Bonsai soil mix, or how to get a black eye at a bonsai convention

I want to talk about bonsai soil. Yes, yes, I know. Talking about bonsai soil is very contentious. I would not be surprised if fist fights have broken out over this very topic. Why so much contention? Everybody has their own recipe for mixing soil for use in bonsai.  Why does everybody have their own recipe? Well, because everybody lives in micro climates, everybody has different species of bonsai at different development different age, and everybody wants to satisfy their own taste. I’ll use an analogy. Come election year you get all kinds of political parties coming out of the woodwork. Yes In the U.S. we have two main parties. However, they are not the only ones on the ballots. It’s the same with soil. Some people prefer some mixes over another and vice versa. There are some media that are more popular than others. Also, there are some media that just a handful of people use.  Time for my disclaimer: I do not endorse any one type of medium nor soil mixture.

Camelia in substrate National Bonsai and Penjing Museum

Ok all, let's get down to the “nitty gritty”. Bonsai need soil because they are plants. Plants use soil to grow roots. Roots absorb water and minerals from the soil. Not to mention that the roots in the soil serve as an anchor. It seems simple enough right? Then why so much contention? Well, imagine all the different places, climates, and plants in the world. Some plants do well in many of these places. Some plants can only be found in very specific places. An Example is the Torrey pine of California. This super slow growing pine can only be found in two very small places in the whole world. Check out this article: https://en.wikipedia.org/wiki/Torrey_pine. Each different species has different needs. because bonsai live in pots some of these species make their way to places that they normally would not thrive in. For instance on a popular bonsai web sight i read about ficus in Denmark. The ficus are kept in indoor grow rooms or boxes with lights and what nots. These ficus would not be able to survive without this.This is an extreme but it does happen. Because these species are out of place the soil mixture has to compensate for the different environment.  

One of the biggest factors in determining soil mixture is water retention. Some plants love water some don't. However, as a general rule most plants will die in soil that is saturated in water. That's how office plants die from overwatering. On the other hand, I was living in Pomona CA. It is not a desert but it is next to one. To top it off it is in a valley. Being that it is in southern california rainfall is minimal at best. I made a potting mix that retained very little water. I lost a few plants last summer because of it.

Tangent. I am going dark on the soil mix for one second. I want to say that all those plants that are drought tolerant and do good with minimum water are not. You heard me, they are not. Well, they are but only after being established. What does that mean? It means that while it is in a pot you better not let the dirt dry out. If you do the plant is a goner. For the plant to be established it has to have been planted, then they have to be watered regularly from six to twelve months. Then and only then can you ignore it. You are welcome.

Ok, I am back. Here is where i might make people uncomfortable. It is ok to use one hundred percent potting soil on your bonsai. I said it, if I had many readers most would be scrolling to the comments to tell me why i am wrong. Their is a qualifier to what I just said. It is ok to use one hundred percent potting soil on your bonsai if that is what your tree needs. I have heard of some bonsai in Japan that are kept in one hundred percent sand because that is what those trees need. If you keep a tree in sand only will you have to water it more often? If it doesn't rain a lot, probably yes. Or, who knows what logic the tree owner used to come to the only sand conclusion, but that is what works for their tree in their situation. Would I do that? If I feel that it is what is best for the tree, heck yes.

That leads me to where we are now. What should I put in my mix? I am not embarrassed to say I do not know. I am ignorant. I know what works for me and my trees. I haven't a clue about what works for you. People can make suggestions, but it is up to you to find out. Oh, by the way, don't be scared to make mistakes. do what you have to do. If it works great then great. If it fails, well now you know what you have to change.  How will you be able to refine your tree if you do not take a risk. Believe me keeping a tree in a pot is a risk in itself. Cutting roots, pruning branches, deadwood these are all risks. You are basically damaging your tree to shape it how you want. Yet, perhaps you should experiment on less valuable plants.  

Black pine growing in sand and Turface.

When you plan your soil mix keep in mind that one of the important points is  moisture retention. A bald cypress can have lots of moisture in the soil. A Mugo pine needs quick draining soil. They both need different amounts of water retention.

Soil can be broken up into two categories. They are organic and inorganic substrates. Organic does not mean that it was raised humanely. It means that is is material that is not mineral that it came from something living. Inorganic is the opposite. Is plastic organic or inorganic? Great question. Most plastics are a blend of both.

The low down on inorganic. I think their are less variables with inorganic substrates. inorganics for the most part retain no water, or very little water. If it does retain some water it does not lose its structure and become slush. For the most part water will cling on the surface but most of it will drain out. The finer the particles the more surface area the more water retention. Sand by volume will hold more water than peas sized pebbles. Lava rock the same size as peas will retain more water than pebbles because of the porous surface. Then there are baked clays like Akadama and Turface. These have micro pores that retain some water but are not as wet as organic substrates. Some people use decomposed granite some other people use perlite.

Organics, they were once alive. Organic things were made to hold water. Think about it. every living thing holds water. Organics for the most part are broken down. Depending on what you need it for will determine how far along the material has been broken down. Orchids like bark chips. I also use bark chips to root ficus cuttings. If wood chips are the beginning of the process to break matter down then humus is the end. Humus is a black muck. Humus is rich in nitrogen. Regarding humus there is debate about what humus is and how its made, so ill stop right here. I break down organic substrates into two categories. The first is organic matter that breaks down fast. Things like leaves, grass, and chopped up garden waste I consider break down fast. The other is organic matter that breaks down very slow. These include coconut husk, peat moss, pine needles. I use bark potting soil for orchids as a slow organic material.

Soil mix for Moreton bay fig

Let me present you with the challenge of using organic soils for bonsai. Depending on the species of tree you will have to repot it from one to three years. Some trees sooner and some trees later. any nutrients in the soil will be absorbed fairly quickly. To replenish nutrients you will use fertilizers in there many forms according to the trees need, of course. So soil is basically just for retaining water. I don't know if you have ever seen an extremely root bound plant? If you have not I will tell you. When you pull the plant out of the pot you see a mass of roots that have grown in a circular pattern following the inside of the pot all in one direction. Moreover, all the dirt disappears. It is all replaced by roots. When I root ficus cuttings I add a bark only soil. some of it is chopped up some are big chunks. after the ficus has rooted I notice that the big chunks of bark have been broken down by the roots. Remember that the finer the soil is the more water it retains.

What does it all mean? It means that you can talk about it all day, but you are going to have to put it to the test. People might have success in your area with specific blends, but until it's tried by you it's not true. There is a lot that I need to learn about bonsai soil. There is a lot that is debateable. I present this as a neophyte. Cheers.

Next post I will experiment with water retention using different substrate.