Well Aerated soil Formulas

Bob1016(9b)January 31, 2012

Let me start off by saying if I gat something incorrect, please correct it. I thought that this would be a good way to take an encyclopedia's worth of information in a more simple form, and it would allow us to look at the big picture of soil components, not just very specific ingredients.

We all know that we can include both organic (peat, moss, bark, compost, etc.) and inorganic (perlite, vermiculite, calcined clay, sand, rocks, etc.) into our mixes. amongst all of these ingredients, there are so many specific properties that it becomes very hard to keep track of. I find that just a few of these properties are the most important.

Absorbent vs Adsorbent

These two terms are often not understood enough by the average person, absorbent materials hold liquids or gases inside their structure. These are materials that we use to hold water in our mixes.

Adsorbent materials have a very high surface area, they literally trap individual molecules on their surface via Van der Waal forces (no you don't need to know that word, but it is fun to say :-). We use these materials for their very high CEC, and they also hold water, but not as well as absorbent materials.

Adsorbent materials include:

Calcined Clays (turface, oil dri, etc.)

Lava rock

Most charcoals


Absorbent materials:



Lava rock


Yes, some of these belong to both. Some charcoals are more absorbent, some are more adsorbent, but all are both. The same holds true for many of the materials listed.

So for Gritty Mix (I wish I could put a trade mark symbol there, Al deserves it), Turface (70% ads. 30%abs.?) would be the adsorbent component, Bark (60% abs. 40% ads.?) would be absorbent, and the grit would primarily provide volume, weight, and aeration (though some bonsai enthusiasts claim that its sharp corners induce branching in the fine roots, not sure of the validity of this).

For 5-1-1, the bark would take the roll of the adsorbent part, peat would be absorbent, and perlite would provide aeration.

BTW, the numbers above (70% ads. 30%abs.) are a guess, I don't know if it is true or not, it just seems that some things are more adsorbent and others are very adsorbent, and so on.

So, I would put forth that a modified gritty mix could be approximated with a combination of ingredients:

1 part turface

1 part charcoal

1 part lava rock

1 part bark

1 part perlite

1 part grit

the turface, charcoal, and lava rock would be adsorbent, the bark, lava rock, and charcoal would be absorbent, the grit and perlite would provide the extra aeration and volume.

I like this way of thinking about potting mixes, and find it very easy. I would love to hear any comment, correction, ideas, what ever.

P.S. this does not address drainage, to fix drainage problems, I quote Al "the PWT tends do disappear as uniform particle size approaches 0.11"", sorry I paraphrased, but I think I got pretty close.

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penfold2(4b, MN)

I don't see the point in differentiating media based on their (ab/ad)sorbency. As long as the soil has proper water retention and porosity, it should perform well in containers. CEC is another factor, but is not as important in containers since we are fertilizing regularly.

Is there something you're trying to correct in the gritty mix? Do you think the CEC should be increased/decreased?

I don't see the need for six ingredients unless there is some useful improvement.

    Bookmark   January 31, 2012 at 7:16PM
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I dont think gritty mix needs to be adjusted except for minut changes in ratio to account for different plant culture and weather conditions. This is not new info, it's just a different way of organizing it. I like to know how each component works, as well as knowing how the mix itself works. I figures I would post this for other people who have the same interest.
The water retention and porosity are functions of the (ad/ab)sorbency (clever), and CEC is very important in any non-NFS setup; it allows the nutrient to wait to be absorbed by the plants roots as needed.
I was using the six ingredient mix to show how you could approximate gritty mix if you cannot find an ingredient, or if one ingredient is only availbe as a small quantity, or if the prices of certain ingredient in your area are not available. Calcined clay is not available everywhere, nor some other ingredients.

    Bookmark   January 31, 2012 at 8:03PM
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