Updated: Jan 7
Often in Bonsai you will hear people talking about sieving Bonsai Soils and separating particle sizes, But why do we do this and why is it actually very important? Lets find out.
First of all lets talk about the tool we use to achieve this technique
A Bonsai Soil Sieve is like most other sieves except they are usually stainless steel rather then plastic and they have interchangeable screens for different particle sizes. You can also get them in different sizes, the ones we sell at Bonsai-En are 210mm Soil Sieve , 300mm Soil Sieve and a 370mm Soil Sieve . All 3 sieves come with interchangeable screens in common particle sizes for Bonsai.
Ok so now what about uses? I'm going to split this up into 2 sections, The first will be organic soils for development and the second will be in organic soils for refinement.
Organic Soils For Development
When we are using organic soils for development although we want a much more dense mix then we otherwise would use in refinement we still don't want this mix getting to dense, after all one of the major factors that needs to be present for good root growth and development is oxygen. If we have a mix that is too dense we can easily end up holding too much moisture and minimising the amount of oxygen that is available to the root system. There are 2 ways we can combat this issue, 1 is to sieve the soil before using it, and 2 is if we add an inorganic aggregate to the mix to help open it up some. I actually recommend doing both. Personally i sieve my organic mix then add some perlite to help it breathe.
When we talk about sieving the soil what are we actually doing and what is the goal? We are usually using a 1mm - 1.5mm screen to remove the finer dustier particles from the mix. These are the particles that will turn to a thick mud if mixed with water, These fine particles can find their way to the drainage holes and clog the holes preventing drainage, they can also cause the mix to become compacted quickly which will cause the soil to struggle to take on water. when an organic mix is wet you want it so that if you pick it up it will crumble apart very easily. You also want the mix to percolate well so when you water your tree you want to see that water moving straight through the mix and not just sitting on top of the surface struggling to move through. Now most mixes will actually percolate well for the first few weeks of watering but the more of that fine material that was left in there the quicker the mix will compact and make it harder and harder to get the root ball completely soaked.
If you are going to use perlite in your mix i also suggest you sieve this before using it. Select a screen that is small enough to keep all the particles in the sieve but to separate out the dust particles. Perlite is a very dusty medium and if you allow that fine dust to make it into the pot then you will have the same issues we have just spoke about above.
In Organic Soils For Refinement
Now we are getting into a different area of sieving soils for Bonsai. Although we do sieve our In organic mediums such as pumice and akadama to get rid of fine dusts for the same reasons spoken about in the last section to prevent fine materials clogging drainage holes and causing compaction, we also need to sieve in organic mediums into different particle sizes so we can now layer our mixes in the bonsai pot.
Generally when creating a bonsai soil mix we have 3 layers in our mix, We have the Drainage Layer, The Body Mix and also the Surface Layer. All 3 of these layers have a different job and work in tandem with each other. Usually when you buy a bag of in organic medium it wont be in a single particle size, it will be in a bag of say 2-4mm or 2-6mm or 6-12mm just for example. The size of the tree we are Re-Potting will determine what size particle we use and in what layer we use that size, the other contributing factor to particle size is how much moisture we need our mix to hold for the particular species we are working with. A finer particle size will hold more moisture then a larger particle, so if you want to hold more moisture you may use a smaller particle size through out each of you layers. So first lets take a look at the 3 layers and what they do.
Drainage Layer : This is a larger particle size layer to help any water that may pool in the bottom of the pot to easily escape, It also allows a higher amount of oxygen to enter the bottom of the Bonsai Pot so that your root system can utilise that oxygen. As you can see this layer favours oxygen over water but only makes up a very small portion of the mix.
Body Layer : This is where the party happens, Most of your root ball will occupy the body layer, You want a good balance between water and oxygen here so you would use a medium size particle in relevance to the size of your tree and pot. like mentioned earlier if you have a tree that favours a slightly more moist environment then you can make the particle size slightly smaller which will increase the moisture holding capacity of the mix.
Surface Layer : On the surface layer we usually use a finer particle size as this will help hold a lot of moisture on the top layer of the soil mix. This will help stop the tree from drying out quick in windy or hot conditions. Having a fine particle layer on the top surface will also help you grow moss directly to the surface of the soil. Having a layer of moss will help roots grow into the top layers of the soil where they wouldn't usually grow without it. Roots want to avoid too much light and air. If you have the top layer of your mix open then roots will avoid that area. Having a top dressing such as moss will help keep the top layer dark and moist meaning you can utilise that extra layer of soil for root growth resulting in a healthier tree.
So now that we know what each layers job is how do we choose a particle size for each layer? In simple terms you would look at it like this, Large on the bottom to favour oxygen, medium in the middle for a balance of water and oxygen and fine on the top for moisture retention. As mentioned before the particle size is relative to the size of your tree. Obviously for a big tree a large particle size might be a 8-12mm particle. but for a Medium size tree that might be a 5-6mm particle. but for example in a shohin tree you might be looking at 2-4mm for your body mix so your drainage layer might be a 6mm particle where in a larger tree the drainage layer might be a 12mm. In a Chuhin size bonsai and up the average body size would be 4-6mm so the drainage would be 6mm+ as the tree and pot get larger.. If you have a massive tree i wouldn't use anything over a 6mm in the body, we have to stop scaling the body mix at a certain point even though the tree is getting larger, we still need that balance of water and oxygen and over 6mm we might start introducing too much oxygen in the body. But at least you have a rough gauge for starting out.
so as you can see Sieving Bonsai Soil is an important part to creating bonsai soil mixes no matter if you are in refinement or development. I'm hoping you now are equipped with the proper mindset when creating your mix's to layer properly and to know what particle sizes to use and avoid having finer material in your mix. Obviously this article didn't speak about what mediums to use in a mix but this is not the right article for that, this was just about the technique of using a Soil Sieve on what purpose it might have in our bonsai work.
Until Next Time, Enjoy Your Bonsai Journey.