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Posted by sandandsun 9a FL (My Page) on
Fri, Sep 13, 13 at 11:51

There is a common misconception concerning perlite: that it aids in providing moisture to roots. This IS NOT true.

And usually the reason for this confusion is because perlite and vermiculite are often confused.

The primary usefulness of perlite is to improve drainage. The concept of drainage is moving moisture through the soil and therefore away from the roots - i.e., less moisture.

There are moisture control potting mixes available on the market. First of all, the name, "moisture control," is meaningless when considered literally - that is, one might want moisture controlled to a minimum for cacti, or one might want it controlled in to any unlimited array of degrees of moisture. The most familiar moisture control mix is designed for more moisture retention - in order to make more moisture available to the roots. Well designed mixes of this sort do not contain perlite because adding perlite defeats that purpose.

As a beginning gardener, one must become aware that plants have environments with climate conditions to which they are adapted. The easiest example is cacti. Cacti are adapted to desert conditions. A boggy environment would spell quick death to cacti - they are not capable of that extreme degree of adaptation.

Some potting mixes exist to allow the gardener to accommodate/compensate for the specialized needs of plants being grown in the gardener's "alien" environment (one that isn't natural to the plants). Specialty potting soils exist for orchids and cacti, etc.

The home gardeners primarily focused on rooting new plants are rare. Folks focused on this are generally in the nursery industry. So, the value of perlite for a rooting medium is irrelevant to the discussion of perlite's qualities in a potting mix or when added to one's garden soil (by coming along to the soil when a plant potted in a mix with perlite is planted directly into the garden).

So the point here is that if we don't mix up the ideas - a good medium to root in vs. a good medium to grow in, and we understand that roses don't want to drown, but that in general they do like water, then we realize that perlite is not user friendly in this regard. And it can be considered dangerous in soil like sand, where drainage is the primary quality.

Follow-Up Postings:

RE: Perlite

Thank you, Chris (sandandsun), for the distinction. That's why I always speak for myself only, for what works in my garden doesn't work for someone else.

RE: Perlite

Hi Chris: I read your post the 2nd time, what you wrote is true and makes sense. Thank you. My mixing perlite into my compact clay was a disaster, it water-logged and killed one tomato plant. Here's an excerpt:

"Is it necessary to add anything for drainage such as perlite, vermiculite, sand, expanded clay pebbles?

It is best not to add any additional materials to your potting soil. Horticultural grade potting soil is perfectly mixed for the best air-water ratio. Adding more non-nutritive substances depletes the water retention capacity of the soil, it unnecessarily "stretches" the soil and reduces the total amount of nutrients available to the plant, and it creates dry pockets in the container.

Adding a large amount of perlite/vermiculite (some growers add as much as 25%!) is a completely outdated practice from 30-40 years ago when there was only a very small selection of horticultural potting soil available for non-commercial gardeners. It is one of the unfortunate harmful practices in cannabis cultivation that many still cling to although they do not understand the reason or consequences.

Especially harmful is to fill the bottom of the container with coarse materials such as expanded clay pebbles. This is where most roots grow down looking for water and nutrients! If they reach a dry and sterile layer of substrate the delicate root hairs shrivel, valuable space is lost where the plant requires rich soil that stores moisture and minerals.

If you are planting outdoor and your soil is too compact a modest addition of perlite/vermiculite or sand helps to increase drainage. Humus, such as from compost, is the best additive because it also provides many microorganisms and nutrients to the soil mix."

Here is a link that might be useful: Mandala Seeds and potting mix

RE: Perlite

I moved the info. about perlite from "hello" into here:

With regard to perlite: great stuff for pots, holds moisture long. My neighbor works for Ball nursery and gave me too many impatients annual flowers. I was too lazy to plant so I threw them on top of the compost pile. They were alive for 1 month without watering.

Perlite was a disaster mixing into my clay. I got this grand idea that if I mix perlite into clay, plant tomatoes, then I'll never have to water. Wrong! It rained tons, tomatoes got wet feet with perlite in heavy clay. One died from water-logged. Here's an excerpt on perlite from link below:

"Perlite is a natural occurring volcanic glass that, when heated to 1600 degrees Fahrenheit, pops like popcorn and expands to about 13 times its original volume. The end product is a lightweight, snow-white, granular material that weighs only five to eight pounds per cubic foot.

According to the Perlite Institute, each particle of perlite is comprised of tiny, closed air cells that do not absorb moisture into the particle, but holds water around the outside. When closely examined, the surface of each perlite particle is covered with tiny cavities that trap moisture, making water available to the plants roots.

Perlite is used in potting soil mixes because it modifies the structure of soil and improves aeration by keeping it loose and preventing compaction. A potting mix containing one part garden loam, one part peat moss and one part perlite is excellent for containers because it holds just the right amount of water and oxygen.

Perlite is also an excellent rooting medium for cuttings. Roots formed in perlite are much stronger than those formed in water because perlite allows oxygen to reach the roots as they develop, enabling the roots to adapt more readily to a soil environment."

Here is a link that might be useful: Iowa State Extension on perlite

RE: Perlite


I reread your posts and noticed that you contradicted what I wrote. You wrote: "My mixing perlite into my compact clay was a disaster, it water-logged and killed one tomato plant."

So you're asserting that the perlite retained so much water that it killed your tomato plant?

What about "It rained tons..." plus clay.

You wrote: "That's why I always speak for myself only, for what works in my garden doesn't work for someone else."

Am I to understand that there is no truth about perlite? Am I suppose to believe that perlite's properties vary by location? That like plants that may not like a climate, perlite will perform differently if people in different locations use it?

Before you say that this is true, I ask you: do you really expect me to believe it?

RE: Perlite

Hi Chris: I don't know how perlite would act if mixed with fluffy potting soil, or mixed with sand. I only know that when mixed with my heavy clay, roots get water-logged.

I got nit-picked and falsely accused in "Public apology" thread that I make generalizations (by Christopher) ... so I have to say "in my garden only." Or "I speak for myself", or "what works in my garden doesn't work for someone else." That's to protect myself.

RE: Perlite

You just wrote: "I don't know how perlite would act if mixed with fluffy potting soil..."

In your previous post you wrote: "With regard to perlite: great stuff for pots, holds moisture long."

It's one thing to contradict someone else - but contradicting youself in less than 8 hours. HUH?

It is also very interesting that you accuse Christopher of attacking you. When I characterized some of the posts in that thread as neutral and well reasoned, his was one of the primary posts that I had in mind when I wrote that. His post reminds me of the one I wrote back in December 2012.
I wrote: "I also have some concerns about your conclusion..."

Thread linked below.

Here is a link that might be useful: Grow deep roots, reduce salt, and reduce watering

RE: Perlite

Hi Chris: You can have this forum all to yourself, I'm leaving. There's no point of sharing my experience when I got hassled like that.

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