Bonsai soil is often a discussion topic and controversy and it is so also in our local bonsai society. My knowledge and experience about the subject is low to brag anything about it. Nevertheless due to one of the discussions I decided to do an experiment.
It is found on many different places that on the bottom of bonsai pot should be put a more coarse akadama (or whatever soil is in use) due to water column which is to be shorter. In one of the books though it was mentioned (can’t remember the book’s name) that there is no use to do that as already small space in the pot is even more reduced for useful fine roots because the coarser the soil the less fine roots can find space and thus the water consumption is reduced. Sounds logical.
I wanted to know, how much the water column really depends on the soil grain coarseness and what height is the water column.
I decided to simulate a bonsai pot with a transparent plastic fruit “box” into which I would put different cat litter grains and will check what water column it produces but first I wanted to know how much water each grain keeps in.
I measured 200 ml of 2-3 mm cat litter, 3-5 mm and >8 mm (should be 8-11mm).
Then I put it into 250 ml of water to soak properly:
and after the free dripping of water back I would have the difference of water levels to conclude what amount of water remained inside the grains:
The photo shows that water was lower around 75 ml. So, I can conclude that cat litter keeps water equal to its ~35% of volume.
The test showed same values for all grains which is logical. The difference of volume space of each grain sizes doesn’t differ that much.
This test doesn’t bring me much value for the next test nor for the question I asked above but I wanted to know.
Now is the time to check water columns. I created the simulated transparent bonsai pot with screen on the bottom which I filled with different grain sizes. I watered the “pot” thoroughly and let the water leak out freely without any pressure. When the dripping stopped I made a photo of the end result. After that I would put the pot under angle, let it drip and make a photo of the end situation too.
Notice: Measuring is not extremely accurate but I rather wanted to know comparison between grain sizes than having perfect measuring “devices”.
Grain size 2-3 mm
In the second photo is visible that the water is visible even above half of the depth of the grains. Water column was around 2-3 cm.
Setting the “pot” under angle didn’t reduce water column much. Yes, the water did drip away to some extent but the water column still remained even on the side which is higher.
Grain size 3-5 mm
Expected result, water column was lower 2-3 times, around 1 cm for this grain size.
Setting “pot” under angle here did make the difference, the water column remained only in the lower part of the pot, in the corner, again, around 1 cm.
Grain size >8 mm
The photos don’t reveal much to be honest but I did not find any water column. After setting the “pot” to the side some small amount of water did leak out but nothing so substantial like in 2-3 mm gran size test.
After all of this, it is visible that the coarser grain does have lower water column. Not a big deal, that was expected. It looks that after 5 mm the water column is almost non-existent. The small particle grains of 2-3 mm have quite a high water column, around 2 cm. For bonsai pot of 5-8 cm that is pretty much.
Now came the moment to try the “soil” with mixed grains. Coarser to the bottom and finer to the top. I put one layer of >8 mm cat litter grains (so it is around 8-10 mm on the bottom) and then finest I have, 2-3 mm over it.
I repeated this test several times. I never managed to see any water column!
What value the final conclusion can bring is questionable 🙂
Nevertheless, I will do it for myself.
My experiments showed that coarser grain to the bottom does remove water column. I didn’t expect it to be removed so substantially but it did. One layer of very coarse grain and the water didn’t stay anymore. That gives at least to me trust into the advice to keep coarser grains on the bottom.
Now, that it reduces the space for finer roots it does. Nevertheless, the lack of water column in my opinion (I repeat this is only my opinion) means to me more than slightly reduced space for fine roots. That can also mean that I just don’t know the subject well and/or I don’t maintain bonsai’s well and I can agree with that 😉
Perhaps this experiment is lacking a mixture of different soil types (like pumice for an example) test. I avoided doing such a “fine” experiment because there are so many different mixtures that it wouldn’t make much difference. I assumed that cat litter, as the most retention-wise soil type, is the most important for keeping the water column.
The water columns is existent because the bonsai pot has the bottom and only holes on some places. If the bottom would be the full screen mesh the water would leak out freely, I guess. BUT, then the pot wouldn’t be a pot anymore wouldn’t it 🙂