Biochar for Stockholm Tree Pits: Improving Urban Tree Growth

Urban trees face some of the most difficult growing conditions. Compacted ground, limited rooting space, poor drainage, pollution and low soil oxygen can all restrict healthy tree development in towns and cities.

To overcome these challenges, landscape architects, councils, contractors and developers are increasingly using Stockholm Tree Pits as part of modern urban tree planting schemes. When combined with EBC certified biochar, these engineered systems can support stronger root development, better water management and healthier long-term tree growth.

At SoilFixer, we supply high-quality biochar solutions designed to improve soil performance in landscaping, green infrastructure and urban planting projects.

What Are Stockholm Tree Pits?

Stockholm Tree Pits are engineered urban tree planting systems originally developed in Sweden to improve tree survival in paved and heavily built-up environments.

Traditional tree pits often fail because roots are restricted to small areas of compacted, oxygen-poor soil. Stockholm systems create a more open underground structure using stone aggregate, soil and organic materials. This gives tree roots more space to grow while also improving drainage and aeration.

A well-designed Stockholm Tree Pit can help support:

• Better root development
• Improved water infiltration
• Increased soil aeration
• Stronger tree establishment
• Better stormwater management
• Longer-lasting urban planting

When biochar is added to the tree pit mix, the soil environment can become even more effective at supporting healthy growth.

Why Biochar Works in Stockholm Tree Pits

Biochar is a carbon-rich material produced through pyrolysis, where biomass is heated in a low-oxygen environment. Its highly porous structure makes it especially useful in urban soil systems, where water retention, nutrient availability and soil biology are all important.

In Stockholm Tree Pits, biochar acts like a natural sponge within the soil profile. SoilFixer typically adds around 5 to 10% biochar into Stockholm Tree Pit systems, depending on the project requirements.

This helps create a more resilient rooting environment for trees growing in challenging urban conditions.

Key Benefits of Biochar for Urban Trees

Improved Water Retention

Urban trees often experience inconsistent moisture levels. During dry periods, compacted or free-draining soils may not hold enough water around the root zone. During heavy rainfall, poor drainage can also lead to waterlogging.

Biochar helps retain moisture while still allowing excess water to move through the soil structure. This balance is particularly valuable in urban tree planting schemes, where trees need reliable access to water without becoming waterlogged.

Enhanced Nutrient Retention

Nutrients can easily leach through sandy, stony or free-draining urban soils. Biochar helps hold nutrients within the soil structure, making them more available to tree roots over time.

This can support stronger establishment, healthier root systems, improved canopy development and reduced nutrient loss.

Better Soil Biology

Healthy soil biology is essential for long-term tree health. Biochar provides habitat for beneficial microbes and fungi, helping to support nutrient cycling and root development.

Over time, this can create a more resilient underground ecosystem, giving urban trees a better chance of thriving in paved or restricted spaces.

Stronger Tree Establishment

The early establishment phase is critical for urban trees. If roots cannot access enough air, moisture and nutrients, growth can be slow and long-term survival may be affected.

By improving water retention, aeration, nutrient availability and microbial activity, biochar can help create better conditions from the start.

Why EBC Certified Biochar Matters

Not all biochar products are the same. Quality, feedstock and production standards can vary significantly between suppliers. This is especially important in urban landscaping and infrastructure projects, where poor-quality materials may affect soil health, water movement or plant performance.

The European Biochar Certificate, known as EBC, is designed to provide a recognised quality standard for sustainable biochar production and sale. The EBC states that its certification system helps guarantee sustainable biochar production, processing and sale, giving customers a reliable quality standard.

Using EBC certified biochar helps provide confidence around:

• Low contaminant levels
• Consistent product quality
• Sustainable feedstock sourcing
• Carbon stability
• Safe use in soil and landscaping applications

For councils, developers, contractors and landscape architects, certification helps reduce risk and supports better long-term project outcomes.

Biochar, SuDS and Sustainable Urban Drainage

Stockholm Tree Pits are often used as part of Sustainable Drainage Systems, also known as SuDS. These systems help manage rainfall naturally by improving infiltration, reducing runoff and supporting greener urban spaces.

Biochar can support SuDS tree pit designs by helping soils retain moisture during dry periods while improving infiltration during rainfall. This can contribute to reduced pressure on drainage infrastructure, improved urban cooling, better climate resilience and stronger long-term tree survival.

Stockholm Tree Pit specialists also highlight the importance of giving trees enough soil volume to mature and deliver long-term benefits to the local community.

Long-Term Benefits for Urban Landscapes

Trees planted in well-designed Stockholm Tree Pits can deliver decades of environmental, visual and social value. Adding biochar helps improve the quality and performance of the soil environment from day one.

Long-term benefits can include:

• Healthier tree growth
• Larger and stronger canopies
• Better drought tolerance
• Reduced maintenance pressure
• Improved soil resilience
• Enhanced carbon storage
• Better-performing green infrastructure

For urban developments seeking sustainable and future-proof planting solutions, combining Stockholm Tree Pits with EBC certified biochar is a practical and effective approach.

SoilFixer Biochar for Urban Tree Planting

At SoilFixer, we work with landscapers, contractors, developers and local authorities to improve soil health and urban planting performance.

Our biochar solutions are designed to support modern landscaping, tree planting and green infrastructure projects where soil quality, drainage and long-term resilience matter.

As towns and cities continue to invest in healthier urban environments, biochar is becoming an increasingly valuable tool for creating stronger and more sustainable planting systems.

Final Thoughts

Stockholm Tree Pits have transformed urban tree planting by creating better underground growing conditions in challenging environments. When combined with around 5 to 10% EBC certified biochar, these systems can become even more effective at supporting healthy, long-term tree growth.

From improved water retention and nutrient availability to stronger soil biology and better drainage performance, biochar offers significant benefits for urban landscaping projects across the UK.

For councils, developers, contractors and landscape professionals looking to improve urban tree performance, high-quality biochar is an important step towards greener, healthier and more resilient cities.

Spring Soil Preparation in the UK: Improve Soil Health with Biochar Compost

Why Spring Soil Preparation Matters

Below the surface, a new season is ready for action

Success in our gardens looks great, but the most important part happens below the surface. According to the Royal Horticultural Society, maintaining healthy soil through composting is essential for strong plant growth.

As temperatures begin to rise and daylight increases across the UK, plants start growing quickly. In order to support this growth, we need our soils in the right mood from the beginning!

After winter, however, soil can be compacted, out of balance and lacking some health. Preparing it early is what sets strong gardens apart.

That’s where SoilFixer’s organic, biochar composts & biochar give you a great start.

Start the Growing Season on the Right Foot

The Benefits of Early Soil Preparation

Winter conditions can leave soil quality struggling. Heavy rain can wash away nutrients, while frost can reduce airflow and structure.

Before planting begins, we need to build our soil back up to a healthy, balanced medium.

By improving soil early in the season, you:

• Create better conditions for root development
• Improve drainage and aeration
• Support consistent, healthy plant growth

Spring isn’t just about planting; it’s about preparing the ground for success. Our SF40 is a wonderful multipurpose biochar compost to create the perfect conditions for your plants to wake up from winter!

Why SF40 Biochar Compost Works

SF40 Multipurpose Compost combines sustainable, locally sourced brash and biochar a handful of horse manure from the farm, creating a more advanced and ‘alive’ soil solution than standard compost.

What Makes Our SF40 Effective:

• Organic nutrients that support natural soil biology
• Biochar that helps retain nutrients and improve soil structure long-term
• Improved moisture management in changing spring conditions
• A balanced growing environment across multiple applications
• Well-rotted horse manure from the farm (any horses which may have taken medication or wormers are removed)

Biochar acts like a sponge within the soil – holding onto nutrients and water while creating space for beneficial microbes to thrive.

What Biochar Does in Soil

Biochar improves soil structure by increasing aeration, retaining nutrients, and supporting beneficial microbial activity. It acts as a long-term soil conditioner, making it especially valuable in spring soil preparation.

Built for Unpredictable UK Spring Weather

UK spring weather can chop and change quickly – dry one week, wet the next.

How SF40 Stabilises Soil Conditions:

• Retaining moisture during warmer, drier days
• Allowing excess water to drain during heavy rainfall
• Holding nutrients in the soil for longer availability

This creates a more resilient growing environment during early plant development.

A Sustainable Way to Improve Soil Health

Choosing an organic, biochar-enriched compost means investing in long-term soil health – not just short-term results.

The Environmental Benefits of Our Biochar Compost:

• Using sustainably sourced, local biomass to make high-quality compost
• Locking carbon into the soil permanently with biochar
• Reducing the need for synthetic fertilisers

Using SoilFixer products is a forward-thinking way to gardening.

Final Thoughts: Strong Soil, Better Results

Get the soil right first and the results you’re after will come.

By improving your soil with SoilFixer compost, you:

• Build a stronger foundation for the season
• Support healthier, more resilient plant growth
• Improve results across your entire garden or project

Because when your soil is truly alive and balanced, everything else follows.

SF60 Super Compost Soil Improver: Uses, Benefits and FAQs

This article looks in detail at the SoilFixer SF60 Super Compost – what it is, how it is made, how it works and the benefits of using it.

The Development of SF60: From Innovation to Super Compost

Tony Callaghan has a long track record in composting – he invented the award-winning, patented hot bin composter (the patent and trademark are now assigned to Engineered Foam Products Ltd). Tony noticed the compost coming out of these bins was more sticky and held more water than normal compost.

This started a research project to investigate what compost is made of and if there was such a thing as a better compost, i.e. a super compost. Having looked at the science around what bacteria do with organic matter, the substances that make up colloidal humus,  Amazonian Dark Earth (ADE), biochar and a host of other things related to how bacteria and fungi really work in the soil, he formulated a theory and set about testing various “additions” and “formulations” of new types of compost.

Finally, there was that eureka moment – a compost that gave better growth results, which survives for many years and sequesters carbon so offsetting global warming gases. The new product (SF60) is 100% organic, natural, peat free and made from sustainable resources.

What Is SF60?

SF60 Super Compost is made in the UK using our propriety composting process and humification agent. The resulting product is a matrix of activated (enriched) biochar, colloidal humus, organic matter (compost) and plant macro- and micro-nutrients.

Formulated ready to use by gardeners and allotment growers who want to improve their soils and container growth mixes to the best they can be.

In our SF60 tests, we have seen an increase in plant growth and crop yield of 100% over the control soil.  (All plant growth testing is complex and it takes many test replications over a number of years before the average increase can be quoted. We believe most users will conservatively achieve +20% increase).

How does SF60 work?

The biochar/colloidal humus matrix acts as a ‘sponge’ for water and essential nutrients – storing them ready for use. The ‘sponge’ matrix supports increased levels of soil microbes. The microbes (notably mycorrhizal fungi) can access the stored nutrients.  Microbes form symbiotic relationships with plant roots and supply nutrients to the roots. This mechanism is sometimes described as ‘improved soil health leading to improved plant growth.

One of the easiest ways we have to describe SF60 is by thinking about sponges and what they do.

• Biochar granules are like hard sponges that absorb water and nutrients
• Enriched /activated biochar is a sponge filled with nutrients and water
• Colloidal humus is a soft sponge that absorbs nutrients and water (X3 more than biochar!)
• SF60 is a matrix of biochar and colloidal humus produced when we add biochar into the composting process.

Once we have ‘nutrient and water sponges’ in the soil we need to connect them to plant roots. Some of this happens when a root hair touches the humus/biochar particle.

However, it now appears symbiotic mycorrhizal fungi (AMF/EMF) are far more important as a transfer mechanism. The fungi and roots create an exchange system. Nutrients are sent to the roots in exchange for carbohydrate sugar from the plant.

Benefits of using SF60

• It holds lots of water – excellent water-holding capacity
• It keeps hold of water for a long time – excellent water retention
• It re-absorbs water very quickly after drying – excellent re-wetting ability
• Macronutrient (NKP) supply – has both fast and slow-release nutrients
• Micronutrients (Ca, Mg, etc) – contains micros nutrients in plant-available forms
• Improved nutrient supply to roots – builds and supports bacterial/fungal roots associations that increase nutrient supply to roots (e.g. mycorrhizal fungi, AMF)
• Reduces nutrient leaching – colloidal humus has high CEC reducing leachate losses on irrigation
• Improves soil aggregation leading to improved tilth and aeration (essential oxygen flow to roots and microbes)
• Results are long-lasting – colloidal humus and biochar typically survive in the soil for over 100 years (note nutrients transferred and removed via crop need replenishing)
• Positive environmental impact: colloidal humus and biochar both sequester carbon and help offset carbon dioxide and hence reduce the impact of greenhouse gases

Is SF60 different to biochars and activated biochars

Yes – very different.

Many biochar suppliers have moved away from supplying ‘raw biochar’ to selling ‘activated (charged, enriched) biochar’. Biochar ‘holds onto’ plant nutrients. If you think of biochar as a type of sponge – if you add a dry sponge, the first thing it does is suck up water and nutrients from the soil. Hence it is better to add a wet sponge full of water and nutrients.

Many seek to charge (activate) biochar by mixing it with compost, others by ‘soaking’ in liquid fertiliser. We have found neither to be fully effective.

We have looked beyond ‘mixing’. We start with soil biology and build a soil improver matrix that supports microbes. Many of these microbes work with the plant root hairs to move nutrients into the plants.  In our analogy above, we are filling the sponge pores with colloidal humus. This holds more water and nutrients and supports microbes.

We believe the most effective way is to add the biochar into the active compost heap and sell an ‘integrated mix’ of colloidal humus and biochar.

What is SF60 made from?

We combine green waste into a proprietary composting process with our humification agent made from biochar, micronized mineral powders, NPKs and some particulate organic matter.

The approximate composition of SF60 is:

Key: CH = colloidal humus, BC = biochar, TM = micronized trace minerals, NPK – nitrogen phosphorus, potassium nutrients, POM = particulate organic matter (i.e. compost). * Soil will contain mostly clay, sand and silt particles.

Is SF60 a fertiliser?

It depends on how you define and use the word! The dictionary defines ‘fertiliser’ as a chemical or natural substance added to soil to increase its fertility.  On this basis, SF60 is a fertiliser – i.e. a natural substance that contains nutrients (micro and macro NPK, Ca, etc) plus organic matter and biochar designed to improve fertility, i.e. plant growth.

However the term “fertiliser product” is defined and certified for use under EU/UK law. Currently, ‘fertiliser products’ are restricted to the chemical compositions of nitrogen, phosphorus and potassium (NPK). Typically fertiliser products have 10-30% NPKs and are usually soluble ‘fast release’ forms.

We are not allowed to call SF60 a fertiliser product, although we could (subject to testing and submission for approval) now call it an organic fertiliser under the new wider EU fertiliser definitions.

To maintain soil fertility, you either need to recycle nutrients or you need to replace nutrients taken away in vegetables and crops. Heavy cropping plots (e.g. vegetables) will deplete NPK nutrients from the soil and SF60.   Once fully depleted, you will have to replenish i.e. add nutrients back to the soil or container mix.

How is SF60 different to biochar, enriched biochar and activated biochar?

One of the easiest ways we have to describe the difference is by thinking about sponges and what they do.

• Biochar granules are like hard sponges that absorb water and nutrients
• Enriched /activate biochar is a sponge filled with nutrients and water
• Colloidal humus is a soft sponge that absorbs nutrients and water (X3 more than biochar!)
• SF60 is a matrix of biochar and colloidal humus produced when we add biochar into the composting process.

There are hundreds of types of biochar and only a few are proving beneficial as soil improvers. Further, how beneficial biochar is in soil depends on how they are used – raw, activated/charged, co-mixed with compost or co-composted (per SF60). Thousands of biochar papers have been published and we now have a ‘meta-analysis’that summarises these papers. It points to biochar + organic matter outperforming ‘biochar’.  We are proud that SoilFixer was one of the first suppliers in the world to identify that co-composting biochar with plant (green) waste using a specific formula of biochar and a specific composting method produced a mix (SF60) that improves soil fertility and plant growth way beyond adding enriched biochar to soil.

Do I need to add SF60 each year?

You can add the total amount needed in one session or top-up each season. The biochar and colloidal humus in the SF60 product remains in soil year after year. You can top-up and build towards 20% target by adding a bit more each year – this gradual improvement spreads the cost over more than one year.

Do remember – the product will help your plants access nutrients. However, if you are heavy cropping, the vegetables taken away deplete the NKP soil nutrients. Hence you will need to top up NPKs via your chosen organic feed/fertiliser. To maintain nutrients level – you have to put back what you take away.

Can I repair degraded (infertile) soil using one addition of SF60, i.e. in one step?

Yes, but the cost might be prohibitive, Adding SF60 can dramatically reduce the time taken to repair soil as you are adding a large percentage of stable biochar and colloidal humus.

Soil is degraded usually by a combination of nutrient depletion and loss of colloidal humus. The spiral gets faster as it proceeds – the loss of colloidal humus also affects water retention and soil aggregation leading to the dust bowl syndrome.

We feel it is fair to state the essential difference between fertile and infertile soil is colloidal humus and its role to glue and aggregating soil particles. As soil degrades, adding fertilisers becomes less effective – due to runoff (leaching).

Again, this is linked to the decline in colloidal humus. It can take 50 years to completely degrade soil and 50 years to repair using compost and no-till regimes. Compost decomposes but leaves very little colloidal humus, hence it can take a long time to rebuild the humus.

We believe adding SF60 can dramatically reduce the time to repair as we are adding stable biochar and colloidal humus.  We are happy to work with the experts who can prove this.

How long does SF60 last?

The scientific evidence suggests biochar and or colloidal humus can survive in soils for hundreds and in some cases thousands of years. The exact lifetime depends on many variables.  Our tests (using CO2 respiration levels), indicated at least 10 years. (It takes much more sophisticated test equipment to measure beyond 10 years!)

Notea; the SF60 product is approximately 60% biochar and colloidal humus, approximately 5-10% macro & micro nutrients and 30% organic matter. The organic matter part will decompose in 1-2 years.

Application rates – based on Area

• The recommended range is 2 to 5 Kg per square metre (Kg/m2).
• which is 10% by volume, 10 parts soil to 1 part SF60
• The minimum is 0.5 Kg/m2
• The maximum recommended is 10 Kg/m2 for SF60

More info on SF60 application rates and making the product go further can be found in our blog

In SF60, the biochar and colloidal humus fraction remain in soil year after year. You can top-up and build towards 20% target by adding a bit more each year.

How to use SF60

Dig (or hoe) into the soil to depth 5-10 cm.

Reasons why should always dig in your SF60:

• Colloidal humus has fantastic water holding capacity, retention time and re-wetting. If left to dry in the sun, it can lose the ability to hold water or take on water (rewetting). The colloidal molecules transform and beyond a certain dryness point this is irreversible – it no longer has any water-holding property.
• Colloidal humus acts as a soil aggregator – it helps the soil particles ‘stick’ together into small lumps. This increases tilth and helps water and oxygen flow. It cannot perform this task when sat on the surface of the soil – it needs digging in.
• Colloidal humus has a very high CEC value. It acts as a ‘sponge’ for soil nutrients, in other words, it holds onto the NKP nutrients and releases them to roots when needed. Your roots are deep in the soil – hence your colloidal humus needs to be in the soil near the roots.

Storage

Store in a cool dry place.  Preferably use within 3 months. When storing, avoid long hours of exposure to direct sunlight as this will dry the product. If the product becomes completely dry (i.e. friable) as it will lose some of the colloidal properties.

If the product becomes completely dry, rather than directly to the soil dry, it is better to revitalise it by adding it into your compost process and then add the revitalised compost/SF60 mix to the soil.

Why is SF60 sold damp/moist?

The product contains soil microbes, colloidal humus and water.  If the product is completely dried, the microbes die and the colloidal humus loses the colloidal property. As the product contains water, in line with the Growing Media Association guidelines, we should sell based on volume (number of litres) not weight (Kg) in the bag.

However, we are an online business and our products are delivered by courier based on very strict weight bands. So we state the weight and an approximate litre volume.

Is SF60 a type of compost?

Not really. Compost and SF60 are both soil improvers and both supply slow-release nutrients. However, compost contains ~95% partially degraded pieces of organic matter and ~5% colloidal humus. SF60 is 20% biochar, 40% colloidal humus, 7% trace minerals and NPKs and around 33% POM (i.e. compost).

Can I use SF60 as a compost activator?

Yes. It contains soil microbes, biochar and colloidal humus – all of these will activate/inoculate compost. However, you are using a fully formed soil additive.  To convert your compost into your own version of SF60, it is better to add the SoilFixer compost humification agent and raw biochar.

Is SF60 a source of trace minerals?

Yes – SF60 contains about 5% of micronized (very fine) powdered trace minerals

You mention SF60 contains colloidal humus? What is colloidal humus?

We define ‘colloidal humus’ as organic (carbon-based) colloidal molecules that absorb water, have exceptionally high CEC, WHC, retention and aggregation (stickiness) properties. Such colloids have significant benefits in soil and compost.

Fertile soil contains a tiny fraction of these vitally important organic colloids. We do not think it overstates the case to say they are fundamental to soil fertility. When these compounds are lost from soil, crop yield (productivity) declines.

Is SF60 some new miracle gardening product or compound?

It can work exceptionally well in certain situations! However, we work hard to avoid hype and sensation and focus on how and why things work. This can mean things get too technical for many readers. Rest assured: colloidal humus and biochar exist in nature. What we do is understand how nature makes them, what combination improves soil fertility and how to make them for you!

Does SF60 kill soil pathogens?

SF60 contains no man-made (artificial) chemical herbicides, pesticides or fungicides, i.e. anything added that kills pathogenic bacteria.

However, it does support healthy soil. Soils, composts and soil improver products can contain both good and bad (pathogenic) microbes. In nature, microbes compete ‘good vs bad’.  Although the exact means is not understood, biochar and colloidal humus appear to promote good soil microbes that form symbiotic relations with plant roots.

This healthy community of microbes and plant roots is stronger and out-competes (suppresses) many plant pathogenic bacteria. Plants are healthier.

Is the SF60 sterilised (pasteurised)?

Sterilised is normally taken to mean the killing of all microbes. SF60 is not sterilised. The product goes through a special type of hot composting at 60C. Pathogenic (‘bad’) bacteria are suppressed and killed-off by healthy microbes.

The product contains ‘good’ bacteria, so it is not sterile (defined as no living bacteria). It would be more correctly described as a pasteurised product – i.e. heat-treated to reduce harmful bacteria.

How much biochar (or SF60) should you add to your soil and compost?

In our Biochar and SF60 product pages, we simplify the application rate to “1 Kg/m2” (1 kilo per square metre of soil)

As we supply 10 and 20 Kg sacks,  the maths is simple: 10Kg will cover 10m2,  20 Kg 20m2

Returning guests please note this blog was revised 26th August 2021

This is a simplified approach. Depending on your needs, it might not be accurate enough. Below we discuss in more depth what affects application rates and how to jump through the various calculations to ensure you are on the right track.

What affects the application rates of biochar and SF60 super compost?

There are five areas to look at:

• Our soil health goal
• Application rate based on the area of soil (how many square metres, m2)
• Application rate based on the volume of soil (i.e. how many cubic metres, m3)
• Being cost-conscious: maximising the benefit whilst minimising the cost
• How you mix (apply) the biochar (or SF60)

Biochar application rates based on soil health

Your goal is likely to be: create the very best soil conditions. Your reference point for good soil is what high-quality loam – a dark rich crumbly soil. What defines and makes the rich loam is humus (not compost – read here for the difference).

Adding biochar (and SF60 super compost) positively affects the texture, tilth and organic matter of the soil. It also affects the microbial population.  Our advice (aligned to other suppliers) is to add 5% by volume of biochar. The 1 Kg per m2 approximates to 5% (but see the note below on soil depth!).

There are many biochar academic studies that report benefits from a wide range of application rates from as low as 1% to as high as 30%. If you had very poor soil you might need more, good soil add less.

Although there is a common understanding that organic matter (compost and biochar) are good, you CAN add too much.  When the organic matter goes above 20% other issues start to occur – it can dry out quickly, become friable and be subject to wind erosion. Alternatively, in very wet conditions, it can become airless and acidic (i.e. a peat bog).

It is very challenging for professional soil scientists and gardeners alike to easily validate what % of soil organic matter level (sometimes referred to as soil carbon)  their soil has and hence the optimum level of biochar or SF60 to add. One thing is certain – you cannot take it out! Gardeners on the whole are patient – building better soil over a number of seasons might be more appropriate.

Application rates for biochar (and SF60 super compost) based on Area

This is the easiest method. For example, apply 1 Kg of biochar per square metre of garden soil. However, it is also the least precise because it does not account for the soil depth. Referring to the excel chart below,  digging into a depth of 2.5cm, 5cm or 15cm produces a five-fold change in the % added (10%, 5%, 2% respectively).

Biochar application for maximum impact with minimum cost

The most powerful biochar and SF60 benefits involve interactions between the microbes and root hairs. 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. When you mix it into the soil you are improving the whole soil – where you choose to plant is not relevant as all the soil is better.

You can use the root-zone impact to stretch and make your budget go further!

If you are planting up and digging holes, it is more cost-effective to apply a thin layer in the bottom of the hole

It is more cost-effective to only mix biochar into the backfill soil

Example: planting 10 trees, 30cmx30cmx30cm (1x1x1 ft)
Each hole would be approx. 27 litres of soil taken out. At 5% that is 1.4 litres of biochar per backfill hole. 10 holes = 14 litres is approximately one 5 Kg of biochar.   If you dug the whole soil and added 5% by volume, that would probably be an area of 20m2. At 1 Kg/m2, that would be a 20 Kg bag (x4 the quantity).

Should I top-dress biochar onto my soil? (i.e. spread a top layer)

Preferably not. You get the maximum benefit when mixed in at the root zone. However, a top dressing when lightly hoed into the top few centimetres of soil will work. Worms will pull down biochar/SF60 into the soil.  Rainwater will flush the smallest particles down to the roots eventually (allow at least a season). Also in very hot summer conditions, biochar and SF60 left on the surface will dry out.  When it gets exceptionally hot and dry (above 60C, below 40% moisture, the colloidal nature of humus is lost).

Application ratios of biochar to soil based on volume calculations (litres, and cubic metres m3)

The volume of soil is the width of bed * length of bed * depth. But the depth of soil you are digging into will/can vary. For example: if you are hoeing in (or trowel planting) then usually you are looking at a maximum depth of 2.5cm (1 inch). If single digging 5-10 cm (3-6 inches) and double digging or ploughing 20-30 cm (12 inches).

Referring to the excel table below, adding 1Kg to a 1x1m square has a different volume of 25 litres, 50 litres or 150 litres based on the depths. Adding 1Kg would be 10%, 5% or just 2% depending on how deep you till it in.

Application ratios of biochar to soil based on weight calculations (Kg biochar. Kg soil)

Mixing ratios based on weight is by far the most complex method. You have to know the bulk density of your soil (which varies from 1-2 g/litre (or 1000-2000 Kg/m3). You also have to decide if you are working with dry weights or wet weights. Biochar can absorb 3-4 times its own weight in water. Soils can absorb 1-3 times weight.

Usually, this method is only used in academic studies. This makes it all the harder to translate the academic papers referring to 1% to 30% by weight back into real-life simple gardening application rates based on Kg/m2.

An example application rate of 10% biochar by weight

Take volume in m3 and multiply by bulk density in Kg/m3

• 1m X 1m X 0.2m  depth =  0.2m3 of soil, multiplied by bulk density of 1200 (kg/m3) =  240 Kgs
• 1m X 1m X 1m = 1m3 * 400 Kg/m3 of biochar = 400 Kg of biochar per bulk bag
• 10% by volume would be 24 Kg / m2 at 20 cm depth

We have an Excel spreadsheet available – please email help@soilfixer.co.uk for a copy.

Can biochar be used to improve green roofs?

The quick answer is a huge yes.

Let’s look in a bit more depth at how biochar is used in green roofs

What is a green roof?

A green roof (also known as a ‘living roof) is a roof on a building that is covered with plants growing in a growing medium. This usually sits on a waterproofing membrane and may include other layers such as a root barrier, drainage, and irrigation pipework.

Why add biochar to a green roof growing medium?

Biochar delivers several benefits

• It fundamentally improves plant growth (see our biochar tests results)
• It retains water – preventing ‘thin’ living roof layers from drying out as quickly, but also helps prevent waterlogging
• It has a low bulk density – 250Kg/m3 dry, compare with compost 500 Kg/m3 or soil 1700 Kg/m3. The extra weight of a living roof is a substantial design challenge

SoilFixer’s 0-8mm Biochar weighs just 250 Kg/m3 dry, 400 Kg/m3 as supplied, and 700 Kg/m3 saturated. Biochar can be sourced in fine granules (0-2mm), medium (0-8mm), and coarse (2-8mm).  We recommend no bigger than 10 mm as larger particles tend to have a detrimental impact on properties like ‘aggregation’ which makes the soil fall apart too easily.

How much biochar is added to green roof soil / growing media?

• For base layers, use 100%, ie just biochar
• For the growing media, we recommend 10-30%

More than 30% will help with weight, but it tends to tip the balance away from strong plant growth

Green roof types and construction

The experts describe two types of green roofs

• Intensive roofs, which are thicker and require more maintenance
• Extensive ones, which are lighter, have less vegetation and require less maintenance.

They come in many forms from slim-line to very thick.

(Rooftop designers have precise maths and engineering formulas for loading and safety – the above is a rough approximation to give gardeners an appreciation of the challenge)

The addition of biochar to green roof soil / growing media is only a few years old –  as more roofs are built and tested, the advice will become more precise.

If you have a shed, garden office or outbuilding, or even wheelie bin storage, you can create your own DIY living roof by adding a water-proof membrane and a barrier to keep everything in place and plant your own.

Best plants to use for green roofs

Shallow-rooted varieties are the best options for planting living roofs such as various species of sedum, succulents, wildflowers, and grasses. Be mindful of the positioning of the roof and how much or how little exposure to the sun it gets, choose plants depending on the conditions.

Benefits of living roofs

● Creates a habitat and food for wildlife

● Contributes to improving air quality

● Provides insulating and soundproofing properties

● Improves drainage

● Adds a touch of nature to an otherwise unnatural surface

How is biochar added to the green roof soil/medium?

Usually, this is added at the start, but it can be added when the growing medium is being ‘repaired/topped up (see degradation below)

Sometimes the biochar is added as a granular base layer and other times it is integrated into the growing medium (see SF40)

Biochar offers multiple benefits but some of these benefits have to be balanced with other challenges. The route chosen depends on the specific challenges the roof installer is looking to address (see below).

What are some of the other criteria a green roof designer might consider when utilising biochar?

• Weight and density of the growing media layer, safe structural loading
• Hydrological  – desired drainage, stormwater runoff, water holding capacity
• Nutrients – how much, amount, slow-release, nutrient runoff
• Minimise long-term degradation – eg media items such as peat, coir, and compost decompose over 1-5 years
• Creating a natural habitat or “urban wilderness”
• Filter pollutants and CO2 from the air
• If a company building, contributing to Corporate CSRs goals
• Enhancing building aesthetics
• Sustainable wastewater management
• Outdoor water use reduction
• Design for Enhanced Resilience
• Local food production
• Green power and carbon offsets
• Rainwater management
• In high urban densities, reduction of the city’s average temperature during the summer.

Are there any experts in green roof tech? Yes

Google ‘Green roof design and installation’. Although we have not worked in partnership with any particular installer, we did like the information presented on  GreenRooftech  website and Green roof training.

Green roofs with biochar: carbon offsetting & positive climate impact

We predict a huge uptake in biochar use for green roofs. All the above benefits are great – but biochar has a trump card – it is now widely recognized as the leading carbon-offsetting technology. Carbon Credits (see PuroEarth, Verra) use a certified methodology that demonstrates biochar-carbon is locked away for at least 100 years and hence offsets CO2 emissions. This marketplace is now growing at +50% year on year.

Not only will biochar be added to agricultural soils and horticultural growing media it will be a number one choice for corporate buildings with green roofs.

The benefits of adding biochar to green roofs also apply to ” Living Walls”

Living Walls are popular where space is limited or the building has a restricted floor space for soil/growing media. Adding biochar to living walls can help improve plant growth and health by providing a range of benefits, including improved water retention, higher nutrient availability, and better root development.

Image: British_Horse_Society_Head_Quarters_and_Green_Roof.jpg. Please note: Our use of the open-source image is not meant to imply any association with or use of SoilFixer Products. It is just a really nice green roof!  Image attribution:  Sky Garden Ltd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons