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What happens to Vermiculite in Potting Mix?

carlson yeung

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posted 6 years ago

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If I understand correctly, stuff like vermiculite and perlite don't really decompose.

If every year, I starts seeds in a flat that contains this stuff and then transplant into my garden, doesn't it accumulate? Like, after 10 or 20 years, I would expect that my garden will be elevated with lots of this stuff to get rid off?

Why shouldn't I just start seedlings in flats of compost for transplanting a month later?

Chris Kott

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posted 6 years ago

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Perlite and vermiculite are essentially pockets for air and water, if I understand correctly. They lighten potting mixtures for ease of transfer and movement, and they open the structure of the potting mix so that it doesn't cement itself into a block.

I don't think you'd be using straight compost for starting even if you did away with perlite and vermiculite, as you'd need less-rich components in the mix so as to not burn delicate seedlings. I have planted into pots of unamended compost and straight topsoil from the garden, and I ended up with solid blocks that wouldn't even let water in, after being so waterlogged that the germinated plants' rotted away.

As to why perlite and vermiculite don't accumulate visibly, it may be because they get blown away in a stiff breeze, so if the dessicated surface layer is all you see, the lighter stuff has already blown away.

It may also be that they continue to break down under mechanical and biological action.

-CK

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Bryant RedHawk

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In the case of Vermiculite, it is expanded (heated under pressure then the pressure is released rapidly) mica, Mica is a linear crystalline form of silica.
Perlite is volcanic glass that has a concentric structure, appears as if composed of concretions, is usually grayish and sometimes spherulitic, and when heated expands to form a lightweight aggregate used especially in concrete and plaster and as a medium for potting plants (Webster's definition)

Perlite will persist in potting soils for as much as 12 years with no signs of degradation, Mica will persist in potting soils for approximately 5 years, flakes will break into smaller and smaller pieces as the soil is handled (worked).

Of the two, the structure of mica (vermiculite) is superior for bacterial breakdown into nutrient particles, Perlite, because of the concentric structure is far more resistant to bacterial break down, acting more along the lines of "biochar".

Both will hold quite a lot of water but perlite will only release this water grudgingly because of its structure where mica will readily release water which is held between the thin layers of its structure.

Redhawk

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Bryant RedHawk

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When you are starting seeds, the biggest mistake people make is to try and use a regular potting soil mix or a straight compost mix.
Seeds have a "yolk" which needs to be used up prior to the new plant taking in nutrients via the roots.
When a seed sprouts in something that has a lot of nutrient value, the baby stem lengthens even though it isn't very strong since no lignin has had a chance to form, this usually results in the stem becoming longer than the cell structure can support.
When the stem is longer than its ability to support itself it fails and bends over, killing the baby plant.

Try to always start seeds in a non-nutrient medium (sand or sand mixed with vermiculite), this will keep the stem to the correct length and there won't be any losses from the stem bending and breaking.

Redhawk

Chris Kott

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Bryant RedHawk wrote:When you are starting seeds, the biggest mistake people make is to try and use a regular potting soil mix or a straight compost mix.
Seeds have a "yolk" which needs to be used up prior to the new plant taking in nutrients via the roots.
When a seed sprouts in something that has a lot of nutrient value, the baby stem lengthens even though it isn't very strong since no lignin has had a chance to form, this usually results in the stem becoming longer than the cell structure can support.
When the stem is longer than its ability to support itself it fails and bends over, killing the baby plant.

Try to always start seeds in a non-nutrient medium (sand or sand mixed with vermiculite), this will keep the stem to the correct length and there won't be any losses from the stem bending and breaking.

Redhawk

Mind blown.

-CK

carlson yeung

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Bryant RedHawk wrote:

Try to always start seeds in a non-nutrient medium (sand or sand mixed with vermiculite), this will keep the stem to the correct length and there won't be any losses from the stem bending and breaking.

Redhawk

I am one of those people that thought seeds should be started in nutrient rich environment. Thank you for correcting my misunderstanding.

New Questions:

1. How long can a plant, for example, lettuce stay in a low nutrient environment before it must be transplanted into the garden?

2. It may not be ideal, but is it ok to use plain garden soil in a container to start seeds?

Thank you.

William Schlegel

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posted 6 years ago

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When I was in soils class in college the instructor told us that a common gardening mistake is to think that adding a bag of sand to a garden's soil will improve it. He pointed out that to do that you would need an enormous volume of material. Think of a 25 foot by 4 foot bed and imagine adding six inches of material. How many cubic feet is that? Now expand that to an entire garden or an acre. This is why when I want to add sand to my garden I buy a ten yard truck load. The same principle I think generally applies to vermiculite and perlite. The square foot gardening book guru Mel Bartholemew advocated either extreme soil amendment or complete soil replacement. So often he was gardening in a 1 foot deep planter. Or alternatively adding an artificial one foot deep soil horizon on top of existing soil. So if you really want to change your topsoil by adding vermiculite to it you will need a lot of volume, and it will be really expensive.

Western Montana gardener and botanist in zone 6a according to 2012 zone update.
Gardening on lakebed sediments with 7 inch silty clay loam topsoil, 7 inch clay accumulation layer underneath, have added sand in places.

s. lowe

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posted 6 years ago

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I have no experience with vermiculite but I have witnessed perlite break down. It turns soft and yellow and then brown and then is no longer distinguishable from the soil around it. As Dr. Redhawk noted above they are both processed mineral substances so they cycle through like everything else.

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