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Compost as potting soil?

 
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I'm sorry if this question has been asked a dozen times before...  I finally was able to save some compost from last fall and can use it for starting seedlings this early spring.  I did three trays of onions yesterday with three ratios of compost to potting soil - 100/0, 50/50 and 0/100.

Before I find out if any of those combinations works better than another, it will be time to start a bunch of other things.  I have enough compost to do all my seedlings but is it a good idea?  It seems loose enough and it doesn't seem like it will compact.  I've heard you want actual soil for the plants to grow in but sterile potting soil doesn't seem to have much soil in it either.  I did see Violet's neat DIY Potting Soil thread but I don't have any organic straw/hay at my disposal.

Should I keep going with straight compost?  Mix it with potting soil to make the potting soil go further?  Amend the compost with perlite or something else?

Thanks!
 
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my DIY potting mix last year was a 50/50 mix of the previous year's compost and biochar. I mixed them up in a pile in fall of 2019 and left it until spring 2020. For starts and potted plants it worked fine, no problems at all, and I have more of it leftover that I'm going to use again this spring.

For filling 72-cell flats I put it through a 1/2" screen to take out the big chunks.
 
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I don't have much experience with this but I did stick some tree collard cuttings into straight compost that I had put in some pots and they're all doing great. They've grown a ton already. I did the same with a cutting of longevity spinach and it's also doing great in pure compost.
 
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Hi Mike- the thread below talks about using compost for potting soil. She seems to have had great results:

https://permies.com/t/153926/DIY-Potting-Soil-Scratch#1207367
 
Mike Haasl
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Thanks Annie!  That's the one I linked to above.  It sounds like she uses half compost and half straw (which I don't have).  
 
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hau Mike, most "potting soils" are not soil at all these days, the types I find are more decomposing mulch. You should find that mixing compost with these mediums is acceptable for growing seedlings out.
 
Mike Haasl
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Thanks Bryant!  Would you have any reservations about using pure compost?
 
Bryant RedHawk
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Pure compost might be to nutrient rich for baby plants, just as it is for seed starting.
 
Mike Haasl
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Gotcha, thanks Bryant!!!
 
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My sister in Boston always fills pots with compost and grows vegetables, herbs and flowers just fine. her compost includes a good portion of autumn leaves, and then kitchen and garden waste. Has worked great for her for many years now.
 
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Hi Mike,

I guess the answer here would be ... it depends.
How did you make your compost initially, what ratios of materials did you use, how long has it been standing to mature?
If your compost presents no funny smells - only smells reminiscent of walking in the forest are acceptable, and is a dark chocolate brown (not too dark), and has decent structure; you should be able to plant into it directly.
Compost only ever burns plants if it is not completely composted, meaning that it is removing nitrogen from the soil for purposes of decomposition, instead of giving it back to your plants.
Once completely decomposed, you should also see the presence of mycelial hyphae in your compost.

I've personally moved away from seedling trays because plastic, and I find them too high maintenance cause they tend to dry out too much and require lots of watering.
My solution is to have a dedicated seedling raised bed, filled Hugel style, topped with a mix of untreated wood shavings (to hold moisture) and compost.
If you're doing very small seeds, I'd suggest a layer of sifted compost on top - for small roots to develop and make its way through a finer media.

If you're compost is happy and well matured, you should have no problem planting directly into it.
I also tend to multi-sow into divets. In nature plants live in competition, this will ensure that the strongest plants will grow the quickest, offering you some natural selection.

Technically potting soil is just old compost that has decreased nutrient cycling capacity, maybe with some added mineral content.

Come to think of it, even the wood shavings might steal nutrients from your plants, so if you're mixing something in, get it mixed up and then leave it to stand for 3 weeks before planting into it.

Perlite and vermiculite work well for retaining moisture in the soil. I'd also suggest putting seedling trays onto the ground instead of on shelves - that also seems to aid in germination.
Many people will suggest adding trichoderma to a germination/ seedling mix to make sure that they are not susceptible to mold... this is a horrible idea.
Recent studies have shown that trichoderma is a cannibalistic fungi that will embed itself in the hyphae of other fungi and kill it... and then we wonder why were losing fungal diversity in soils.

Maybe other things that you need to consider would be the number of transplants and repotting you're doing.
Best thing is to sow directly into the soil where you want to grow, second best is least amount of transplants... even better is self sown and spending time thinning out and transplanting.

Again, guess it would depend on your system and the production turnover you're looking for, and the amount of land you're working with.

Best of luck with your grow!
 
Mike Haasl
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Thanks Leigh!

The compost came from garden debris, fallen leaves, coffee grounds and chicken poop.  The leaves and poop (70% leaves, 15% grounds, 15% poop) composted over the winter of 19/20 into something brown and relatively crumbly.  It was then moved out to the compost pile all summer and mixed in with the garden debris.  Near the end of the summer I spread most of the compost on the garden but sifted some and saved it for this purpose.  It's been sitting in a big garbage can since then (average temp of 40F).  It's brown and if it wasn't for the white beads and being a bit heavier, it looks a lot like my organic potting soil.  It doesn't smell bad at all and is somewhere between a dark and milk chocolate color.

I'd love to direct sow a bit more but I seem to be transplanting more and more.  I have a 90-110 day growing period so that's a big limiter.  And a huge garden...

 
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I did a few combinations of seed starting soils, with and without compost and even some only compost (I can't make compost to save my life, so mine was bought organic compost)  Anyway,  the strange thing is it didn't seem to make a difference. For my little experiment the seeds sprouted and grew all about the same.  My conclusion was the veggie seed I started aren't that picky. Being in 100% compost didn't seem to bother them.  Maybe I just lucked out, or being bagged it had lost a lot, I don't know.  Last year was my first real attempt at starting seeds indoors.  I did  learn once the seedlings have there first set of true leave they need some kind of nutrition.  Seed starting mix doesn't have any at all (nothing is needed to sprout, just grow and produce), so if you only use that make sure you fertilize at that time.  That is why I like to add compost because it gives the seedling more of what it needs.  when I transplanted the seedlings into larger containers I used 100% compost and everything did very well, again who knows why.  I listen carefully to what Bryant RedHawk say, he knows his stuff, so this year I will be mixing the compost with some good organic soil.  Maybe I will get even better results.  Good luck to you, Happy gardening.
 
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I've tried compost as potting soil before. At first its excellent, then the worm poop appears more and more at the top of the soil until all is eaten, now finally the last worm slags out at the top, having eaten everything and smiles at you disgustingly.
 
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Elaine Ingram has mentioned straight compost being her preferred medium. She knows her soil, but I would think better drainage and aeration would be advantageous in my rainy winters (80-100”/yr).

Geoff Lawton recommends 50%-70% sharp river sand:30-50% well finished compost
50-50 is for trees, 70-30 for veggies. This worked as well for me as high end potting soil (roots organic) in a side by side trial starting tomatoes.

 
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