Alliums and Biochar: The Results Are Finally In

Allium crops drying on the patio. The lower 2 rows are the ones grown with biochar  
At last the crops are dried and everything's been weighed to find out whether growing alliums with biochar added to the soil makes a difference.

The Experiment

I created 2 identical raised beds, each filled with home-made compost from the same bin. One raised bed had a large bag of multi-purpose compost + biochar instead of some of the home-made compost. This was well mixed throughout the soil.

Each bed was planted out with the following:
  • 10 garlic cloves 'Solent Wight'
  • 15 shallot sets 'Jermoor'
  • 24 onions sets - 8 each of brown, red and white (from a mixed onion bag)
The alliums selected were matched as closely as possible in size and weight for each bed. They were then left to their own devices and were weeded as and when needed. The same amount of water was poured evenly onto each raised bed when watering duties beckoned.

The Results

In a nutshell, I'm declaring it a draw. The shallots were better grown in home-made only, the garlic was better grown with biochar and there was hardly any difference at all between the onions.

Here are the more detailed results:

From this year's and last year's results it would be easy to conclude that biochar makes little or no difference in a small garden situation. However, my experiments are too small-scale for a firm conclusion and also I haven't repeated them to see if the same results are obtained each time.

I've also been reading a very interesting summary of the work of Justus von Liebig with biochar in the 1840s. One of his conclusions was that whichever soil element is deficient, this is what limits a crop's potential. I've endeavoured in my experiments to create identical conditions as far as possible and used fresh compost for my non-biochar beds which is also humus rich. Thus it's quite likely that my experimental controls weren't deficient in carbon and so were acting in a similar way to the biochar ones. This would help to explain my inconclusive results. Biochar may be more effective for using in situations where soil fertility and/or good growing practices haven't been maintained.

Biochar is also reputed to remain effective for many years, so if I could create 2 identical beds where just carbon was depleted over time in the control bed, it would be interesting to see if there were any significant differences in the results. Sadly, I believe this is too advanced an experiment for my humble allotment!


  1. Interesting VP and as usual with your experiments most thorough. I do wonder though if beds can be identical - maybe in dimensions, depth they can but other factors come in such as position, shadows etc. Where your two beds adjacent to each other and will you be trying again next year? Whatever the results you've got a good crop to enjoy eating :)

    1. Hi Anna - good questions. The raised beds are next door to each other, so they have the same aspect, lack of shading etc.

      As for next year, the biochar will still be in 1 bed, but I'll be giving the experimentation a rest.

  2. Interesting results. I'd like to see what difference it could make to my appalling poor soil at home. Hope you enjoy your garlic and onions!

    1. TBH CJ, I think there are cheaper ways of achieving the same result. Hence my remarks in the conclusion.

  3. After all the fuss about biochar a couple of years ago, this makes for interesting reading, especially since your yields were not noticeably bigger with biochar. (Sorry, that sounds a bit negative, didn't mean it to be!) I think ANY soil improvement has to be worthwhile but until the cost is reduced I'll still be trotting down to the City Farm for a few bags of well-rotted horse manure! PS Did you see the Greg Wallace programme with the farmers last night? Amazing to see how plants responded so well to different coloured lights - worth watching, even to see tiny turnips being sold for 25p each!! Enjoy your harvest!! xx

    1. Hi Caro - as you know getting our soil right should be a gardener's number one priority. However, there is more than one way to do it. I could spend around £10 on a bag of biochar, or I could make my own compost for free and also invest in £15 for a trailer load of manure. I'm pleased to have had the opportunity to try this new product, but I know where my future lies...

    2. Oh, and I forgot to say the biochar doesn't go very far, but my compost + manure will do the whole plot

  4. Interesting experiment, as you say it's difficult to draw conclusions but both crops look good.

    1. Thanks Damo - coming from a vegetable shower that's very high praise indeed :)

  5. Hmmm, perhaps that's why it makes a difference commercially? Perhaps if you use it on land that hasn't been cultivated to that extent before, it has more impact? I must say, I'm not convinced of its (mega) benefits for the average keen gardener or allotmenteer, who has probably been chucking muck on their plot for a few years, and I think your trials are very valid for people like us. I agree with Anna: an impressively thorough experiment.

    1. Hi Victoria - I seem to be a lone voice saying the results are inconclusive. There are plenty of growers who are much better than me who are singing its praises. However, I think you have put the finger on the main point, most gardeners take great care of their soil, so the positive impact that biochar has would be lost.

      It's interesting reading Jane Perrone's recent blog post on Horticultural re how her raised beds are constantly shrinking down. I believe this happens to anyone who doesn't look after their soil, it's just more apparent with raised beds as they're still reaching an equilibrium. We had a plot holder up at the allotment who added no organic matter to his soil at all, relying on chemicals to do the job. His plot was a good foot below the grass paths around his plot.


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