Wood chip potting mix - micro greens

I work at a small scale, bio-intensive-ish market garden.

We're keen to start making our own potting mix - there aren't many organic suppliers around, and the main one recently stopped supplying bulk (instead they now use small plastic bags!). We're also not keen to be using loads of peat etc.

We've done a few trials with just straight compost (our own and imported from a commercial organic compost maker) vs the organic potting mix on micro greens, one of our crops that keeps us financially viable. We found that straight compost doesn't do well - signficantly worse. I suspect this is likely due to drainage, as micro greens are barely accessing the mix for nutrients, most (all?) of it's growth comes from stored energy.

I've seen reference to other market gardens making woodchip compost and mixing with vermiculite for potting mix.

Agricology published this interesting trial, where woodchip + vermiculte and lime performed admirably compared to commercial potting mix. Better in some aspects, worse in others. But promising enough.

However, I can't find too much info about the actual process. We aren't at tractor scale, and try to avoid using loads of fossil fuels (we could pay someone to turn piles). So we'd be wanting a low maintenance (never turned?) composting system.

Is it worth incorprating other material in the composting mix - this would require a lot of labour? Obviously it would take a long time with such a high ratio.

Would there be any disadvantage to simply getting a BIG pile delivered and leaving it alone for 18+ months?

What species of trees would we need to be careful about?

What minerals would be need to add? Vermiculite makes sense for drainage. But most potting mix recipes I've seen are much more complex than Iains Woodchip+vermiculite recipe. What sort of ratios might we aim for?

Has anyone tried something like this? Has anyone tried it with microgreens, where the potting mix is barely more than a growing medium [we would also be using it for all our other starts - most of the typical annual crops you'd find on a market garden)?

Texture is of course the other concern - we use soil blocks for some purposes and "chunky" potting mix is tricky to use here. But I figure easily dealt with with sieves. But we'll also need an efficient way to sieve lots of material, as we do use a lot of it.

Hi Jon, I used to grow micro greens for a local farm. I’m not sure of your location or what kinds of wood chips you’d be getting. Having said that I believe that any chips that are fine enough could be used after a sufficient amount of time. I think that I would start with arborist wood chips if available. They are simply everything ground together, leaves included.
Vermiculite tends to retain moisture so opt for perlite if drainage is an issue.
I wonder how a mix of compost, perlite and chips from the bottom of the pile would work?
If you have enough spent growing medium sell some. I used to bring some home to seed my new hugelkulturs. As long as there was no mint or other invasives in the mix it worked great! I was always surprised by how many seeds didn’t germinate the first time around.
PS, welcome to Permies!

Scott Stiller wrote:Hi Jon, I used to grow micro greens for a local farm. I’m not sure of your location or what kinds of wood chips you’d be getting. Having said that I believe that any chips that are fine enough could be used after a sufficient amount of time. I think that I would start with arborist wood chips if available. They are simply everything ground together, leaves included.
Vermiculite tends to retain moisture so opt for perlite if drainage is an issue.
I wonder how a mix of compost, perlite and chips from the bottom of the pile would work?
If you have enough spent growing medium sell some. I used to bring some home to seed my new hugelkulturs. As long as there was no mint or other invasives in the mix it worked great! I was always surprised by how many seeds didn’t germinate the first time around.
PS, welcome to Permies!

Thanks - is there any sort of wood chip species to watch out for? E.g. I seem to recall species like eucalyptus having growth inhibiting hormones.

Did you ever make your own mixes for micro greens?

I’m not sure about eucalyptus but I use all sorts of “bad wood” in my hugelkulturs. After it breaks down sufficiently it’s all good. I even use mimosas and privet but they need to be pretty far gone or they’ll root. Mimosa is so soft it only takes a summer but privet will take eighteen months.
I never made my own potting mix because they kept me pretty well supplied.
There was one mix where they added too much peat and it was soggy. To make up for it I turned the temp up under the trays. It dried faster and kept mold at bay while the trays were covered.

Scott Stiller wrote:I’m not sure about eucalyptus but I use all sorts of “bad wood” in my hugelkulturs. After it breaks down sufficiently it’s all good. I even use mimosas and privet but they need to be pretty far gone or they’ll root. Mimosa is so soft it only takes a summer but privet will take eighteen months.
I never made my own potting mix because they kept me pretty well supplied.
There was one mix where they added too much peat and it was soggy. To make up for it I turned the temp up under the trays. It dried faster and kept mold at bay while the trays were covered.

Thanks Scott. We're going to get a big pile delivered to one of the bosses houses and let it rot. There's a reasonable chance I won't even be around to see the results of the trial, given how long it'll need to decompose - there's only so long I'm keen to work for someone else as a market gardener (or at least for one person), but we're all cautiously excited about the possibility of becoming less reliant on the commercial options. Unfortunately it's tricky to get free woodchip here, as there's arborists have cottoned onto the fact it's a valuable product and not a waste stream! But that's probably a good shift in realising all resources have value, at a broader level.

We had two large heaps of woodchips delivered this year. They had stood outside in the rain for a couple of months before being moved inside for 6-8 months before we got them. (The intention was they should be boiler fuel but they started to rot) We have now had them 5 months and they now consist of about 30% small rotted material and 70% chunks. if sieved I think they would make a good material, BUT and there is always a but, they are full of bugs and fungi, great for the garden, probably not good for microgreens.
They are some form of forestry chips, probably the tops of one of the spruces sitka at a guess, and not very finely chopped the largest pieces are as big as my hand and the smallest like dust.

I agree that fungi and bugs wouldn’t be good for micro greens but I believe they could be dealt with fairly quickly. Once the chips are sifted and mixed with other medium I would put the mix in grow trays and heat it without the lid. Most micro green trays are heated from the bottom so your current setup could be used. When the mix drys out enough the problem should be taken care of. The growing media can be dry when seeding because it’s watered after and a lid placed on.
If I still did this I would definitely be running some experiments along this line.

Hmm, perhaps more research needs to be done on what makes a good micro green medium. It's not my "department" at work, and the person who normally runs it didn't have too many specific answers when i asked about it. It seems moisture management is the main thing, to my observation. The seeds are literally just on the surface, weighed down, and harvested before they even really access the nutrients of the soil (to be honest, I kind of find it an absurd food - we all do, but it does subsidise growing more meaningful food for our community, and without it, we probably wouldn't have jobs, sadly - so I guess I need to give a thanks to trendy food fads!)

Maybe an experiment with the different growing mediums available would answer your questions.

I don't know what all is available so for an example:

Wood chips
starter mix
fine compost

Three different mediums, add seeds, and watch how the microgreens do.

Anne Miller wrote:Maybe an experiment with the different growing mediums available would answer your questions.

I don't know what all is available so for an example:

Wood chips
starter mix
fine compost

Three different mediums, add seeds, and watch how the microgreens do.

Indeed - this is the plan. I'm just crowdsourcing more info and such as it'll take 18-24 months for woodchip to decompose to a state they can be used, so keen to get info on different considerations to incorporate into the trial