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I am making a list of mediums that I can use to make soil more loose and improve airiness.

Soil will be used primarily in pots. I am interested in both home made mediums and commercially available, but more in the home made ones.

Currently on the list: 1. coir 2. forest soil

What other mediums can be used?

UPDATE: I use garden soil for pots (and so far had zero diseases and pests), but I need to mix it with something to keep it loose over a long time.

What about straw? Is it okay to add it to soil?

  • Hi there Sanjihan. Potting soil has very very little soil. It is mostly all the stuff Bamboo has listed. Before you add the already 'sterile' stuff like vermiculite, pearlite, coir...any soil, compost...cook it for half an hour at 180 to 200 degrees. Others say less time than that but to be on the safe side...half an hour. Do you have an oven out of doors? Pretty stinky. Just to be environmentally responsible, grins, don't use peat moss. Here is an article that agrees with the half hour and to test the temperature of the soil to 180 degrees. Also microwave and pressure cooker info. – stormy May 17 '17 at 18:18
  • Hey stormy. "vermiculite, pearlite, coir..." Could you expand the ellipsis :) This is the stuff i am most interested in – sanjihan May 17 '17 at 18:52
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    @sanijhan grins, I had to make potting soil ONCE for a project and after that I will never do that again unless I make it through some apocalypse and want to grow in pots! I used peat and very little sand. Sand is just not that great for drainage in my mind, too...heavy. I had wonderful loam soil in my garden that I used. Coir is great stuff that wasn't available at the time and we didn't know peat was a precious commodity. Here's another good article. I was pleased I actually got the time and temps for oven sterilization. No microwaves at that time either. Ages me huh! – stormy May 17 '17 at 19:17
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From Bamboo's reference, the one that will aerate your soil:

  • Sphagnum peat moss

A very stable ingredient, peat takes a long time to break down and is widely available and inexpensive

  • Coir fiber

A byproduct of the coconut industry, coir looks and acts a lot like sphagnum peat

  • Composted pine bark

Composted pine bark lightens up soil mixes by increasing pore sizes and allowing air and water to travel freely in the potting soil mixture

  • Perlite

A volcanic rock, perlite is heated and expanded to become a lightweight, sterile addition to potting soil mixes

  • Vermiculite

Vermiculite is a mined mineral that is conditioned by heating until it expands into light particles used to increase the porosity of soil mixtures

  • Also charcoal (actual charred wood, not "briquettes" that are mostly coal dust.) – Ecnerwal May 18 '17 at 15:01
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This link (10 Ingredients to Make Your Own Potting Soil) lists ten ingredients for making your own potting soil. The only thing I would add is, where it talks about compost you've made yourself, it does not make it clear that the compost should be produced using a hot, aerobic method, which will kill major pathogens and weed seeds - cold, anaerobically produced compost should only be used on open garden soil in the ground. You will note that soil (garden or forest) needs to be sterilized prior to use in potting mixes, and guidance is given about that.

Of the ingredients listed, those with aerating properties to lighten the soil include perlite, vermiculite, peat and coir.

I note you also ask about commercially available potting soils - I'm lucky enough to live in a country where John Innes potting soils are available; they contain sterilized loam among other things, and they are the king of potting soils, especially for long term planting in containers.

  • I had a set of a very old gardening encyclopedia that had invaluable information in it. I kept coming across references to the different kinds of John Innes potting soils and how good they were. I'm envious of you and by reference, where you live with so many public gardens within easy driving distance. – Jude May 17 '17 at 23:18
  • @Jude I'm mystified as to why JI potting soils aren't made elsewhere in the world, surely it must be possible. Not sure about easy driving distance - the distance might be easy, but the driving won't be, small country, too many cars, too much traffic. – Bamboo May 18 '17 at 0:01
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In Africa we use crushed peanut shells.

  • Very clever indeed – sanjihan May 18 '17 at 7:30
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A commercially available choice is Akadama. It is used in Bonsai cultivation.

  • I always thought this stuff is man made. Thanks – sanjihan May 18 '17 at 7:31
  • It appears that Akadama isn't so perfectly round shaped as the very similar looking brown spheres you often see in stores. Is this the same product, just somehow rounded? potager-au-balcon.com/wp-content/uploads/2014/12/… – sanjihan May 18 '17 at 7:37
  • It is a hard baked Japanese clay, but I remember reading that it is porous. However it breaks down over time and is pricey. The picture you posted looks like a solid media. I sent the photo to some friends to see if they know what your picture was. I have never seen that substrate. – Rhizoqueer May 18 '17 at 16:00
  • These appear to be clay pellets used for hydroponics. – Rhizoqueer May 18 '17 at 16:15
  • Akadama can be baked but it is normally used raw. It's composed of naturally formed rounded clumps of clay which are mined and sorted by size. It turns back into regular clay with use, though. Bonsais are re-potted regularly. Clay pellets and vermiculite may look similar but have completely different properties. – jbcreix May 19 '17 at 4:23
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Polystyrene beads (beanbag chair filling, the denser variety). enter image description here

Costs what, $8 for a person-sized bag? I've used them (with peat, hydrogel, etc) where lots of root space was needed while keeping the weight manageable. Lasted 6 years so far.

Use none at the topmost layer, more at bottom.

  • Interesting. Thanks. You might also be interested in ejnet.org/plastics/polystyrene/health.html. Not sure how this applies in agroculture – sanjihan May 18 '17 at 11:43
  • @sanjihan I doubt my bamboo would suffer much from this, and I'm keeping it outdoors. The beads still look unchanged, and all data I find says the stuff is almost nonbiodegradable. Plus, after all those years, I think much of the residual monomer would have evaporated already. Still, yes, good thing to keep in mind for the future, thanks. – kaay May 19 '17 at 6:09

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