I am not getting many large particles from sifting, so am looking for an alternative material for the initial drainage/aeration layer in the bottom of the pot. How about using pea gravel sifted to 1/4 - 3/8 inch?
It should work but be a little heavier. Round pea gravel will not pack as tightly as the irregular particles of pumice or lava, but more like akadama. I know that Ryan recommends using the same mix as the main soil mix but in a larger size. However, I normally just use pumice in the larger size and it works well.
The reason for using the same material would be not to break the water column. Different layers could effect drainage by different absorption or capillary function.
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I agree that using the same material will lead to a more continuous water column. But one of the reasons to use an aeration layer is to provide a break in the water column due to the particle size difference so air can come into the bottom of the pot. With the thin (1 - 1.5 particle thick) aeration layers we use and the fact that some of the upper soil will fill the inter particle voids there will be a minor break in the water column even with a very different layer like pea gravel.
I do the same as you for most trees especially in taller pots a maple in a shallow pot will however at least get akadama mixed in.
i assume you know the information in the following article but it is something everyone growing bonsai needs to understand. @Chibon
Is everyone ready for some backyard bonsai science!!
I decided to look at the effect of different aeration layers on how the water drained out of the pot shortly after watering. I used the retained water weight rather than trying to look for a perched water table height and washed the aeration layer to avoid including the absorbed water on the aeration particles in the retained water weight. Please excuse some of the odd text formatting, but this is how it copied in from Excel. Please note that I am bit of stickler for particle size vs. screen type since the particles that pass through a 4 mesh screen will be less than 1/4 due to the wire diameter, but almost everyone will call them smaller that 1/4”. There are pictures at the end.
My experimental conclusions from this study are:
- There is a fair bit of variability between tests of the same type. Not sure if I want to spend the time to do 3 or more of each to get some level of statistical significance.
- Just Boon mix without an aeration layer retained the most water which justifies using an aeration layer.
- The aeration layers using medium particles that are only a bit larger than the main soil were closest to the mean regardless of type. Visual analysis (no close-up picture) indicated that there was a clear line between the aeration layer and main soil.
- Retained water was lowest for large particles and visual analysis indicates that the main soil particles dropped into the gaps in the aeration layer. This makes sense since there is about a 2 to 3X difference in size.
- Retained water in pea gravel was higher than in the same size particles of pumice or Boon mix indicating a more perched water table. The top surface of the pea gravel felt and looked smoother than other aeration layers - the particles were rounded and smoother.
My conclusions from the bonsai practice standpoint are:
- The aeration layer should be just a little larger than the main soil to ensure a perched water table so air can get in through the drainage holes.
- The aeration layer should be thin, particularly in shallow pots, to ensure that most of the soil mass is in the perched water table and remains moist.
- The particle type for the aeration layer is probably not too important to create a perched water table as long as #1 is followed. Pea gravel is probably an acceptable aeration layer.
- Based upon observation of roots in the aeration layer, the type of particle will probably have an impact on the roots that grow there. An aeration layer with the same composition as at the main soil will probably give finer roots, but we will probably trim them off anyway. My guess is that the roots in pea gravel will be the stringiest based upon what I have seen lava aeration layers. I plan to use pumice unless I have recycled mixed particles of the right size.
Marty
| 7" x 4.75" x 3" clear plastic pot. 1.25" hole with 8 mesh screen. Bottom outside groove holds about 15 g of water and weighed 31-33 g depending upon how wet it was from previous test. | |||||||
|---|---|---|---|---|---|---|---|
| Particle sizes used | |||||||
| Small Boon mix (AKA 1-1-1) screened to -4 mesh and +8 mesh. | |||||||
| Medium aeration layer screened to -3 mesh and +4 mesh | |||||||
| Large aeration layer screened to -2 mesh and +3 mesh | |||||||
| Procedure | |||||||
| Aeration layer material was washed and drained over an 8 mesh screen. It was then placed in the bottom of the pot to just cover it with only small gaps - 1.5 layers thick and weighed. | |||||||
| 500 g of Boon mix was added, smoothed, gently tapped on the table, and weighed. | |||||||
| Pot was watered from all sides until it ran fairly clear from the drain hole | |||||||
| The wet pot was allowed to drain for 2:30 where watered on the slatted bench and moved to a dry spot for another 2:30. | |||||||
| The drained pot was weighed to obtain the wet weight. | |||||||
| Results - Weights in grams | |||||||
| Aeration Layer | Empty | Wt. /aeration | Aeration | Dry filled | Wet drained | Retained water | Diff from mean |
| Med Pea Gravel | 31 | 229 | 198 | 729 | 934 | 205 | 2 |
| Med Pumice | 33 | 122 | 89 | 621 | 806 | 185 | -18 |
| Med Boon mix* | 33 | 121 | 88 | 620 | 821 | 201 | -2 |
| Large Pumice | 33 | 138 | 105 | 638 | 790 | 152 | -51 |
| Large Pea Gravel | 33 | 261 | 228 | 762 | 961 | 199 | -4 |
| Just Boon mix | 33 | 33 | 0 | 533 | 774 | 241 | 38 |
| Med Pea Gravel repeat | 33 | 235 | 202 | 735 | 972 | 237 | 34 |
| Just Boon mix repeat | 32 | 32 | 0 | 532 | 765 | 233 | 30 |
| Med Pumice repeat | 34 | 129 | 95 | 630 | 842 | 212 | 9 |
| Large Pumice repeat | 34 | 130 | 96 | 630 | 794 | 164 | -39 |
| *Most of the pumice in the Med Boon mix floated while most of the straight pumice sank due to different batches. |
Wow, great idea! Very helpful to know about the aeration layer particle sizes and composition from an actual experiment. Think I will give pea gravel a try. Thanks much.
I was not advocating the use of pea gravel, but showing that it will work. The only place I can see where pea gravel gives is an advantage is when the pot needs to be heavier.
Marty
I just repotted some trees that the previous owner grew in 100% small pumice, what would stay on a 1/16 & 1/8 inch screen combined together with 1/4 in pea gravel, river rocks ( smooth not jagged edges) as the aeration layer.
The roots in the fine pumice where spectacular as expected but in the smooth gravel their where no fine roots at all, just large course roots grew there. The trees were American Larch.
The root growth in the small pumice was mainly small feeder roots, the kind of roots we want.
The roots in the pea gravel were large, course with no fine feeders attached.
I just saw the post and since I was doing this last night in the kitchen I thought I would chime in.
Large pumice and akadama is easy to source. Large lava is difficult to find on the east coast.
As Marty mentioned large pumice may be a good choice even large pumice and akadama should work just fine.
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Okay, thanks much for that info
@MartyWeiser Wow. This was fantastic information. I honestly never considered pea gravel. I’ve been struggling to find affordable pumice, so I’m going to try larger pea gravel for some repots this spring and mark accordingly so I can compare for the future repot.
@CMP your observation about the fine roots in the fine pumice being nice and fine while the roots in the pea gravel being coarse makes sense. The fine roots are growing in a well oxygenated perched water table while the coarse roots are growing in moist air. This is analogous to trees in nature sending out big, heavy roots in search of conditions where they can have lots of good feeder roots. How many times have we been told to fertilize our landscape trees out at the edge of the canopy. A place which gets both rain and sun.
I was thinking about this a bit more today. I think we want a perched water table! A pot with a perched water table has moist to wet soil (or aggregate in our case) above a layer or mist air in the aeration layer. If we have a thin layer of moist air in the aeration layer there will be reasonable gas exchange (both oxygen in and carbon dioxide out) between the aeration layer and the smaller soil above assuming the drain holes are adequate (the diffusion rate of gas molecules is typically 1000 times or more faster in the gas phase than in water). Of course, if we fill the bottom half of a shallow the pot with gravel we don’t leave much good soil for root growth and it can remain too wet if the water table extends to the surface.
I think I have heard that you want slightly coarser soil in shallow pots compared to deeper pots which matches up with the article posted by @Robert. The height of the water table in a coarse soil will be shorter than in a fine soil since the capillaries between the particles are larger and surface tension cannot support as tall a column of water against gravity. In a shallow pot this allow the top of the soil to dry a bit faster. We add moss to the surface to create a moist aeration zone on top the pot to match the one on the bottom. In a deeper pot the finer soil will slow the water drainage from the top of the pot and keep those roots a bit moister. This last paragraph has no direct experimental in bonsai, but makes sense to me based upon other data and theory.
Thanks much. It seems counterintuitive that smaller trees would do well with larger particle sizes, but that doesn’t mean its not true..
Also - I am wondering how much the aeration layer really matters, given that:
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after placing the tree on the mound and pushing it around to get it into place which disturbs the bottom layer
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and chopsticking the tree in at the bottom layer which mixes particles
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and that smaller particles naturally fall between the spaces in the larger particles anyway,
…after all that, how much separation of particle size really remains at the bottom of the pot? And then how much of a perched water table actually remains?
The aeration layer makes sense, but I wonder if anyone has done actual tests w control experiments of real bonsai trees grown with and without aeration layers.
@Chibon Good point about how we actually repot on top of the aeration layer. I might try to simulate that process once other tasks slow down. A controlled study of actual bonsai would probably require the use of 10 - 20 bonsai with the same genetics and general structure. That probably requires starting at least 50 along the development path from cuttings several years before conducting the actual study over 2 or 3 years. I like doing experiments, but that one is a bit longer term than I like.
For the coarser soil in a shallow pot thought I was thinking of the height of the water table in a fair sized maple or similar. In a small shohin or mame pot I think smaller particles are good to hold as much water as possible to minimize how frequently they need to be watered.