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SUSTAINABILITY | Read Our Commitment
Most garden centres now stock peat-free multi-purpose compost. (More accurately they stock peat-free soilless growing media tailored for plant growth. Few of them actually contain "compost" - ie the end result from a managed process to break down organic matter into a useful soil amendment. If you need more help with the different types and names of compost have a look at this post.
Many of our readers will know that SoilFixer pioneered adding biochar into the active composting process. Our SF60 Soil Improver is a fantastic soil improver and helps gardeners towards the goal of the famed Amazonian Dark Earth soil composition and fertility. However, SF60 on its own is not a good growing media - we would not sell it as a multi-purpose compost for use solely in containers.
At the start of 2020, we set out to tweak our production to produce and test a biochar peat-free multi-purpose compost that:
Results Summary SF40 versus leading brands
It would be great to make a growing media that gave substantially bigger and better plants than the leading MPC brand. In reality, this was unlikely. The leading brands have spent 15 years perfecting what goes in them. They mix multiple materials (coir, wood fibre, wood chip, peat), specify different particle sizes, water retention chemicals, water surfactants (re-wetting agents), and slow-release fertiliser prills - everything your plants need to give you the results you want - great container plants.
SF40 has matched and in some cases bettered leading brands. However, biochar brings something new to the party. It can be made in the UK from UK waste wood materials. Biochar locks (sequesters) carbon for hundreds of years. It will survive from 100 to 1000 years in soil and containers. There is a realistic prospect of 'topping up" and reusing SF40 every year for many years.
Even if you are unlucky and the container plants get infected with disease, correctly composted (eg in the hot system for minimum 90 days) the pathogens can be eliminated and the SF40/compost re-used.
We make our compost and biochar here in the UK. This gives us the flexibility to alter the ratio of biochar and the particle size. Working closely with our biochar production partner has given us the capability to make biochar more cost-effectively. This has opened up the possibility of changing the ratio of biochar and having a cost-competitive biochar multi-purpose compost. We shortlisted two formulations for a head-to-head test. We code-named the mixes "2:1" and "SF40". Even we were surprised!
(Note: we take care to do controlled and comparable tests, but we are always upfront that plant growth tests are very challenging with multiple variables to control. Test results that will stand up to academic rigour can take many years to complete and include thousands of replications. We are always enthusiastic, but also careful not to go over the top in our claims based on one or two results).
SoilFixer tests, 2:1 mix vs SF40 mix, radish, tomatoes and Sunflower
Radish
Germination and first few weeks:- the SF40 germinated far more.... and faster
Short-term: (2-10 weeks, mainly radish tests) - we observed X4 more total plant growth via SF40 over the "2:1" mix
Tomatoes and Sunflowers
We started to see more early growth in height and spread of the sunflowers and tomatoes planted in the SF40 over those in the 2:1 mix.
Towards the end of summer, we noticed the 2:1 plants were taller (2.4m vs 2.0m). We then noticed a flaw in the planting - the sunflowers in SF40 nearer to the house were shaded for longer each day. At the end of September, we cut down and weighed the seed heads, stems, leaves and root ball. When all the weights were added together, there was no significant difference. Both formulations performed equally well for sunflowers.
We planted 3 tomato plants in 2:1 mix and 3 in the SF40 - these were mingled in with the sunflowers. The tomatoes did show a significant difference. Again we weighed the fruit, stems/leaves and root ball. The SF40 root balls were 300% more in both length and total weight. The fruit in the SF40 was 50% more - but when we cut down the plants - we noted that much of the ripe fruit allocated to 2:1 plants was probably from the SF40 plants that have spread and tangled. Overall the SF40 plants showed 75% more growth than the 2:1 mix plants.
SoilFIxer SF40 versus leading multi-purpose compost results
Germination 11/12 plants for SF40, 5/12 plants for the leading brand MPC - double
Total growth 94g for SF40 versus 59g for a leading brand (that's x1.6 difference)
Biggest weights, top two radish weights: 17g and 16g for MPC, 16 and 14 g for SF40
(We note the major difference is that fewer plants germinated in the MCP so fewer plants went on to grow to maturity. We would expect an MCP to be suitable for most seed germination, but do accept many leading brands suggest a weaker fertiliser ratio for seed germination over full-on potting up and container growth).
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Germination - 2 seeds of each - all germinated
During the first few weeks broadly about the same growth rates. We observed that during hot days (25-30C) the SF40 retains water far better than the MPC. Despite watering all the pots in the early morning and evening with the same level, by the end of the day, we noted a significant dropping and water stress in the MPC compost.
Long-term, tomatoes at 3 months
We are starting to see differences. The tomatoes in the leading compost appear taller and leafier. Those in SF40 are slightly shorter and have greener leaves and do appear to be developing more fruits. (One other tester has also reported similar with tomatoes. We hypothesize that this is due to SF50 having more potassium and the leading brand having more nitrogen. (Virtually all multi-purpose composts used as growing media will have a slow-release fertiliser pellet in (eg Osocote - which are small white/milky pellets - not the annular bright chips - they are perlite!). (We also noted a flaw in the test design. A week 3, we moved the seedlings into full sun. But the MPC plants shaded out the SF40 at all times of day.
What did our bloggers and testers find?
@the_organic_plot, @saralimback, @theworkinggarden, @emilysgreendiary, @allotment102, @cotswoldpotager, @cj.grows
Pat P – RWB Gardening club
Tomato production is waning now and I've cut the plants back a lot as [gone] mad in my greenhouse.
A very good crop despite the excessive heat. They certainly fared as well as all the others, if not better but that may also have been due to the tomato varieties. I grew the same in each pot of biochar and all did well.
Sally M (Instagram and writer)
Very pleased with SF40 - tested it against my own compost with one squash plant in each pot. Perhaps not the best year to grow squash in pots given the heat as both pots suffered from water stress despite watering regularly - I used a layer of wool fleece over the compost in both pots to try to retain water which helped. Plant growth was roughly the same in each - similar length of shoots, the colour of leaf etc. Some of the leaves were damaged by scorching. I didn't get much of a harvest - one squash off each as the flowers appeared during the heat wave and didn't set fruit.
Marie T (Instagram)
Sorry for the delay in response. Unfortunately, I have broken my foot so it had been a bit difficult but not to worry hubby has been on it so I will get more info for you. Thanks again honestly just love the stuff!
Benjamin P (Instagram)
Our trial of the SF40 began 10th June, using 35 litre containers and Tomato ‘Magic Mountain’ as our subject. We planted one in the SF40 (A) and the other in our preferred peat-free potting compost (B). Throughout the trial, plants were watered, tied into supports and pruned/pinched out, but never fed.
Initial growth saw A to be smaller and tighter in size, though the foliage was a darker healthier shade of green than B. A also developed fruit trusses a little earlier than B, and there were more of them resulting in A cropping first and for a longer period of time. During the length of the trial (June till October) A produced 103 tomatoes as opposed B which produced 85 (see monthly counts below). Both still have lots of tomatoes still to ripen and so plants will be moved in under glass to avoid the cooler wetter conditions.
Though this was an isolated trial of just 2 plants, I was encouraged by the results and love how the compost can be ‘reactivated’, rather than replaced with fresh compost. I would be keen to use the product again and very interested to see results from a larger trial.
Tomato harvest
Chris J (Instagram)
I have to say that my tomato growing experience this summer has been slightly disappointing though that has absolutely nothing to do with the SF40. The tomato plants I have tended and cared for have hardly borne any fruit but the self-seeded plants that got among the blueberries have been bending under the weight of the fruit.
In terms of SF40 compared to the Woodland Horticulture peat free multipurpose the plant has been double the size, a much darker healthier foliage and double the amount of fruit. A mighty 8 tomatoes.
During watering, the SF40 soaks it up like a sponge whereas it pours through the other and then sits in the tray for several days attracting mosquitoes.
Repotting with SF40 was significantly easier too, the root structure much thicker holding the root ball together during transplanting where the other compost would crumble off at the top and be sludgy and probably anaerobic at the bottom
Using a biochar rich compost has reinforced my belief that biochar needs to be part of horticultures future and it will definitely be part of mine.
I'm hoping to be able to get one or more bags for next season to use on its own and to enhance my compost mixes for cacti and houseplants.
Many thanks for allowing me to participate in your trial.
(Dr Ian R feedback/report is posted separately).
Summary SF40 to leading brands
It would be great to make a growing media that gave substantially bigger and better plants than the leading MPC brand. In reality, this was unlikely. The leading brands have spent 15 years perfecting their MPC blends. The science that goes into some of them is amazing - they mix multiple materials (coir, wood fibre, wood chip, peat), specify different particle sizes, water retention chemicals, water surfactants (re-wetting agents), and slow-release fertiliser prills - everything your plants need to give you the results you want - great container plants.
Biochar brings something new to the party. It can be made in the UK from UK waste wood materials. Biochar locks (sequesters) carbon for hundreds of years. It will survive from 100 to 1000 years in soil and containers. There is a realistic prospect of 'topping up" and reusing SF40 every year for many years.
Our goal is to produce comparable plant growth per the leading peat-free brands. We are currently doing internal testing against what we regard as the leading UK multi-purpose compost. We have one tray of radish seeds and one container of tomatoes.
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Post By Tony Callaghan On 29/11/2022
Biochar Multi-Purpose Compost (SF40) A ready-to-use multi-purpose compost used as a general..
£19.99
