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Beginner's Guide to Soil
American Bonsai Society
A beginner’s guide to bonsai soil
By Randy Davis
First of all it is important to understand natural environments of your plants -- that is the kind of locales they prefer and the typical soil found there. That information is necessary before you can formulate your bonsai soil. Most starter trees come in nursery containers with “commercial soil,” which is adequate only for short-term growing. A tree destined to become a bonsai and live in a container for an extended time needs special soil. To enhance your partnership with your chosen tree, you need some understanding of its natural habitat. Knowing if your tree originally comes from a high-mountain area; an arid desert-like area; an open meadowland; or a tropical area does make a difference. Another important factor is your local climate and habitat. Understanding what the tree likes and what your environment provides will help guide your choices for soil formulation. A further consideration is your tree’s stage of training. Is it in long-term training, requiring it to grow freely; a sort-term development stage; or just being repotted on its normal cycle? This is important.
Soil components - Soil for the serious bonsai enthusiast will usually consist of three major components; a structural component, a humus component and a clay component. This is quite different from the soils used in the nursery industry and rightfully so, as they have very different objectives. Every Bonsai person has a different soil formulation that suits their particular needs. I have seen some experienced people use nothing but a humus component and sand; and it works for them. A beginner might use that same soil mix only to have everything die. In the end, it is whatever works for you and your growing environment. The most important thing is to keep your bonsai growing strong and healthy. In the ABS care-sheet section we will provide some general guidance to proportions, but you will need to make appropriate adjustments based on your individual situation.
Structural components - The structural portions of your soil will generally be the largest component in your mix. They are inert, rather coarse and provide air space between the particles to allow for good drainage. And they do not decompose quickly if ever. Typical structural components are granite, high-fired clays, ground lava and similar materials. I include coarse sand here also but only because it’s usually a small part in any finished soil mix for those in dryer climates. I personally happen to keep a little of each of these components around so I can fine tune my mix to the specific type of tree I’m repotting.
Granite – Granites are generally available anywhere in the continental United States with the exception of the large urban centers. It is most often found at farm-supply stores and sold as chicken or turkey grit in 50 pound sacks. This granite usually come in three sizes: large, medium and small grain, of which the medium grain is the most popular for general bonsai use. Be careful when you’re buying this stuff and make sure it’s actually granite and not ground limestone. You’ll be sorry if you end up using limestone in your mix as 99% of plants don’t like it in large quantity. I personally find granite works better on pines and junipers than the other kinds of structural components.
High-fired clay – High fired clays have become quite popular and are widely available across the country. This product was developed for golf courses or sports fields as ground conditioners. The ones that I’m familiar with are Turface, from Turface Athletics Inc., Marbet’s Mule Mix from Southern Athletic fields, Inc and Haydite. All of these come in 50 pound sacks at a reasonable price. They can be found in various particle sizes but I like the mid-range size for most applications. Because they are high fired they do not break down over time. These clays and the granites can be reused if you sift and sterilize them before reuse.
Ground lava – Lava is generally available, but not in large quantity and you have to hunt for it. When you can get it, it works well and is high in trace mineral content but you’ll probably have to sift it thoroughly before you use. I don’t use lava as a major part of any mix. I do like its mineral content and will use it along with granite or high-fired clay to make a finished structural mix for trees that need additional trace minerals.
Humus components - Where you live has a great deal to do with what sort of humus will be available to you. The most common ones are pine bark in the Eastern and Central US, redwood or fir bark in the Western US, and peat moss which is available everywhere plants are sold. Humus provides your mix with an organic component that helps in water retention and will hold some amount of fertilizer in it as well.[PM2] For bonsai trees in training, I use a large humus component combined with some sand. This encourages fast growth but you must ensure that there is at least 25% air by volume in the mixture.
Clay components – The clay component of your mix is really nothing more than natural clay. The most widely used is “akadama” clay from Japan and should be found at your local bonsai vendor. I include akadama here and not in the structural components because it is a low fired clay that will break down over time. Just between you and me, I do what I’ve always done for my clay. I dig it out of the ground. Early on in my bonsai training, my boss and bonsai master made his underling (me) go out in the back of the nursery and dig a 3x3x3-foot hole in the ground so we could use the clay from the bottom of it. It is old school, but it works for me. The reason for the hole being three feet deep, by the way, is that at that depth you’re usually out of the zone of clay that has any fungal infection. If you don’t want to dig that deep, you must sterilize your clay before you use it, particularly on anything in the Prunus family. It is the small particles of clay that retain most of the fertilizer that you apply and the plants fine feeder roots grow into it to get their nutrition. I use it in virtually all of my bonsai mixes except training mix.
Using these three basic components, you will be able to make a suitable soil mix for any kind of tree. You need to experiment a bit to learn what is best suited to your particular environment but you’ll eventually come up with one that works best for you. Those in dry climates will want to add more humus. Those in hot, moist climates will want to add more structural material. Paying attention to your trees and fine tuning your mix will pay off for you in the end. Feel free to experiment with other materials, but know exactly why you are using them and, above all, keep track of your results.