How to use biochar in containers

Biochar can be added to a growing media to help improve drainage and water and nutrient retention.

What is biochar? And is it the magical ingredient some claim?

Before reading further, you should know that it is not essential to know about (or use) biochar to grow successfully.

However, if you want to create a growing mix that can be re-used successfully for many years, biochar is worth experimenting with. Particularly if you can find a local supply. 

What is biochar?

Biochar is wood charcoal, finely ground, specially made for growing. As an addition to a growing mix it can be valuable because it:

  • helps to retain water,
  • gives structure to the soil, improving aeration and drainage.
  • provides a home for soil life and a reservoir for nutrients.
  • lasts for tens or even hundreds of years – and locks CO2 into the soil.

You can buy biochar pre-treated with nutrients and microbial life or ‘raw’. If you buy raw biochar, you need to add it to your womery or compost heap before using it in a growing media (or it will ‘suck’ nutrients out of the soil).

What difference does it make in practise? Amongst the success stories you can find online, others (including some professional growers) have found that biochar may have little positive impact and sometimes even be detrimental to growing. The result depends on both the type of biochar used (the way it is made and ‘activated’ varies) and the mix it is added to. It is not completely straightforward. 

My own (limited) tests

I’ve tried biochar for a few years now, mixing 10 – 15% into the mix in some of my containers. To be frank, I haven’t noticed much difference in the growing – although this is just an observation, not a rigorous test.

That’s the less good news. The better news is that the structure of the growing mix with added biochar did look noticeably better at the end of the season. This is important as sustaining a growing media from year to year (without throwing it all away) is one of the biggest challenges of container growing. Few of us want the effort, hassle (and waste) of replacing compost every year – particularly if you live on the eighth story!

I’ve also gone on to re-use this mix successfully for three years now. 

How you can use biochar

To try biochar for yourself, the easiest way is to buy some treated or ‘charged’ biochar – and mix 10 – 25%  into your growing mix. If you have several containers, label which ones you’ve added biochar to.  You can buy treated biochar from some garden stores and online.

If you buy some untreated biochar (look for a local coppice worker who makes it – or buy online) add it (in layers) to your compost heap or wormery before you use it for growing. The composting process will fill the the pores in the biochar with soil life and nutrients.  The biochar will also help to improve the structure of your compost or worm compost.

This is raw, untreated biochar I bought from an urban coppice worker in Manchester. This needs adding to a wormery before using.
This is raw, untreated biochar I bought from an urban coppice worker, Mike Carswell, in Manchester. This needs adding to a wormery before using.

Developing a more effective biochar

One person working at the cutting edge of biochar is Tony Callaghan of Soil Fixer.

Tony’s experiments suggest that the performance of biochar depends on how it is charged with nutrients and life. He says that most commercial biochar is activated by mixing it with ready made compost. But he finds that biochar that is part of the composting process performs better.

The kale in this picture is growing well in a container with a mix of biochar and compost, made by Tony Callaghan.
The kale (cavelo nero) in the foreground is growing well in a container with a mix of biochar and compost, made by Tony Callaghan.

Tony has also found that adding biochar can help the formation of more “colloidal humus” in the compost. I’d never heard of colloidal humus before – but it sounds fascinating. The details are complex, but in essence colloidal humus holds on to water and nutrients about five times better than normal compost. So potentially a very good thing for a growing mix.  In a  normal compost heap, little if any colloidal humus is made. Tony is now pioneering a mix of biochar and other ingredients (he calls a humification agent) that will encourage the formation of more colloidal humus in compost. 

Biochar in a nutshell

In summary, biochar is an interesting product. It has many potential benefits in container growing and compost making. It can be made sustainably and locally and locks up carbon. It has potential to help create a growing mix to sustain year after year – and as an ingredient to add to wormeries or compost. However, how it works out in practise will depend on the type of growing media you are using and the quality of your biochar. It’s a case of trial and error.

Your turn

If you’ve tried biochar in container growing – with good or not so good results – I’d love to learn from your experience in the comments. 

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