Getting to Grips With Biochar

I've been trialling various biochar products since the spring, courtesy of Carbon Gold and now it's time to make some sense of the results.

The trials I set up were:
  • Seed germination rates with rocket seed - biochar seed compost vs John Innes seed compost. Plants were raised  indoors and each tray given the same amount of water (via a spray mister)
  • Seed germination rates and salad leaf productivity using the seed mats I showed you in March (plus an update picture here) - biochar vs New Horizon peat free compost. Plants were raised outdoors and no supplementary watering was required owing to the weather!
  • Salad leaf productivity in my new growing area - biochar vs New Horizon peat free compost and no supplementary watering was needed
  • Tomatoes grown in pots - biochar vs New Horizon peat free compost
  • Allotment cucumber and courgette growing - biochar vs home-made compost in allotment soil and no supplementary watering needed
Owing to the poor summer the tomato trial had to be abandoned. This also happened with the courgette trial because it turned out the courgette plants were different varieties. Strange seeing the seed was from the same packet!


Seed germination - I've already reported on the results of the rocket seed trial here. The germination rates with the salad seed mats were 50% higher for the seed mat grown on New Horizon compost. There was no observed difference in seed germination rates for the cucumber and courgette trials. 

Salad leaf production - a mixed set of results. The salad leaf mats yielded 25% more with biochar and the new salad growing area yielded just over 53% more for the leaves grown in the New Horizon (i.e. non biochar) compost.

Cucumber - the home-made compost + allotment soil cucumbers were larger and more prolific than the allotment soil + biochar ones. However, owing to pest damage the cropping season was too short for this trial to be conclusive.

Other observations - there was no noticeable difference in plant condition or colour between the biochar and non-biochar raised plants. All plants seemed equally susceptible to pests (!). I didn't compare root growth during the trial.

Comparison with results elsewhere - Carbon Gold have announced the results of trials conducted by professional growers, which are positive all round. The Big Biochar Experiment independently run by Oxford University has reported (by email) positive results using biochar for wheat germination plus improved onion, carrot, potato and pepper yields. They have yet to confirm if these have statistical significance. 


I'm wondering if the difference I saw in the rocket and salad mat germination trials might be due to the seed size involved, with the larger courgette and cucumber seeds unaffected by the type of compost used. Another factor may be the differing water requirements for the growing media used - the biochar compost is based on coir, which usually needs less water.

The positive result for the salad leaves produced from the seed mat grown on biochar could have been due to its poorer germination rate. The mats were pot grown, so the seeds which did germinate on the biochar weren't so crowded as their New Horizon grown cousins. NB the salads grown in my new salad area were evenly spaced and the positive result for biochar grown leaves was reversed. This could explain the difference in results.

The trials I've set up this year aren't really scientific enough (or large-scale) to draw any firm conclusions concerning the effectiveness of biochar as a component of a growing medium. I need to set up some further trials where the growing medium is the same with biochar added to one half of the trial area or pots used.

I've shared my preliminary findings with Carbon Gold who've asked when I received my samples (which were May if memory serves). Apparently there was a poor batch sent out - I don't know if mine were part of that.

One general observation: growing media containing coir needs a different watering regime because it retains water differently to other components. Ordinary gardeners like me need more guidance on what to do. This doesn't just apply to the products I've reported on here.

I've also set up a garlic and autumn onion trial on the allotment on two of my new raised beds. The biochar trials continue...

... and here are my variable results. I also found an interesting article referring to work conducted in the 1840s as part of my research for that post. It could be that my soils aren't deficient enough in carbon for the biochar to make a difference.

Further information:

General biochar information via Wikipedia
Carbon Gold biochar trial results pdf (sadly no longer available)
Oxford Big Biochar Experiment website


  1. Interesting reading VP. Not the best year for trials. Will wait to hear more about this product.

  2. I wonder whether there are so many variables affecting plants that any difference made by biochar is just too small to show up in a small trial like this one. In other words, biochar might be slightly better (as found in the Oxford trial, apparently) but that small difference gets swamped by random variations in seed quality and so on. In that case, your trials might be more relevant to gardeners than bigger, more controlled trials.

  3. Anna - there's plenty of trailling going on, so you're bound to hear more.

    Hi Rachel - you're quite right. This needs much more rigorous scientific investigation to see whether biochar makes a significant difference to yields. Whether mine and any of the other trials reported have this rigour remains to be seen.

    I also believe gardeners need much more guidance on effective use of this and other peat replacement products coming onto the market. I'm trialling a couple of other coir based composts and the watering regime is completely different to what I'm used to. There's no guidance on the compost packaging at all, so I'm learning by trial and error.

  4. Rachel makes a very good point, and one that I think is missed by many 'scientific' trials. For very small scale growers, the slight advantages achieved in small-scale growing may be insignificant compared to the individual knowledge and experience of the grower based on local conditions and their own skills base, so unless you're growing on a commercial scale, variations in soil/seed/heat/water may be impossible to calibrate against other systems and even other years.

  5. Allotment Blogger - welcome :)

    That's quite right. I also wonder about how much more likely the larger scale growers comply with the 'ideal conditions' advised by the producer, which can't be duplicated that easily by ordinary gardeners.

  6. I, too, wish there was better information about watering requirements for different composts etc. Given the vagaries of seed quality, growing conditions etc I certainly think it is worth doing more experiments, and I also agree that experiments such as you are doing, on a domestic scale, are likely to be far more relevant to us hoi poloi. What is the difference in price between New Horizon and Biochar? Because when push comes to shove, the increased performance would have to be considerable and repeated before I would consider paying extra...

    1. Janet - I've tried quite a few new composts this year, most of which seem to have a high proportion of coir which reacts with water quite differently to the growing media a lot of us are more used to. None of them seem to have guidance on the revised watering regime lots of gardeners will need to adopt.

      I believe the 2 products are comparable in price. However, I've just seen in The Garden magazine that this summer's poor summer has affected peat harvesting. It could mean we see peat free and peat composts becoming closer together in price. Looks like the clouds may have a silver lining after all!


Your essential reads

Garden Bloggers' Blooms Day: 'Just Add Cream'

Salad Days: Mastering Lettuce

Merry Christmas!

Jack Go To Bed At Noon

Happy New Year!

Please read if you follow this blog by email

Happy Easter!

Chelsea Fringe 2014: Shows of Hands - Episode I

Seasonal Recipe: Nasturtium 'Capers'