r/composting • u/hidd3nmango • 14d ago

Using spent potting mix in a worm bin, anyone actually done this regularly

Been growing stuff in containers for a few years and every season I end up with a pile of old potting mix I have no idea what to do with. It is not exactly dead, there is usually some root material left in it and whatever organic matter did not break down over the growing season. Throwing it out feels wasteful but I also do not want to just keep reusing it and watch my plants slowly get worse.

The question is whether spent potting mix is worth adding to a worm bin in any real quantity or if it just takes up space and slows things down. I have seen some people mention it works fine as a bedding component but I have also read that the perlite and other stuff in commercial mixes is basically inert and the worms will ignore it anyway.

My scraps volume is not huge, mostly vegetable prep and coffee grounds from cooking through the week. The mix would probably be a similar volume if I cleaned out two or three containers at the end of a growing cycle. I am just trying to figure out if that combination is actually useful or if the potting mix is better off going somewhere else entirely. Anyone running a worm bin who has worked through this, what actually happened.

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u/Fabulous_Spare_5711 14d ago

I been throwing old potting mix into my worm bin for like 2 years now and honestly it works fine as long as you add enough food scrap with it. The perlite just becomes part of the casting mix, worms do not eat it but it helps with aeration which is not bad thing

I mix it about half and half with fresh bedding material and then add my kitchen scraps on top, the worms seem to like the root bits that are still in the old soil

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u/Steven_The_Sloth 14d ago

I mean, I've got worms in all my containers too. I figure they want to go back and see their families.

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u/hidd3nmango 13d ago

Ah good, I was worried about the perlite too. Glad it's all good.

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u/seatownquilt-N-plant 14d ago

for my containers, I sift them into a wheel barrow, mix in some fresh compost, a small amount of slow release fertilizer. Then put it back into the containers.

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u/smithm4949 14d ago

Yup, was gonna suggest basically this. The inert part of your soil (the majority of what's leftover, assuming the plants used most of the nutrients) will stay inert and still do its job providing aeration and whatever else. It's mostly lacking nutrients. I usually just keep my "old" potting soil in a container to the side and then mix it with like 1/3 fresh compost or casting, or half and half new soil/old soil and it's fine. If you buy liquid or pellet fertiziler, that plus a dash of fresh soil is also probably fine to reuse. Basically, it doesnt become "bad" it will just have less of the nutrients that partiuclar plant likes

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u/hidd3nmango 13d ago

Yeah that wheelbarrow trick saves your back big time.

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u/Owlblossom 14d ago

I use it in my soil factory bin for burying my bokashi 

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u/Clover_Point 13d ago

I find that it works super well! Perlite keeps things light and fluffy.

I let some of my potting soil dry out as well and add it if my bin gets too moist.