Shop Products
Houzz Logo Print
penfold2

More Soil Substrate Comparisons

penfold2
12 years ago

Inspired by xerophyte_nyc's previous experiment on soil substrates, I decided to carry out my own. I focused on drainage components since that is my biggest concern, and is always an issue for succulent growers. The materials I used are lava rock, pumice, granite grit, and perlite.

{{gwi:632386}}

While the original sizes of the materials prevented me from using perfectly identical particle sizes, I did sift them to obtain very similar sizes. This should provide a meaningful comparison, but one should keep in mind that particle size has a very strong influence on these numbers, so they should be looked at primarily in relative terms. Also, mixing different particle sizes tends to yield results similar to those of the smallest particle size. So, for example, adding perlite to peat will not increase porosity until the mix is mostly perlite. This is why I try to use equal particle sizes in my mixes.

After sifting, I filled each cup with substrate, then measured the amount of water required to fill the cup and substrate, then measured the amount of water that drained from the cup when the drainage hole was uncovered. From these two numbers I determined the values below.
Porosity (% air space)
Water Capacity %
Saturation (water/porosity)
Perlite
61
22
36
Pumice
53
17
32
Lava
61
18
30
Granite
44
8
18

There are a couple interesting things to note here. Perlite, pumice and lava rock appear to have very similar characteristics, with perlite holding just a bit more water than the others. From the looks of things, I'd say they could be used nearly interchangeable. And although I didn't expect granite to hold much water, I also didn't expect it to be so far behind the other materials, especially since it appeared to have a slightly smaller particle size than the rest. Yet it held only 8% water. Obviously it has no internal porosity, but its smooth surface may also help it to shed water when compared to the irregular surfaces of the other materials.

Comments (11)

  • greenman28 NorCal 7b/8a
    12 years ago

    Thank you, Penfold!
    This is a very helpful comparison. Indeed, the Perlite and the Lava are closer
    than I thought. I'd assumed that the Lava retained less moisture, but now I know better.
    I have a couple bags of unopened red Lava (Scoria), as well as quartz (my granite substitute),
    and I think I'll be doing some succulent re-potting in the next few weeks.

    Grey pumice, red scoria, white perlite....

    {{gwi:587144}}


    Josh

  • penfold2
    Original Author
    12 years ago

    Looks great, Josh. I wish I could find a local source of pumice and scoria like that, but around here there is no pumice, and scoria comes in big chunks. You west coast people have all the good stuff, and great nurseries to boot.

  • wantonamara Z8 CenTex
    12 years ago

    Thankyou , I have found a source of the ted Lava sand so I have already been using it because I thought it would be better than the perlite about floating away.

  • wantonamara Z8 CenTex
    12 years ago

    I wonder how expandable shale rates here? That also is available readily here in Texas. And I wonder if decomposed granite difers significantly from granite. That is cheap as dirt here. It is dirt just 20 miles to the west of me. If I find some time should do this test but probably I won't since my time is spare time is disappearing fast..

  • meyermike_1micha
    12 years ago

    There is something about those ingredients that looks absolutely beautiful, doesn't it?

    No wonder I love the way my soilless mixes come out after using these ingredients.
    Adding that uncomposted fir bark adds so much more flavor to it all. I love these mixes for my succulents!

    Mike

  • westes Zone 9b California SF Bay
    5 years ago

    Does anyone have this water holding and capacity data for peat moss?

  • rina_Ontario,Canada 5a
    5 years ago
    last modified: 5 years ago

    Westes

    I do not remember seeing any; perhaps because most (not all) ppl do not use peat in mixes for succulents and are aware that it holds water for quite a while (unless hydrophobic). Perhaps you can test it for yourself? - I know you are aware of a thread by ewwmayo on the subject. Here are his instructions for testing:

    Required for testing:

    • Digital scale, preferably in grams.
    • Clean and dry pots (4" is adequate, needs to have one drain hole).
    • Dry and clean plastic tub that can hold pot + water.
    • Enough dry soil mix to fill your pot to the brim. (Peat in your case)
    • Duct tape to seal bottom of the pot.
    • Water.

    Steps for testing:

    1. Weigh the dry empty tub.
    2. Use a small piece of duct tape and seal the drain hole of one pot. It must be water-tight!
    3. Weigh the dry empty pot.
    4. With the pot on the scale, fill it with water and record the weight.
    5. Empty the pot and dry it.
    6. Fill the dry pot with your soil mix to the brim.
    7. Weigh the dry pot with your soil mix in it.
    8. With the pot with soil on your scale, fill it with water and record the weight. If the ingredients take some time to absorb water, you may need to wait a few minutes and top up the water to the top of the soil mix level.
    9. Carefully remove the pot and put your dry empty tub on the scale.
    10. With your pot over the tub, carefully peel a corner of the tape to let the water drain into the tub. If your hole is too big the mix will fall into the tub and skew your measurements.
    11. Weigh the water drained with the tub.
    12. Weigh the drained but still wet mix and pot.

    Formulae:

    • Total Porosity = weight of water needed to fill pot / empty pot volume * 100.
    • Air Space = weight of water drained from pot / weight of water needed to fill pot * 100.
    • Water Holding Capacity = (weight of water needed to fill pot - weight of water drained from pot) / empty pot volume * 100.
    • I am sure you can also google other instructions.
    • If you test, could you post results pls?
      • ETA:
        I found some data from sungro. They list following:
    • Total porosity: 89 - 94
    • Water holding capacity: 74 -77
    • Air capacity: 12 - 20
  • westes Zone 9b California SF Bay
    5 years ago

    @rina, I was probably not ambitious enough to do my own test, especially since I tend to hate peat moss. :) But talk about delayed gratification: after a lot of details on how to do the testing you supplied the data. Thanks :) Do you have a URL for that Sungro page?

    A Turface test by user @ewwmayo shows Turface saturation (sat) at:
    whc 52% / por 69% ~= sat 75%

    The Sungro peat moss data shows peat moss saturation at:
    whc 74-77% / por 89-94% ~= sat 81-83%

    I calculated the saturation (sat) for peat as 74/89 and 77/94 to establish an approximate range. Was that supposed to be 74/94 versus 77/89, which would be a much bigger range?

    So based on this peat moss looks like it has a similar saturation (sat) percentage with Turface. What are the aspects of peat that make it so preferable in the 511 mix? Is it holding onto the water longer?

    I'm not sure I understand how to define water holding capacity (whc) in the data above, where peat moss appears to have an advantage. One website I read had these definitions:

    field capacity (usually) ~= 50% of saturation = maximum water holding capacity of soil against gravity

    permanent wilting point (usually) ~= 50% of field capacity

    water holding capacity = water content between field capacity and permanent wilting point

    Using these definitions, it is hard for me to understand this data since none of the whc data points are ever less than 50% of the saturation.

  • westes Zone 9b California SF Bay
    5 years ago

    @rina in your formula above you have:

    Total Porosity = weight of water needed to fill pot / empty pot volume * 100.

    I do not have a direct measurement of the weight of the water needed to fill the pot. I have the weight of the water that drained from the pot. I could calculate the weight of the water that filled the pot as:

    Weight of water-filled pot - Weight of pot with dry soil

    Is that what you meant by "weight of water needed to fill pot"?

  • rina_Ontario,Canada 5a
    5 years ago

    Westes

    I am sorry for misunderstanding - those directions are from ewwmayo. I just posted part of his post and results from -THIS THREAD- . You may have read it.

    1. I would think that you are correct - but re-read his Steps for testing (just above the Formulae)


Sponsored
Hoppy Design & Build
Average rating: 5 out of 5 stars9 Reviews
Northern VA Award-Winning Deck ,Patio, & Landscape Design Build Firm