Can You Reuse Bonsai Soil?

When a bonsai tree is repotted the old soil is often just thrown away or put in to garden beds.

This is fine, but you could potentially reuse the soil for other bonsai. If the soil has not fully broken down it is perfectly okay to reuse it.

If you want to reuse your bonsai soil you will need to process it, but this doesn’t take much effort and it should also save you some money as well, since you’re not binning it and constantly using fresh soil.

Can You Reuse Every Type Of Bonsai Soil?

Not all bonsai soil is equal and not all soil components breaks down the same way. This means that the individual components in your soil will determine how much of your bonsai soil can be reused. (You can read more about – A General Guide To Bonsai Soil)

A portion of your mix is probably made from non-organic material and these are potentially made from materials that don’t break down. Components like lava rock, turface, perlite or grit don’t break down. They will pretty much last forever and can be used over and over again.

Other components like pine bark or akadama will break down over time. However, they might not be fully broken down when you repot your tree and could still in good shape, so there is a possibility they can be reused. You will have to access this on case by case basis.

You will have to study each of your soil components over time and try to work out how long they will typically last in your garden. You just have to use your common sense here and over time you will get a better feel for how the soil breaks down and how long it might last.

You have to do this yourself as you may find that something that lasts 3 years in my garden …might only last 2 years in yours.

It all depends on the quality of the soil component, the weather conditions (the amount they dry out, get wet and even freeze all affects how quickly they break down) and the species you have had planted in it. (A tree with crazy aggressive roots will break down soil faster than a slow growing tree) so its hard for me to say X component will last X amount of time.

Reuse Soil Sensibly

If you have soil that is partially broken down, you will also need to evaluate what you will be using the soil for next and plan ahead to make sure you use it sensibly.

This is also not an exact science as you can never really be sure how long soil may last, but you can usually give make a good guess.

For example: If you have some akadama that is a year old and you think it can last roughly 2 years in your garden (meaning it’s probably got another year left before it is too broken down). 

You need to make sure you use it with a tree you know is only going to be in a pot for 1 year.

If you decide to use it with a tree you know is not going to be repotted for 3 years, you’re just setting yourself up disaster as the soil is going to be too far broken down in only 1 year.

Just use your common sense and you should be fine.

How I Process My Old Soil, So That It Can Be Reused

When I repot my trees I take all the old bonsai soil and dump it in a large container.

I then just leave it there and let it dry out. There is no rush as this will not get used until next year. (Sometimes I put it in the sun to speed this up over summer)

This old bonsai soil is a mess. It’s full of roots and moss and also really dirty, but that’s fine. Just let it all dry out.  

Once its bone dry it’s relatively easy to remove any big bits of roots. Just pick them out and throw them away. Don’t worry too much about the little bit of roots that are still left.

I then sift the soil. It is going to have a lot of fine particles and dusty like material, so you are going to lose a lot of it, but that’s totally normally. This is basically a lot of the broken down material so its good to get rid of it.

Once you have sifted everything out, you should be left with good usable soil.

If you are happy with the particle size, then you’re good to go.  If you feel it’s still got too much fine material, then sieve it again through a different size mesh and you should be able to remove anything you are not happy with.

You should basically end up with something that looks pretty similar to your fresh soil.

What I End Up With After Processing My Soil

My bonsai soil mix is made up of molar clay, perlite and compost.

The molar clay and perlite will not break down. However, they can get a damaged and little bits will chip off.

I find sifting the soil will leave me with just good size particles of molar and perlite that I can reuse.

Nearly all the little chips and the compost will pass through the sieve. Obviously the soil may still be covered in a fine dusting of compost that has dried on. However, I’m not worried about this. It’s such a low percentage that it will not affect anything.

I pretty much just end up with a mix of just perlite and molar clay. I then just look at it and move it about with my hands to get a feel for it. 

I ask myself questions like “is it a 50/50 mix?”  …“Is there more perlite?”  …“Is there still too much compost or fine particles?”

Once I’ve got a rough idea of what it is made up of, I then correct it as needs be. I can add more fresh components to make the mix be the ratio that I want it to be. 

As I am usually making a large batch, I find I will not have enough from just the reused bonsai soil, so I have to top it up with some fresh components to get the amount I need.

I then just repeat this every year, topping up with fresh soil where its needed.

When Not To Reuse Bonsai Soil

Firstly, you shouldn’t reuse your bonsai soil again if it has broken down too much. It will be obvious if it can’t be used again it when compared to fresh soil.

Bonsai soil that is reusable should look and feel pretty similar to fresh soil, just maybe a bit dirty or discoloured.

If it has become mush, or just too fine and only really small particles, this is the point where you should be putting it in the garden beds.

Secondly, if your tree has died for an unknown reason or it has had some sort of fungal disease, do not reuse the bonsai soil.

There is a chance that whatever killed your tree is living in the soil and its best to just get rid of it in case it infects another tree. 

Should You Cook Old Bonsai Soil?

Some people suggest that you should cook your soil, either in the oven, or on a BBQ. This is to sterilise the soil so you can reuse it. If you want to do this, go ahead, but I certainly wouldn’t recommend it.

For me, a sterile soil is a dead soil. The microorganisms found in the soil actually help your tree. If you have already had this soil in your garden for a while it is going to be full of beneficial local microorganisms. I feel leaving these in the soil is just sensible.

If you look at the repotting advice that is often given, you will hear “don’t wash your roots”… “ don’t remove all the soil” or even “add back in some of the old soil” this is all based on the fact that you don’t want to remove the bacteria from the pot. This advice is actively trying to get you to keep it in there.     

It also seems like a total waste of time, you are cleaning it for it to get immediately dirty again. I’d imagine that within minutes of you cooking it, bacteria in the air is just going to land on it again and start growing.  So why waste your time.

“The Bonsai Soil Mix Won’t Be Exactly What You Want”

As I explained above, I look at my soil and physically get my hands in to it to evaluate it. I am never going to get 100% accuracy on what my mix is, but I can get a good feel for what it is. I can add extra fresh components until the mix looks right to me and is what I want.

Although knowing and understanding your mix is important, I think that if you are measuring it out exactly to the gram, that is a bit too much. You don’t need to be this strict and precise.

You can just add your components free hand and still say you have “equal parts”, as long as it feels and looks right, it will be fine. It doesn’t take you much experience to really get a feel for your soil and you will find it easy to mix it up free hand.

Conclusion

Reusing your bonsai soil is really easy and really simple. If it’s still in a good condition, just use it again. If it’s a mess, bin it.

Nothing bad will happen if you use old soil that is still in good shape. If anything it’s actually probably better as it is full of good bacteria.

If you then mix it in with fresh soil, you won’t even see any difference, it all just becomes one big pile of good quality soil and you didn’t have to buy twice as much.