Often when buying a bonsai tree it arrives in clay-based soil that overtime can become quite compacted. An essential method for bonsai tree care is to repot your plant into a new mix of soil.
The three key aspects to consider to create the best soil for your bonsai are:
- Water retention
- Air Spaces
So lets dive right into these aspects:
Firstly, drainage is very important as we’ve discussed when it comes to watering your bonsai. Bonsai trees are more likely to suffer from over-watering than dehydration and a key part to this is making sure your soil allows an adequate amount of drainage to prevent roots from decaying.
A soil must be very free draining, so much that when watering your bonsai the water will not just sit on the surface. It must drain straight through the soil and start flowing out of the drainage holes after at least half a minute.
While soil must allow water to pass through, it must also be able to hold a certain amount of water to provide enough moisture until the bonsai tree is next hydrated.
Typically a garden mix soil will largely contain fine particles which over time becomes compacted, this can be prevented by using larger particles whether it be organic or mineral to help retain moisture. We will get into this more in a moment.
Lastly, allowing your bonsai trees roots to breathe will help maintain their healthiness. When a soil is compacted this becomes an issue and roots will struggle to take in their required nutrients that requires oxygen to do so.
So…what soil is best?
A japanese soil called ‘Akadama’ is the most well known and sought after bonsai soil mix on the market. It retains water well but also allows sufficient drainage and ample air spaces. It is a hard, coarse, clay-like soil that has a granular structure with particles ranging from 1-6mm (up to 1/4 inch).
To get the best out of your bonsai it is best to use Akadama or to mix your own soil to control the ingredients you size and the particle sizes.
Mixing your own soil
Repotting a bonsai into your own soil mix is good practice when caring for your bonsai as they often become compacted in the original soil when buying a bonsai. As mentioned above, mixing your own soil is a good alternative to using the slightly expensive Akadama soil.
The main two ingredients are: organic matter e.g. peat, leaf mould, composted bark and grit e.g. coarse sand. Water retention comes from the organic matter, which should not be too decomposed so that it crumbles into dust easily when dry, instead it should be moist and when squeezed tightly it will spring apart when pressure is eased.
Lumps larger than 5mm (1/4 inch) should be broken up or discarded during the sifting process, as well as particles smaller than 2mm which would clog up the base of the pot. This is where a soil sieve will come in handy. The matter must be sieved when damp, not wet which will clog the sieve, and not dry where particles will be wasted by crumbling into dust.
To keep your new soil mix breathable and free-draining, adding grit that has been sieved for particles sized between 2-5mm (up to 1/4 inch) depending on your pot size is a necessity. One important note is to only use prepared horticultural grit or sand, not builders or beach sand.
River sand is the most preferred type of grit due to it’s rounded grains that prevents it from fitting together and in turn does not become tightly compacted. Crushed granite or flint can also do the job and is more easily sourced, however their grains have flat sides. This can be countered by using a more whole organic matter such as composted bark rather than the leafs or peat.
Now to mix the organic material with your grit. Combine the two in roughly equal proportions while the organic mater is still slightly damp for similar reasons as the sieving process. If your bonsai tree is more adapted to drier conditions then it is safe to add more coarse grit, however if the opposite is the case then it is best to avoid adding more organic matter. Instead reducing the aggregate size of the grit to around 1-3mm (1/8 inch) or replace some of the grit with a proprietary granular soil conditioner. Adding a soil improver such as calcined clay or crushed pumice helps to keep your soil both absorbent and free-draining for a longer period.