Reusing vs composting potting mix (Promix HP)

9 years ago

I had DH pull my leftover starts and compost the soil from the cells earlier this year - so he decided to do the same with the 13 potted tomatoes I had (2-4gal each)! A wheelbarrow full. It's just dumped on top of the bin, on top of some tomato seedlings that sprouted when he threw the water/skin/seeds from ice water bath (peeling tomatoes) a month ago. He hasn't turned the bin in - forever?

This stuff is expensive, so I had wanted to save the used mix in a bin and use it mixed with fresh next year, not for seedlings, but for larger containers. I wasn't sure about tomatoes (the ones in containers didn't do great this year) but I was thinking of starting some bulbs for Mother's Day plants, or maybe just trying carrots in a container next yet (I wouldn't reuse the mix for tomatoes or peppers).

Can I just take it back out of the compost bin and save it in an 18 gal tote, is letting it freeze going to be good, do I have to sterilize it at all if I'm using it for carrots or ornamentals, or should I just leave it where it is? We use the compost on the house vegetable garden each year, mostly cucurbits, I guess I can be careful to not plant tomatoes there at all next year but I really did want to, my main tomato growing area needs to be rotated next year.

The bin's pretty full, it's about 3x3 (made of old pallets) and I'd say the compost is 4ft deep. Actually, this compost probably won't be used next year, but the year after, since he doesn't turn it often and we have a 3-bin system, the finished stuff will be spread on the veggie garden next spring, the middle bin turned into the last bin, and this will be turned into the middle bin so most of the Promix will end up on the bottom of the middle bin and won't get turned into the last bin until later in the year, possibly used then but most likely not until spring of 2016. Since the middle and 3rd bins are the same size as the first, it takes a while to fill the 3rd bin.

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Comments (3)

Pyewacket
9 years ago
I have reused old potting mix - I mix it 50/50 with new. Sometimes I solar sterilized it and sometimes I didn't. But given yours has been dumped in the compost bin already I wouldn't suggest digging it back out.

I would worry about it picking up fungi, insects, molds, etc. I wouldn't want to use that in a potting mix without solar sterilizing it, and by the time its warm enough to do that again you'll probably have most stuff potted up anyway.

It is a risk - one that I have gotten away with on and off for decades, but still a risk - for ending up with disease or other pests in your new planting even BEFORE your old mix was introduced to your compost heap.

Solar sterilization is too much work for me these days so I just don't do it any more. If it were me I would just start maintaining the compost heap (turning, testing temp, etc) and get that puppy working. I would not try to use the potting medium that was dumped in there in a container again.

But potting mix you make yourself is MUCH cheaper than the bagged stuff. Miracle Gro in particular is mostly peat anyway, plus the last bag I (accidentally) bought had bigger chunks and sticks in it than I usually see in bales of peat moss. I don't hate it the way some folks hereabouts do - but I do think its generally overpriced for what it is. It (and its brethren) sure is convenient though.

The 5:1:1 (pine fines/peat/coarse perlite) bandied about this forum a lot has lots of anecdotal evidence behind it. Lots of people use it. I haven't, but I've no real objection to it except for the perlite component (which I could cheerfully replace with pumice anyway) - its just that its been hard for me to find something suitable AND affordable in the way of bark, and all that screening is a lot of work.

I may have found something I can use now for the bark component, but it took me years to home in on the right stuff and I'm not even sure it IS the right stuff yet. It used to be pretty common and easy to find graded pine or fir bark but so much of the "mulch" now is dyed, ground up old houses and pallets, it's gotten harder and harder to find.

But anyway. 5:1:1 is what I'd suggest if you can find the ingredients. Home Despot carries a coarse grade of perlite bagged by Thermorock but it seems to be variable and will probably need to be screened and washed. You may lose a lot in the process, you may not. If you can find horticultural grade coarse vermiculite from a nursery, it MIGHT (or might not) be cleaner and more consistent in size. At a real nursery they should be using it themselves in their own greenhouses - in which case they should have an open bag sitting somewhere that you could examine if you asked nicely. Can't do that at Home Despot.

That said, basically almost any mix will likely work out OK for your purposes(one season use) that includes 2 or 3 from the following list:
- Peat (make sure its damp all the way through...
drew51 SE MI Z5b/6a
9 years ago
I myself like using compost in pots for vegetables. i don't use it for other plants, but technically peat is a compost. You can find many selling manure in peat as the vegatative content of the compost.
Also compost is much faster draining than peat. I prefer a living mix with bacteria and fungi, so compost is a must.
All these are outside though, for vegetables. I may experiment with compost for ornamentals as the results with vegetables has been extremely good.
At my cottage I recently harvested some very unique compost. From the center of an oak tree. Great stuff!

I got buckets of the compost
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Pyewacket
9 years ago
I have to take exception to the idea that "peat is compost". It is not. Compost is compost; peat is vegetative matter that is actually NOT breaking down as compost does. That's what makes it peat. If it were compost, it would have broken down and disappeared tens of thousands of years ago, if not longer. Peat bogs are OLD.

A large part of its value in a potting mix is the fact that it is water retentive. I see that as a plus, except in a few extreme types of conditions.

I have found that compost compacts and retards drainage. I used to always put compost in my containers, organic gardener that I am. Sometimes I had great results and sometimes not good at all. I finally stopped using it in containers because I figured out that compost is NOT fertilizer; it has relatively low fertility in fact. It's value is as a soil conditioner. Good compost is just wasted in a container.