Biochar and SF60 Application Rates

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 [email protected] for a copy.