bond with colloidal humus to create a bigger and more robust colloidal matrix. Biochar absorbs soluble nutrients and water.
Sources of biochar information
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.
Custom (bespoke) biochar formulations
The biochar industry is young with many research projects still underway looking at many different biochar formulations - everything from a super high surface area "activated carbon' down to brown-char (partially burnt wood). 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 compost
- Various crushed and sieved grades from 0-25mm (eg 2-8mm, <2mm)
- Supply the biochar fresh and dry, or aged and wet
Energy recovery when making biochar
A core premise of biochar to be used as a soil additive (rather than charcoal) is hat biochar production is made in a sustainable environmentally non-polluting fashion. This means low (minimal) production of ‘smoky’ gases (VOC, CO, CH4, particulate matter (dark smoke), and that the heat produced during pyrolysis is recovered and used (rather than just going up the chimney stack).
Most approved biochars will, therefore, come from expensive ‘biochar pyrolysis’ equipment rather than from charcoal ring kilns. The kiln we use is a Kon-tiki. This is a low capital cost, small throughput unit that in the words of the Ithaka Institute can democratise biochar production worldwide allowing many small scale producers to make a positive impact. The Kon-tiki has been proven an exceptionally clean burn – it is, therefore, a huge step up from traditional charcoal production methods (clays and ring kilns). Our R&D Kon-tiki kiln does not have any heat recover, but when we move into our new premises our goal is to fit a heat recovery unit. On balance, Kon-tiki kilns have been approved for small scale biochar production.
Can I use activated carbon as biochar?
Activated carbon sometimes is known as powder activated carbon (PAC) or granular activated carbon GAC) can look like biochar. Can it be used instead of biochar?
Our primary goal is to improve crop growth. Based on our technical expertise many GAC/PAC simply do not confer the properties we believe essential to improve soil fertility. This does not mean you won't get some chemical benefits (read all about activated carbon) and of course these forms of carbon are stable so sequest (lock) carbon. We believe you can get even more from proper biochar.
Can any biomass be used to make biochar?
Yes and no. Process conditions (time and temperature) and the starting biomass affect the final structure of the char produced. Our view is that only high lignin woody materials make highly beneficial biochar.The biochar we use is based on our research around understanding what compost is (and is not). How compost and colloidal humus affect soil and then how biochar and other powders affect humification.
We will be adding more information on biochar, colloidal humus, composting and soil additives to our FAQ and blog in due course so please register and come back to ready more soon.