Potting mix ingredients substitutions?

15 years ago

Based on limited testing, I've decided that Al's mix drains a bit too much to be good for growing vegetables from seed in self-watering containers. The soil within 1/2 inch of the top always seems to be fairly dry. Rainfall or top watering can run through and head to the water reservoir. Found that out the hard way.

It also is a little too loose for lightweight root systems.

However, my container Dahlia in a 5 gallon bucket really likes it. Haven't tried a lot of herbs yet.

I also think Ed Smith's book on self-watering containers that said to use 1/2 part potting mix and 1/2 part compost compacts way too easily.

There are tons of different potting soil recipes out there. I also have tons of potting mix ingredients, and wanted some opinions about substitutions based on what I have.

Here are some guesses. Correct me if I am way off.

- Peat might break down 3-5 times as fast as composted pine bark.

- Vermiculite might break down at 5-10 times as fast as perlite. "Fresh" vermiculite might be a little bit more water retentive than fairly fine perlite.

- Compost generally start as pretty fine/compacted particles but varies a lot depending on type of material used for the compost and how aged it is. Depending what you have, it can either supply too much nitrogen too quickly (fresh manure?) or take away too much nitrogen (partially composted hardwood wood fines??).

- Hardwood bark fines break down much faster than pine bark fines.

- Turface is about equal to perlite in retentiveness and porosity but lasts a lot longer.

- 1 part pine fines and 1 part average, non-compacted compost might equal 2 parts of fresh, non compacted peat moss.

- 1 part very coarse sand and 1 part pine bark fines might equal 2 parts of perlite in porosity/water retention. No idea on breakdown of pine bark vs perlite.

- Peat and Pine Bark are both fairly acidic, but I don't know which one is more acidic.

So how crazy am I with these guesses?

Thanks,

Alice

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Comments (2)

tapla (mid-Michigan, USDA z5b-6a)
15 years ago
Sorry, but there is much in the post that would need correction.

- Peat might break down 3-5 times as fast as composted pine bark.

Conifer bark retains it's structure about 4 times as long as peat. Add to that thought the fact that the bark particles used are usually considerably larger, it actually outlasts peat quite a bit longer than that.

- Vermiculite might break down at 5-10 times as fast as perlite. "Fresh" vermiculite might be a little bit more water retentive than fairly fine perlite.

Vermiculite loses it's loft and compacts very quickly - almost immediately. so should be used sparingly in containers.

- Compost generally start as pretty fine/compacted particles but varies a lot depending on type of material used for the compost and how aged it is. Depending what you have, it can either supply too much nitrogen too quickly (fresh manure?) or take away too much nitrogen (partially composted hardwood wood fines??).

All finished compost is fine because mainly what remains after all the cellulose is eaten by soil biota is lignin. Unfinished compost breaks down at varying rates and causes varying degrees of N immobilization. Sometimes it's a serious problem; more often it's not, but it depends on how 'finished' the compost is.

- Hardwood bark fines break down much faster than pine bark fines.

True - but there are other issues like high levels of soluble salts and the tendency of pH to raise quickly during the composting process of hardwood bark. Heat build-up from the composting process can also be a consideration.

- Turface is about equal to perlite in retentiveness and porosity but lasts a lot longer.

Both last indefinitely, but Turface has much, much greater porosity than perlite.

- 1 part pine fines and 1 part average, non-compacted compost might equal 2 parts of fresh, non compacted peat moss.

They're different and there are too many variables to assume anything.

- 1 part very coarse sand and 1 part pine bark fines might equal 2 parts of perlite in porosity/water retention. No idea on breakdown of pine bark vs perlite.

See comment immediately above.

- Peat and Pine Bark are both fairly acidic, but I don't know which one is more acidic.

Peat

Al
aliceinvirginia
Original Author
15 years ago
I was making a lot of random guesses in the original post.

I'm trying to accomplish several things:

1) Find something that doesn't need to be watered so heavily that my downstairs neighbor starts screaming at me again. She's a smoker and wanders around right below the edge of my balcony.

2) Get a potting mix for self-watering containers with much better wicking than the pine fine based mix.

3) Not use vermiculite, which seems to be a mainstay of the potting mix recipes on the internet.

4) Be able to use the potting materials I have (pretty wide variety) without going out and buying a lot of stuff.
- I've got maybe 4 cu ft each of peat and pine fines.
- Probably a cubic foot each of turface, perlite, and sand that is mostly coarse.
- A little leftover leaf compost and worm castings.
- A bag each of composted manure and leaf compost.

- 1 part pine fines and 1 part average, non-compacted compost might equal 2 parts of fresh, non compacted peat moss.

They're different and there are too many variables to assume anything.

OK. Is there anything I can do to compare them?

Are there any recipes out of the following links that you might recommend? The Cornell Epiphytic Mix seems similar to Al's mix.

http://www.backyardgardener.com/soil.html
http://attra.ncat.org/attra-pub/PDF/potmix.pdf

I might write more about this when I get home cause it is getting late to be stupid and stay here at work. I missed the good light for gardening.

Thanks,
Alice
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Potting mix ingredients substitutions?

Comments (2)

tapla (mid-Michigan, USDA z5b-6a)

aliceinvirginia
Original Author

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Potting mix ingredients substitutions?

Comments (2)

tapla (mid-Michigan, USDA z5b-6a)

aliceinvirginiaOriginal Author

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aliceinvirginia
Original Author