Soil mix test results

[GavinG]

NERD ALERT!! I can’t believe I’ve done this.

I decided to test some soil mixes as I have recently moved and lost access to my usual
components. Grant and Leigh are testing the individual components, but how soil
mixes perform is not necessarily the sum of their parts, due to the way the
components and the particle sizes interact.
The recipes in this test are not necessarily viable bonsai mixes – I chose them to show
how varying one component while keeping another constant affects the performance
of the mix. Some of the results are inconsistent, and some I frankly do not understand.
I hope to replicate these tests in midsummer, a more sensible time to measure drying
effects.
I do not recommend ANY recipe – you can use the information here to tailor your
mix to your own region, microclimate, preferences, trees, and degree and style of
insanity. Please test your own materials in your own environment.

LIST OF MATERIALS:
potting mix – RichGrow Native Mix
bog peat – Brunnings from B*$%##!^%s. Not a good choice environmentally
coco peat – I threw the packet out. The small one from B*$%##!^%s
perlite – RichGrow, looks 2-3mm
vermiculite – Vermigro, looks 2-4mm
diatomite - Mt Sylvia, 2-7mm
zeolite - Canberra standard, greyish, looks 2-7mm
brick dust - Canberra standard, looks 2-7mm, sieved for dust through fly wire
coarse sand - looks 1-5mm, from Turtle Landscaping, Rouse Hill
In retrospect, the potting mix should probably have been sieved.
All components were used bone dry, except bog peat which was used from the bag
due to wetting issues.
Coco peat was broken up dry – if you soak it in a bucket before you use it, it expands
wildly, so you will need to roughly double the volume of coco peat you use to get the
same effect.
While perlite would seem to be a useful component, the dust is fine, nastily irritating,
and light as air. If you’re not likely to mask up, don’t use it.
I know vermiculite mushes up and is not good for bonsai mixes, but I had it, and
intend to use it for propagation. Its drainage/retention figures certainly looks good for
short-term use.

PROCESS:

Components were mixed by volume, poured into a dry clean 120mm pot (140mm
high, for those interested in water tables) and lightly shaken, not tamped. They were
then weighed dry, dunked for one minute to allow good saturation, and drained. The
time it took for the drips to become sporadic was recorded, as an extremely rough
estimate of how freely the mixture drained.
The mixes were then weighed after sitting for half an hour (to allow for further
seepage) and left in a sunny window to dry out. As the newspaper they sat on became
quite sodden overnight, they were weighed again after a day, as the water that seeps
out overnight is not going to keep your trees alive over time. The amount of water that
was absorbed and retained you find by subtracting the dry weight from the wet
weight.
Abbreviations: potting mix pm, diatomite d, coarse sand cs, brick dust br, zeolite ze,
perlite per, vermiculite ver, coco peat cp, bog peat bp.

RESULTS:
recipe drying time water absorbed water retained 24hours lost %
Potting mix series:
3pm 1d 50 seconds 376 grams 320 grams 15%
1pm 1d 40 280 214 24
1pm 1cs 35 336 182! 45
1pm 1br 30 244 130! 47
1pm 1ze 30 326 130! 60
1pm 1per 40 408 282 31

Peat series:
1bp 3d 15 162 78 52
1cp 3d 25 342 242 30
1cp 3cs 35 440 274 38
1cp 3br 25 436 314 28
1cp 3ze 30 358 296 17

Diatomite series:
1d 1per 15 208 92! 56
1d 1ver 15 298 234 22 propagation
1d 1cp 40 640 510 21 not viable
1d 1ze 15 84! 30! 64
(1d 1pm) 40 280 214 24 as in pot mix series

SO WHAT DOES IT ALL MEAN? DISCUSSION.

WARNING – THESE RESULTS COME FROM A SAMPLE SIZE OF ONE.
TEST YOUR OWN COMPONENTS. FIND OUT WHAT THEY DO.
It surprised me that I often had to mix 700ml of each of two components to fill a 1300 ml
pot. When large particles and small particles mix, the small ones fit into the spaces
between the large ones – this affects drainage. It seems that it doesn’t matter if you put lots
of large particles in for drainage - if there is a fine component it will just clog the large
stuff.

DRAIN TIMES:
All potting mix recipes were slowish to drain. 3pm 1d (25% diatomite) was the slowest to
drain of all the mixes I tested. If you want quick drainage, sieve the fines and include at
least 50% coarse drainage.
Diatomite, brick dust, zeolite and coarse sand all drained at much the same speed, with
coarse sand tending slower, probably due to a smaller particle size.
Coco peat had better drainage and better water retention than the potting mix recipes.
If you want very rapid drainage you will get very little water retention. In particular, 1d 1ze
and 1d 1per were diabolical.
Although no good for bonsai, 1d 1ver had very fast drainage with the best water retention
of the diatomite mixes.

WATER RETENTION:
The amount of water lost by seepage over the first day was surprising. In particular,1d 1ze
and 1d 1per retained very little water, and their performance in summer heat must be
considered marginal.
In general, coco peat retains more water than potting mix, in smaller proportions, and
drains more freely.
Of the moderately fast draining mixtures, these are the most effective water retainers, from
best to less: 1cp 3br, 1cp 3ze, 1pm 1per (slowish to drain) 1cp 3cs, 1cp 3d, 1pm 1d.
Diatomite, brick dust, and zeolite appear to hold very little water.
Coarse sand holds more in the mixes, probably due to the increased surface area due to the
smaller particle size. Water just sticks to surfaces.
Perlite appears to hold more water, but did not drain well with potting mixes, and did not
retain water with diatomite. Go figure.
From the data, I cannot separate diatomite, zeolite and brick dust due to inconsistencies.
For instance zeolite lost massive amounts of water in 1pm 1ze and 1d 1ze, but retained
very well in 1cp 3ze. Go figure.
Please remember that the coco peat was measured dry –when soaked in a bucket the same
amount of peat becomes much larger. I would suggest 20% wet coco peat will still give
good water retention and good drainage. I will probably be trialling 1pm 1br and 1 wet
coco peat 3 diatomite, adding some Osmacote for nutrients.

Many thanks to those who have made it this far. I hope the information can be useful.

Good luck!

Gavin

[blkrota]

That is some good information, Good read.
I am lazy and just buy it pre mixed in the large bags from the main bonsai nurserys in Sydney and use what ever one best for that tree.

Respect for doing the leg work.

[NBPCA]

Thanks Gavin

Thanks for you input in the soil testing department.

I have most of the individual materials sieved, graded, in the pots and drying thoroughly together; ready for tests soon.

I think most people are underestimating the effect of particle size on drainage. Smaller particle mixes hold much more water.

Grant

[GavinG]

Sorry about the jammed up numbers in the tables - I spaced them all out in the box, but the spacing got lost in transmission. Just tried to edit - the spacing is there in the edit, and disappears again when its submitted. Go figure. If anyone is interested, I can email the original PDF file.

Gavin

[63pmp]

I'm not sure what you are trying to measure here.

The amount of water retained in a mix is not a reflection of the air filled porosity, nor the amount of water available for plants. Also 1 minutes submergence would not be enough time for components to become completely saturated or stabilized, especially with something like composted bark or perlite. It is the water that drains out of the pot that has to be collected and measured; this volume gives the air filled porosity.

Australian Standards for potting mix requires the moisture content of the potting mix before testing to be wet enough that it just releases water on a firm squeeze. The way it is wet up is also explained, which allows all componenets to absorb whatever water it will. That means the components of the potting mix have to be saturated saturated before testing begins.

Regards

Paul

[GavinG]

I am trying to measure the amount of water retained in the mix. How much is available to plants baffles better minds than mine.

The air-filled porosity test as described completely ignores the fact that after the water drains, a considerable amount remains stuck to the particle surfaces (this is what we rely on for our trees!) - this amount was previously air-filled space, but is not measured by the test, as the water has not drained.

While an extended soak may saturate components better, bonsai growers do not soak each of their trees for ten minutes each day through summer. I would have preferred to water the test mixes by hose, as this is what happens in practice, but this would not have allowed the drainage speed measure.

I have no idea how they justify measuring potting mix water retention buy testing a mix which is already to some degree wet. Makes no sense to this amateur.

Please post the results of your testing as it becomes available.

Regards,

Gavin

[GavinG]

Another attempt to make the results table readable:


RESULTS:
recipe_____drying time___water absorbed__water retained 24hours__lost %
Potting mix series:
3pm 1d ____50 seconds____376 grams______320 grams______15%
1pm 1d ____40____________280_____________214____________ 24
1pm 1cs____35____________336_____________182!____________45
1pm 1br____30____________244_____________130!____________47
1pm 1ze ___30____________ 326_____________130!____________ 60
1pm 1per___40____________408_____________282______________31

Peat series:
1bp 3d _____15_____________162______________78_____________52
1cp 3d______25_____________342_____________242_____________30
1cp 3cs______35_____________440_____________274_____________38
1cp 3br______25_____________436_____________314____________ 28
1cp 3ze______30_____________358_____________296____________17

Diatomite series:
1d 1per______15____________208______________92!____________56
1d 1ver______15____________298_____________234_____________22 propagation
1d 1cp ______40____________640_____________510_____________21 not viable
1d 1ze_______15_____________84!_____________30!_____________ 64
(1d 1pm)_____40____________280_____________214_____________24 as in pot mix series

Hope it makes it through the system.

I will weigh again after 1 week and 2 weeks.

Gavin

[Gerard]

NERD ALERT!! I can’t believe I’ve done this.

A massive task you have taken on Gavin.
I am too lazy to do this kind of testing but rest assured we read these articles with great interest.
Thanks for your efforts.

[Bretts]

Hi Gavin
Physics and a simple test tells us that after the initial drainage and the water tension finds it's balance the pot will drain no more.
I believe the newspaper you are sitting your test pots on are acting as a wick drawing water out of the pot.
I think you should suspend the pot or at least have it sitting on a non porous surface. After a pot has drained to a balance of water tension and gravity if it has no plant in it drawing the water out then the only way out for the water is evaporation. Unless you tilt the pot changing the water tension gravity balance again.
I have always imagined that evaporation was minimal but I would be interested if this could be tested.
Great to see you thinking about this stuff

[Jamie]

interesting that the zeolite mixes have the highest percentages in these test, hey brett

[Bretts]

I might say you where right one day Jamie but not yet Grant and my tests both showed zeolite holding less water. I have looked hard and see that Gavin found his potting mix and zelite mix held more water than his diatomite potting mix but as I mentioned above that the data sheets say it holds less. Grant and myself found it holds less so I have to wonder why Gavin got different results when mixed with potting mix. I think he mentioned he did not sift so maybe that is it?

[NBPCA]

Hi all,

I will probably commence the testing of all the separate components that I have on Tuesday next week.Quite a selection I must say.

I am also testing for diiferent grain sizes as well. For instance CRG 3.2 - 6.4 mm and CRG 1.6 - 3.2mm separately and onr combined.

Also Decomposed Granite in 3 grades etc.

Grant

[63pmp]

The air-filled porosity test as described completely ignores the fact that after the water drains, a considerable amount remains stuck to the particle surfaces (this is what we rely on for our trees!) - this amount was previously air-filled space, but is not measured by the test, as the water has not drained.

The most important physical property of a potting mix is the amount of air it retains after draining. The amount of water it contains is of secondary importance. Without air around the roots the plant dies, end of bonsai. Obviously there is a correlation with the amount of air in a mix, and the amount of water. Lots of air means not much water and the plant needs frequent watering. No air means lots of water, and the plant drowns. Different plants need different amounts of air, compare water lillies with aerial orchids. Air filled porosity is the number that is needed.

I think its great that your putting a lot of effort into understanding the materials you're working with, its a very important aspect of growing bonsai. However, when you post up your work on a public forum you are going to get good and bad responses, that is always going to happen, don't let it get up your nose. In this case a bit more research would have yielded more pertinent results. The real number we need is the amount of air in a mix after draining.

As for posting up the results of my research; well its meaningless, as no one else is using my mix. Also, I cant see the point of testing individual components, simply because once blended the porosity changes, as you already found out . If you want me to post up a method for determining Air Filled Porosity, then sure, be glad to help, but actual results? well that's of no use to anyone.

Paul

[Jamie]

i think the reasone why the zeolite i got held so much water was actually the water table it created as it was quite a fine grain, i would say no more than 3mm, a very evan particle size, thus create more water retention. with the potting mix/zeolite mix i would hazard a guess that the small particle size of the zeolite and the organic potting soil created a high water table and caused it to retain more moisture/weight.

jamie

[GavinG]

Here, as promised, the final results of my small soil mix test. When I started, I had the impression that Grant/NBPCA would only be testing components, not mixes, and my interest was in how quickly complex mixes drained, how much water they retained, and how long they stayed damp. Because mixes consist of particles of different sizes, their interaction is complex.

I measured water retained after a ten minute drain, and after one day (as posted), after one week, two weeks, and four and a half weeks. (Yep, slack.) They were sitting on a sunny window shelf in a mildly heated house in Canbrrrrrrrrr.

I won't post the results, as it all becomes a number salad after a while, but they can be sent to anyone masochistic enough to ask.

In general, the two fast-draining mixes (1di 1per and 1di 1zeo) were bone dry after a week.

All the potting mix blends retained useful water after a week, with the best being 1pm 1per and 1pm 1di. All were virtually dry by two weeks.

The peat moss blends are somewhat unrealistic - they had 25% DRY peat moss (as I was measuring how much water they absorbed and retained) which translates to about 40% or 50% by volume when it gets wet and expands. I doubt that growers will use a 50% coco peat mix, but it was interesting that they all drained quicker than the potting mix blends, despite being overloaded with absorbent material. All the coco peat blends retained around half their water after two weeks, and astonishingly, most retained around 20% after four and a half weeks!

As far as the different grits go, it seems that particle size is more important than what it's composed of. Small grains retain more water, due to greater surface area, but slow the drainage down. There was no clear indicator that zeolite, diatomite, or brick dust retained more water than the "impervious" grits/sands, despite what's been said.

So now I'm running a second series of tests, with a sifted potting mix series (to see whether this improves drainage times) and a dampish coco peat series at about 25%, which more like how people measure their peat out - hand-squeezed from the bucket where it's been soaking. I will measure how much water these mixes retain by subtracting the fully dried weight (after about 4 or 6 weeks weeks) from the original drained weight. Backwards I know, but it makes sense to me.

Happy potting!

(Have you people seen the minimum temperatures in these parts, night after night? Crazy place to live.)

Gavin