Can You Make Soil Blocks Using Only Compost?

Curious if you can make soil blocks using only compost? I tried it, and I've got the results.

The idea came to me out of shear financial pragmatism.

I usually make my seed-starting mix using 3 main ingredients:

• Cured, Sifted Compost
• Coco Peat
• Perlite or Sand

...But the coco peat and the perlite cost money and have to be continuously re-purchased.

The compost is free, and piles up (literally) on its own in the background.

Would it be possible to make soil blocks completely from compost alone? Maybe I could reduce my soil block cost down to free. I was as curious as you are!

So I tried it.

A Note About Curing Compost

If you're using homemade compost for your seed-starting mix, you'll want to make sure it's thoroughly sifted and then cured before using it.

Unfinished / uncured compost can be chunky, buggy, and cause more plant burn than the finished stuff. It can also be hard for plants to root in it if it's too chunky.

Soil Blocking With Compost

The process was exactly the same as typical soil blocking.

The compost blocks held together fantastically!

Compost has a great structure for blocks and they did not dissolve or crumble through repeated waterings

Seed Starting Conditions

The seedlings started in the compost blocks were started indoors, where the rH averaged around 30% during the dry season.

Soil blocks looked great, and the watermelon seedlings I planted sprouted well.

I did not get any mold or anything funky during germination - though I did get a few flies and fungus gnats which hatched from the compost.

Important Note About Nitrogen Burn

Fresh compost can be extremely high in Nitrogen and other nutrients.

It has a tendency to burn some seedlings because of this. We call this "hot" when the nutrients are so high they'll cause toxicities in your plants.

Final Verdict - Can You Make Soil Blocks With Compost Alone?

I would say sort of, with some large caveats.

Compost, is incredibly high in nutrients – too high, really, for seed starting.

It's really tempting to want to use 100% cured compost for your block mix, though I actually found this to be pretty far from ideal. Check out my findings here for context.

How Much Compost to Use in Soil Blocks

From some recent experimentation, I'm finding that I need a very small percentage of compost in micro blocks.

I'm finding that I need a high percentage of compost by the time I get up to 4" nursery pots.

I've dialed in to the idea that each block stage should contain progressively more nutrients from progressively larger compost ratios.

Seeds have enough nutrients to germinate and last the first few days of life, and must remain so wet to germinate that a lot of compost is a liability.

Molds, algae, bacteria can thrive in the anaerobic environment of a moist soil block with a humidity dome. For this reason, I'm using very small amounts of compost for micro blocks, and more compost after blocking up.

These are my current ratios of compost to coco peat. You can add sand or perlite to these, or anything else you like:

3/4” Micro Blocks

• 80% coco peat
• 20% compost

2” Mini Blocks

• 66% coco peat
• 33% compost

4” Cubes

• 66% compost
• 33% coco peat

Have you tried making blocks out of nothin' but compost? I'd love to hear how it worked out for you!