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Which biochar should you buy?

Tony Callaghan 01/05/2019

Which biochar should you buy?

This blog looks at different grades of biochar and the benefits they can bring to soil health and to the garden.

Basic Biochar

We champion the 0-13mm granules as the “basic” (standard) grade of raw biochar. 

Wood-based biochar granules around 0.5 to 2mm work best, with 13mm being the largest. Which is good because this happens to fit well with industry-standard sieving and grading machines!


We opt for wood as the core material because not only does it generate biochar with pores – it generates biochar with a range of pores – know as macro, meso and micro pores. If you compare the pores sizes to the typical size of microbes and fungi – then it is a small step to link the medium and large pores acting like walls and home, protecting the microbes from predators.

Biochar acts like a sponge collecting nutrients from the surrounding soil. If added “raw” to the soil, it can take a few weeks or months to become inoculated with nutrients, water and beneficial microbes. During this period it has been shown to hinder growth. Biochar suppliers, therefore, recommend biochar is inoculated (activated or charged) before use.

How do we make the raw standard wood-based biochar better?

Better Biochar - activated

“Raw” biochar should be activated (i.e. enriched, inoculated, charged) before use. There are several methods of achieving this: soaking biochar in liquid fertilisers, mixing it with solid fertilisers or combining it with compost. (By fertilisers we mean macro plant nutrients (nitrogen, potassium and phosphorus). The source could be natural organic (e.g. chicken poo pellets, bone meal, seaweed) or man-made chemical fertilisers.

If we think of biochar as a sponge, this type of “charging” fills the biochar pores with water and attaches the nutrient molecules to the active carbon sites in the biochar. An analogy would be a full kitchen of food ready for making meals (in our case a meal for the plant roots).

How do we make the best biochar?


There is no doubt that biochar added to soil provides a huge number of benefits. Biochar mixed with compost and or fertiliser nutrients work faster and avoid some issues. However, we and others (e.g. Hans Peter Schmidt) have found that when biochar is added to the composting process and the resulting compost-biochar matrix is used – then the biochar works even better.

Why does Biochar Super Compost work better?

Specific types of biochar-humus matrix actively support symbiotic rootzone soil microbes. These symbiotic root zone microbes and fungi dramatically improve plant growth and plant health.

Through careful choice of biochar added to the compost and careful control of the composting process to produce colloidal humus – we get Biochar Super Compost. In our process, we increase the beneficial colloidal humus from 2% to 20%. Colloidal humus is a super sponge:

  • it can absorb x10 own weight water (versus x2 in soil and compost)
  • it has a higher affinity for adsorbing nutrients (CEC 600 vs 20 in soil)
  • it is sticky – it aggregates soil particles into tilth which improves water and oxygen movement to the roots.

We have suggested the real success of biochar is related to increased colloidal humus. We call this biochar-humus matrix “Biochar Super Compost” (SF60).

It would be wonderful for us if all gardeners bought and used SF60. Many gardeners do not buy fertilisers or compost – they recycle and make their own organic compost/fertiliser by composting garden waste.

By adding the biochar super compost agent (aka the compost humification agent) home composters can make SF60 – in 90 days with a hot composting system or 9-12 months if using a ‘cold’ composting dalek or wood pallet box.

The steps below summarise the basic, better best and matches these to our products that can help you.

  • Raw biochar added to soil
  • BC charged with liquid nutrients then added to soil
  • BC mixed with compost then added to soil
  • BC added to green waste, composted and the compost/BC mix is then added to soil
  • Biochar Super Compost Agent (aka compost humification agent) added to green waste, composted and resulting biochar super compost added to soil

This article very much looks at the performance of biochars. Many people will of course also be interested in the environmental impact and sustainability of the biochar source. You can read more about these aspects at our sustainable biochars blog.

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