DIY Potting Mix?

Hi folks,

I hope you can help me out here.

With the lockdown, and more time on our hands, I find myself needing to plant lots of seeds and avoid the shops. So I'm after my own DIY potting mix from what I have to hand:

Compost - well rotted (some)
Woodchips - Fresh (lots)
Woodchips - rooted 1 year (lots)
Sand - some
Garden soil
A few bags of organic fully rotted "chicken manure fertiliser" that were purchased a year or so ago. Looks like well rotted straw bedding, that has been sieved to a fine texture. Loose and dry.

Any suggestions for using the above?

I'm kind of thinking

1/3rd soil

1/3rd compost

1/3rd rotted chips

a few generous handfuls of the chicken manure stuff

Sound like a reasonable recipe? Plenty of organic material to hold moisture. Some good nutrients from the manure, but not too hot so they burn. This would be mostly for getting seeds started in the green house before planting out.

peat moss, vermiculite or perlite and loam/top soil works well. its helpful to dig your loam from beneath sod roots to eliminate weed seeds

I don't have peat moss, vermiculite or perlite.

We are on lockdown, so I need to source materials from on my land.

Personally, I would cook some of those fresh woodchips into charcoal, then soak the charcoal in a bucket with water and a small amount of chicken manure. If you have an aerator you could use while it's soaking, that would be best. Otherwise just try to stir it every now and again. After that's soaked for a day or two, mix it with roughly equal parts soil and compost.

I'm not an expert, but when I was doing some research on making my own seed starting mix I read that for vermiculite you can substitute Pine fines, ( fine ground pine and bark)   coconut coir, or cotton waste.  Substitute for perlite is Styrofoam(I don't like this one) sand, pumice, and rice hulls.  
When you have to go out for grocery's if you have a super walmart they have all the stuff you are looking for.
My seed starting mix was 2 parts compost, 1 part vermiculite, 1/2 part sand.  It was across the board equal to the seed starting mix I bought in the store.  The only difference I noticed besides being a heavy mix was it needed to be watered more.
When you plant in the ground you get what you have.  Some times we get caught up on the details and worrying that we are doing everything right, and maybe it's not that big of a deal.  I say use what you have.  Mix it up in small batches.  Unless your brand new at gardening you have a general idea of what it should look like and feel like, so do what seems right to you.  You never know you might end up with something new and improved.  Even if you don't, most plants just aren't that picky.  Good luck

Good plan re biochar - I actually have some in a heap that I had forgotten about!

Had some success. I found, in a neglected corner, a huge pile of old leaf mulch. AT least 5 years old. Well rotted and loose and crumbly. Also, my pile of biochar is bigger that I remembered.

Both have been screened into buckets, down to a nice loose texture. Feels pretty good so far. I've mixed up a small batch and added an extra scoop of the fertiliser. Fingers crossed the plants like it.

Plenty more in the heaps, so I should be good for months if this work out.

I was wondering the same thing while I was pricing potting soil. Thanks for the tips and encouragement Jen Fulkerson! Sometimes I need to remember that experimenting is a good thing and you don't have to do everything the Official Correct way. I'll be running experiments with different ratios of peat moss, top soil and sand and a bit of manure compost as thats what I have on hand. Will let you know how it goes!

Sounds like some awesome potting soil!

For various reasons I wasn't planning to grow any veg this year just flowers and some herbs. Plans have changed.

All I have is topsoil. Not particularly weed free, but really nice tilth. The stuff just under about a season old woodchip mulch. So I've used it, and so far so good. For what it's worth, topsoil grew monarda from seed to flower last season, and pretty decent volunteer tomatoes. And so far watering has been easy, no prolonged puddling.

I'm relatively limited on nutrition I can give them too. Coffee grinds and compost tea, are what I've got on hand.

I suspect transplanting will be a little difficult, and I already suck at it. But we shall see.

Michael,

Sounds like a good mix.  Were it me, I would mix some of the char/biochar into the mix and not on the surface—maybe make the mix 30-30-30-10% char.

I would consider using some woodchips on the top as a mulch (you don’t need much), but also those wood chips will begin to decompose and can be added back into your garden.

Just a thought,

Eric

Re using wood chips.

I decided not to. Most of our chips are fairly coarse, and it would take a lot of sifting to get nice fine decomposed material for making a fine tilth for a potting mix.

Hi, similar issue here. I'm on an island 12 miles off the Maine coast and not planning on going in shore any time soon but definitely growing as much food as possible this year.

We're low on compost but I'll dig under last year's pile. I have access to seaweed, sand, sea shells. Egg shells. I'll go ask a neighbor for some chicken manure. No Coco coir. Crappy top soil. Will scavenge the meadows for better soil.

Any advice or suggestions welcome.

I’ve had equal success to expensive potting soil starting tomatoes in the Geoff Lawton mix (1/3 composted chicken bedding:2/3 sharp river sand). You can go to equal parts compost:sand if the plants are heavy feeders or like heavy soil, or you just want to water less often.

This is exactly what I've been trying to figure out the last few days, as well as how to make my mix soil block friendly. My compost is from the deep litter system in my chicken run, in which I've added a fair bit of biochar as the base. I sifted my compost through quarter inch hardware cloth, and the resulting mix has some very finely shredded straw.

I made a test batch of blocks from pure compost just to see how they'd turn out. Once I dialed in the moisture the blocks held their shape. I formed them in a prototype seed tray with hardware cloth as the bottom so that a) the soil blocks can air prune properly, and b) I can bottom water by placing the trays in a tray of water. I did a 5 minute bottom water test and found that the blocks held their shape with very minimal degradation. The fine straw shreds seemed to be reinforcing the shape, much like straw does in cob... whether it negatively impacts plant nutrition has yet to be seen.

The inspiration mostly came from this video, where by his own account, his soil block mix is 90% compost. I plan to do another small test batch where I wet the material down with the diluted clay like he uses to provide a little extra holding power (one of my blocks did eventually developa sight crack with repeated handling.

Two other videos I took inspiration from were this one and this one, both from experienced growers I follow, both with a very DIY/zero budget ethos, but not pertaining specifically to soil blocks.

I don't have many of the ingredients suggested in these videos, but I plan to do a few small test batches to see how the blocks hold together, and how well seeds germinate and grow in each of the mixes. One will be my original compost (which includes biochar and straw fines), one will add clay, another (if I can figure out a way to process it down to an adequate size for soil blocks) will add shredded and sifted tree bark which is readily available on the property, still others will be a combination of the above, as well as some mixed with coffee grounds added in, since that's another thing I have in abundance.

Oh, and here's what the original blocks look like.

I love what you're doing, Matthew. I'm about to try something similar.  My only concern is, are you using palette stringers as the frame wood? They can harbor all kinds of nasty chemicals, insecticides in particular.  How did you form the blocks?

Rob Lineberger wrote:I love what you're doing, Matthew. I'm about to try something similar.  My only concern is, are you using palette stringers as the frame wood? They can harbor all kinds of nasty chemicals, insecticides in particular.  How did you form the blocks?

It's repurposed pressure treated lumber from a demo project... Not my preferred, but it's what I've got. I designed my trays to have a quarter inch of clearance on each side, so the soil blocks aren't actually touching the wood.

The blocks were formed with a purchased 2-inch block maker, but I'll have to come up with something DIY for my 4-inch blocks

I can understand that! I too use pressure treated lumber.  So far so good.  

Here are the 8 different DIY soil block mixes that I'm testing out. I'll let them dry a bit and then do a 5-minute bottom water test to see how well they hold their structure (which is one of the most important things with soil blocks.) I'll then do a germination test with Japanese Giant Red Mustard, which was free, courtesy of Baker Creek Heirloom Seeds (and thus, something I'm totally willing to sacrifice for science... and which might actually survive through winter, which would be a bonus.) Since none of the mixes contain store bought perlite or sand (which isn't available on the property), water retention and root penetration may be major issues (though, if we're being honest, those are the same conditions most things have to tolerate in the garden here anyway.) I'll mist daily (or multiple times a day) until I get germination, since bottom watering often isn't sufficient to get water to the very top where seeds are waiting to germinate, at least, not without overwatering and destroying the integrity of the blocks. Hopefully all of them perform reasonably well, but hopefully there's also a clear winner that can be used going forward. Coffee is the only thing I'm testing as an additive for fertility beyond what's in the base compost. Whatever mix makes it to the next round will be compared to the same mix with added amendments, though the only thing I have to test at the moment is my homemade Complete Organic Fertilizer. I might try to whip up another batch of nettle tea in the meantime to compare that, but because of my setup, I might not be able to do more than one initial application. 🤷🏻‍♂️ We'll see.

Wanted to pop in and give an update on my soil block test.

As you can see, seeds planted in pure compost blew the rest out of the water. I used one block of each "recipe" to grow mustard, spinach, kale, and cabbage, with spinach being the only non-brassica. The mustard grown in pure compost was actually a victim of its own success. I missed one watering, and because the mustard in pure compost was the most developed, it was hit hardest by the lack of water and ultimately died. Of the surviving plants, the ones grown in pure compost are noticeably larger.

A few notes about my composting process, which may make a difference in either plant growth or the stability of the soil blocks (which wouldn't be an important factor if you're growing in pots.) I produce my compost using the deep litter method in my chicken run. All organic matter (food scraps, yard and garden waste, etc.) is put in the run with the chickens. Anything they don't eat is scratched up and mixed in with their manure (a sizable layer of carbon rich material like straw is maintained on top to balance out the nitrogen from the manure/food scraps.) Once or twice a year, this material is sifted through quarter inch hardware cloth. Anything that's too big to pass through the screen is left to continue composting. The resulting compost is fine material, rich with composted manure. A certain amount of fine straw makes it through the screen, and just like with cob, this straw may help to reinforce the shape of the soil blocks. I did not have any problems with my blocks falling apart, but I could not guarantee the same for blocks made of compost made by some other means. Curiously, I anticipated that the bits of straw might have some affect on nitrogen availability and stunt the growth of the plants, but that does not appear to be the case. I'm including a picture of my compost so you can see the relative size and quantity of straw that makes it through the screen.

Definitely want to try this, will have to wait until next spring to have my first batch of "compost" (aka, 90% lawn trimmings, some table scraps, and the remnants from an abandoned chicken coop).

We just got chickens, so we're looking forward to adding a regular supply of wood shavings and manure to the pile!

question about the compost: did you worry about bugs/grubs?  or do the chickens take care of that?  I just finished sifting a bucket of compost from a traditional hot compost pile that had been sitting since last summer, and there were a lot of small curled up white grubs.  I'm a bit worried about using this in seed starting mix.  Anyone have experience with this?  I was thinking of spreading it on a tarp and letting the chickens pick through it...  Any thoughts?

Lina I have no experience with this because I suck at composting.  I think letting your chickens pick through it is a great idea, they do the work for you, and get a snack in the process.  My only concern is if they poop in your compost if it will be to strong and kill or do harm to your seedlings.  Maybe if you sift if when they are done?  Good luck 😃