Soil Confusion!

[Bonsai Nut]

Over the years I have learned a number of things regarding soil mixes and repotting.

First, though many soil mixes can work in the short term, soil mixes with a lot of organic components break down quickly and tend to hold too much water in a container. I will often use a mix with sifted pine bark fines and aggregate for trees that are in development in pond baskets or Anderson flats (mesh bottom), but if a tree is in a bonsai pot it is ALWAYS in a 100% inorganic mix (in my garden here in NC) due to the rainfall we get. I have kept trees in 100% pumice with good results, and now use a 50/50 pumice/akadama mix here in NC. I use the hard akadama because it maintains moisture without breaking down. If I could find lava, I would probably use it, but it simply isn't available here in bulk. I can use expanded shale or expanded slate here to replace lava if I thought it was important, but I don't really think it is.

Second, the timing and method of repotting is just as important as soil selection - particularly if you are working with trees that are coming out of an organic soil (trees from nature, or trees from a landscape nursery). You have to ensure you remove ALL the soil when you repot - particularly the oldest soil that is in the center of the rootball directly below the trunk. It is hard to do, particularly if you have a tree that has been slip-potted three times, and it is easy to convince yourself that you got "most" of the old soil out and "most" is good enough. It is not. Old soil in the center of a rootball and an organic soil mix is a recipe for root problems / fungus. A clean rootball and inorganic soil makes for a happy, healthy tree and easy watering.

Once a tree is established in inorganic soil, repotting is a breeze. Repotting a tree that has become rootbound in an organic soil mix is a pain in the ass - and more stressful for the tree than it needs to be.

 

[Gabler]

You have to ensure you remove ALL the soil when you repot - particularly the oldest soil that is in the center of the rootball directly below the trunk.

I've heard that we should not be removing soil from the center of the rootball below the trunk, particularly the akadama beneath old pine trees. Why might I have heard that advice?

 

[Goodbrake]

The ants aren't in my trees. They're in a pot where I dumped some extra DE. The product is "Floor Dry," NAPA part no. 8822. It's specifically labelled on the bag as "diatomaceous earth oil absorbent."

I've used this stuff before and some of my trees are still in it, but I've found that it holds a ton of water, which isn't always a good thing. How has your experience been with it?

 

[Orion_metalhead]

I use it as a component and not on its own anymore. I would never use it with conifers.

 

[Gabler]

I likewise use it as a component of an aggregate in order to increase water holding capacity. It does a good job absorbing water, and unlike Turface, it gives the water back to the tree.

 

[Wood]

I've heard that we should not be removing soil from the center of the rootball below the trunk, particularly the akadama beneath old pine trees. Why might I have heard that advice?

I think the context of organic/field soil is important to what @Bonsai Nut is saying here. Organic and field soil creates moisture and fungus problems, and leaving those in a rootball is problematic. Leaving good inorganic soil in a rootball is not.

I've helped out with a couple repots at the national arboretum that have involved excavating old soil like surface or Brussels' aggregate from the core, but leaving a healthy amount of good soil elsewhere untouched. If the center of the rootball is in good condition and good soil, no need to change it

 

[Orion_metalhead]

unlike Turface, it gives the water back to the tree.

Can you expand on this?

 

[Goodbrake]

Can you expand on this?

I've heard this too, from Ryan Neil's lecture on soil. Apparently Turface absorbs water, but it just holds onto it and the plant's roots can't absorb it, so it does no good. Other professionals like Michael Hagedorn report poor root health in trees planted in Turface.

 

[Bonsai Nut]

I've heard that we should not be removing soil from the center of the rootball below the trunk, particularly the akadama beneath old pine trees. Why might I have heard that advice?

You may have misunderstood what you heard. It is not recommended to remove all the soil under an old conifer AT ONCE because there are often beneficial microflora and microfauna in the soil. So instead of doing a complete repot and soil replacement every four years, you repot half the tree every two years. However you still swap out all the old soil - you just do it in stages. The same can be said for trees in organic soil with very fine hair-like roots (for example azaleas). The roots are often so fine and so embedded in the soil that you cannot tease the soil out without breaking/removing the fine roots. So it is often recommended to use the "pie" method of removing pie slices of soil every year, gradually working your way around the tree so that after a period of time (say three years) you have replaced all the old soil. But again - you need to get rid of ALL of that old organic soil. Once an azalea is well-established in bonsai soil, it is much less of an issue.

 

[Gabler]

Can you expand on this?

I have not personally tested Turface, but professionals report that Turface is so good at holding water, the trees can't get it out of the particle. I have personally tested 8822, and even when it looks dry, trees are not prone to wilting. I infer that it is better at supplying moisture than Turface.

 

[Goodbrake]

Here are Hagedorn's articles on Turface.

Life Without Turface…

…is full of sweet birdsong and rainbows. I feel the need to comment on Turface, which is still, unfortunately, a common bonsai soil component. I used Turface and Oil-Dri for years before star…

crataegus.com

Further Thoughts on Turface

A couple years back I wrote a post which became one of most viewed posts I’ve ever written. It was about Turface and similar soil ingredients like Oil-Dri and Profile, and my skepticism about…

crataegus.com