by Terry Erasmus April 08, 2017

Background

Let’s get one thing straight at the beginning of this post; there is no such thing as a best growing medium for bonsai! There are growing mediums which are better than others, but the best one for you will be somewhat of a personal thing and will be the medium which best suits:

  1. The species of trees which you are growing, their size and stage of development
  2. Your climate or more specifically the micro climate where you keep and grow your trees.
  3. The type of pots your trees are growing in; glazed or unglazed, wood or plastic
  4. A watering routine which for you is sustainable and practical.

History of growing mediums

You may have noticed that I use the term ‘growing medium’ and not soil. The term, as far as I know, is a modern one as the term ‘soil’ is a more historical term and I believe has certain negative connotations. I remember many years ago, when I began growing bonsai, the ‘soil’ I was encouraged to us was just that. It was generally a mix between compost and some coarse grit. I remember also the type of root development I used to get, long coiling roots with few fine feeder roots. Let’s look at some mixes which I have found in some of the bonsai books in my library for context.

Rudi Adam’s mix

General Mix suitable for most species

10 Litres sieved loam

10 Litres milled bark

15 Litres sieved quarry dust

0.5 Litre milled seaweed

0.5 Litre superphosphate

0.5 Litre bone meal

(Bonsai in South Africa. Rudi Adam. 1992. pg. 26)

John Yoshio Naka’s mix

General deciduous mix for young (4 – 5 years after shaping), large (2 ½ to 4 feet) trees

1 part Soil

1 part Mulch

1 part Sand, large particles (larger than ¼” but smaller than 1/8”)

1 part Sand, medium particles (Smaller than 1/8” but larger than 1/16”)

(Bonsai Techniques 1. John Yoshio Naka. 1973. pg. 90)

Paul Lesniewicz

Standard Mixture

1 Part forest soil or peat

1 Part loam

1 Part coarse, gritty sand

(Bonsai: The Complete Guide to art and technique. Paul Lesniewicz. 1985. pg. 98)

Carl Morrow

General Mix

1 Part coarse river sand, silica sand or fine stone chip (2-4mm)

1 Part good quality compost

(Bonsai Success in Southern Africa. Carl Morrow and Keith Kirsten. 2004. pg. 85)

Now I am in no way suggesting these mixes are bad, outdated or that trees will not grow in them. I am sure the professionals who developed these mixes did so over many years through much trial and error. However, I also believe that bonsai is an ever-evolving art form which is influenced by factors including fashion, technology and others.

There are many professionals today who have continued to study and experiment with growing mediums. New mediums have been developed, discovered or simply brought to our attention. It may therefore be beneficial to us if we heed these new developments and determine whether any may offer us advantages over what we currently use.

The role of a growing medium

I am sure many of you have seen some very trendy ideas around such as bonsai growing in a type of aquaponics system where fish by-products are used to feed the roots of the bonsai, or glass containers in which bonsai are grown in water only, with only a patented nutrient solution added to the liquid to keep the plant growing healthily. You’d be justified in asking why we even need a growing medium in which to grow trees in the first place. The answer is that we don’t really. Plants will grow in pretty much anything providing you satisfy their most basic of needs; nutrients, oxygen and water.

Chinese elm bonsai root system

Image caption. A not so healthy root structure of a Chinese elm as the growing medium used was too water retentive.

You are asking the wrong question if what you are asking is simply “Can I grow bonsai in this or in that?” Or stated differently, “Will it live or die?” The consideration should not be whether you can or cannot, but rather how the tree will respond to what you intend using. It is my opinion that we can grow better bonsai today due to knowledge on the subject being so readily accessible and in no small part because of our greater understanding of growing mediums.

If you look at bonsai exhibited 30 years ago, and compare them to what you can see on any international exhibition today, with a focus on ramification and general health, you will note a dramatic improvement. Of course, this is not exclusively due to the growing medium; these improvements as mentioned above are also related to a general increase in skill. What cannot be refuted though is that you cannot support a highly-ramified tree on a few pipe-like storage roots yet such a root system is precisely what you will encourage through the use of water logged mixes.

Rockspray root ball bonsai tree

Image caption. Much healthier, fibrous root system of a Rockspray bonsai tree due to it being developed in a drier, faster draining mix.

What should a growing medium do? In my opinion it should perform the following:

  1. Provide a stable environment for roots to develop. Young roots are extremely brittle and break easily, this is also why we should always wire our trees into their pots.
  2. It should retain sufficient moisture until the tree can be watered again to prevent complete drying of the roots, and subsequent debilitation of the tree. This must also be sustainable for the individual artist; a person in fulltime employment cannot very well drive home every few hours to water!
  3. In contrast the medium should dry sufficiently between watering’s to prevent the roots from being continuously immersed in liquid or, at a microscopic level, that the gaps between particles are filled with water.
  4. Oxygenation of the medium should be plentiful as this will lead to a healthy root system.
  5. It should rapidly absorb nutrients and in turn slowly release the same to the feeder roots. For example, a tree could theoretically grow in glass marbles but as glass is pretty much impervious you are going to have to water and feed constantly so would it make a good growing medium? I think most of us would agree that the answer would be no.

Categories of growing media

There are two categories of growing medium; organic and inorganic.

Although pretty much self-explanatory, organic mediums are derived from plant or vegetative material such as peat, bark, composted leaves and such, and inorganic refers to a mineral substance of sorts which is processed in some way for instance crushed, baked or exfoliated.

Function of organic components

For these paragraphs I will have to write from a more personal perspective and not necessarily from scientific fact per se. If I am wrong in any of these statement then you are welcome to correct me in the comments below.

In my opinion organic components have historically been used and are still used as we find it hard to believe that plants can survive in anything but organic rich soils. We see trees growing in the ground and we believe that we need to reproduce the same environment in a pot, however we are mistaken in this assumption as container gardening, to use a broad term, is not the same as growing something in the field.

I believe it would be fairly accurate to claim the purpose for utilizing organic material in a growing mix for bonsai is to:

  • Provide nutrients to the tree over a period of time.
  • Retain moisture so the roots do not dry out.
  • Provide structure to the medium, assuming it is sifted of the fines which usually accompany organic mediums.

What we need to realize however is that organic material does not remain in the same state for long. It is by it’s very nature decaying material. Let’s consider some of the changes organic material may experience:

  • Its structure may change from relatively solid to powder or dust
  • Its pH may become can change become increasingly more acidic perhaps
  • Its water retention characteristic may change and it might become increasingly more water retentive, or less so.
  • Its ability to absorb nutrients and release them may change. It has been said that pine bark consumes a lot of nitrogen as it decomposes. I would assume this is not unique to bark but to any decaying material at first. 

Wild olive bonsai tree root development

Image caption. Yamadori {wild collected} Olive root development of 1 year in a coarse mix of organic and inorganic materials.

Functions of inorganic components

Inorganic quite simply refers to material which is not organic in nature and rather is some form of mineral. Such material is usually mined and or processed mechanically to some degree before it is sold and will not decompose, at least not in our lifetime.

Inorganics in a growing medium offer certain attractive advantages over organic materials:

  • Although they generally do not retain much water some can rapidly absorb moisture and just as quickly release it to the roots of a tree.
  • Their structure in a bonsai pot does not change so their ability to provide good aeration in the soil will not diminish over time.
  • These components are usually sterile and will not introduce any fungal diseases which may infect your tree, particularly just after repotting when many roots are exposed to infection.

Very generally speaking and from what I have witnessed to date, most growers will use a combination of organic and inorganic material in their mix, the ratio being dependent on how much and often they can water, the demands of the specific species they are potting, what they can afford and more.

Components detailed

Organic

Compost

With all the kitchen scraps and other garden refuse we produce at our homes why not make your own compost. The advantage of making it yourself over what you are most likely to be able to get at the local garden center is that you can determine what is in it. When you have been to the refuse dump, I'm sure you have seen those large garden refuse processing machines branded with the names of companies that produce various types of mulch, compost, mixes and other? Its fairly obvious then that the refuse being dumped there is what is going into these mixes, including weeds and whatever else - the worst of which I can think of is onion weed bulbs. If you do decide to make your own, bear the following mind:

  • you can pretty much use anything in it except citrus peels and onions.
  • There are many ways to accelerate the decomposition of homemade compost and several products are available on the market for this purpose.
  • Ensure the compost is well decomposed before using it and
  • sift out particles which are too big or too small.

milled bark for bonsai growing mediums

Image caption. Composted bark mixed with LECA produced these roots of a collected wild olive tree rather rapidly.

Bark

Bark is a popular additive for a lot of local enthusiasts as it is fairly easy to get hold of and if you are buying it straight from a saw mill it can be very cheap for quantities you are unlikely to be able to use in your lifetime! Bark is able to retain nutrients and release them later to the roots of your bonsai, which is a good thing of course. One of the problems with bark however is that it utilizes a lot of nitrogen as it decomposes and it will bring your soil pH down making it more acidic. These two factors alone should be sufficient cause for you to second guess whether bark is such a good bonsai growing component. I have used bark quite extensively in the past for many different species of trees but what I have never witnessed are roots growing into the particles, so essentially bark is utilizing space in the container stealing it away from available space for root development.

Mulch and Orchid bark

These forms of bark are generally going to be too large for use in bonsai mixes, unless perhaps you have very large containers in which you are trying to root large collected trees. Should you opt to use this component bear the following in mind:

  • Bark is lifeless - sterile, so any nutrients and micronutrients will need to be added in order to provide them to the developing roots.
  • As mentioned above, as bark decomposes it will leach nitrogen so you will need to add extra to ensure your tree gets some also.
  • Bark alters the pH so if the tree growing in it does not like an acid medium then it is best not to use it.
  • If you do use the mulching type of bark to reduce moisture loss from large developing pots {where you have used a very fast draining medium such as LECA to encourage rapid root development} ensure you do not make the layer too thick or it will reduce oxygen exchange which in turn will reduce root development.

General bonsai growing medium

Image caption. Our General Soil Mix contains 50% composted bark and 50% crushed silica stone.

Composted bark - Seedling mix

Decomposed bark will very dark brown to black in colour. A product which is quite readily available from your local garden center is called Seedling mix. This is essentially composted bark. Usually it will take at least a year or more to get to this point from freshly milled at a sawmill. I have in the past used this product quite a bit in mixes but it is important that before use you sift out the fines which are present and of course particles which are too large {+- >6mm}.

Should you not remove these fines they will settle to the bottom of your container where they may:

  • impede drainage as they fill the gaps between larger particles
  • retain excessive water which may prove to be a problem to the roots in this lower region of the container as it takes longer to dry which means the roots here never dry.
  • reduces aeration which are important to the development of healthy roots.

Decomposed bark is what we use in our General Soil mix as it is economical.

Peat Moss

Decayed, dried sphagnum moss is known as peat moss. It readily absorbs and retains water, unlike coco peat, and it increases the ability of a growing medium to absorb and later, release nutrients to plant roots. It is considered to be a sterile medium, so you need to provide all the nutrients which you believe your bonsai tree will need. Most peat moss is relatively fine fibred which may result in it compacting at the bottom of a bonsai pot. I have successfully used peat for some years now and find it to be a very nice medium to work with.

Peat moss for bonsai trees

Image caption. Our peat moss is high quality and can be combined with the inorganic of your choice.

I also offer two peat based products:

  1. The Professional Mix is a combination of coarse fibred peat moss and LECA in a roughly 80:20 ratio. I have many trees from pines to maples growing in this and I have found the root development to be very good as a lot of fibrous roots are produced.
  2. However, we also offer a peat moss product in 5 and 10L bags which you can mix with other ingredients to product a growing medium tailored to suit.

Professional mix for bonsai trees

Image caption. We also sell a ready-mixed product where LECA and peat have been combined for our Professional Mix.

Coco-peat

You can buy this product in garden centres as a compressed block, and sometimes is sold Coir also. It is a product derived from the coconut husk and is essentially a by-product of an industrial process which has been remarketed to the agricultural market. It needs to be soaked in a bucket of water for some hours to become usable. This should set off the alarms in your head, if you are considering using this in a growing medium for your bonsai. Once it dries, it becomes hydrophobic; meaning that it repels water. This is a big problem in pot cultivation as many times the top level of our growing medium will dry but the lower regions of the container will remain relatively moist.

Coco peat blocks

Image caption. Typical bulk block of coco peat.

When using coco peat, you will find that when you water again the water will simply run over the particles but the particle will resist absorbing it. In my honest opinion, stay away from using this ingredient in your bonsai mix unless it will never be allowed to dry out.

There are also no attractive aspects about coco-peat that I can think of which should make you want to use it in your bonsai mixes other than it may provide some structure to your medium, but if this is the goal then there are better ingredients.

Composted leaves

You might be a little nervous or even embarrassed gathering leaves from the ground in a forest or from the side of the road – I have been as it does attract some attention from passing traffic. Should you feel there is a risk of introducing your bonsai trees to a fungus or some bacterial spore which may do some damage there is this slight possibility but nothing you cannot take steps against should it manifest itself.

Composting leaves

Image caption. Leaves should be well composted before you can use it, and this can take several years depending on leaf type.

Should you be growing pines it is not necessarily such a bad idea to collect rotten pine needles from a pine forest as you will find plenty of mycorrhiza growing in amongst the needles. There is also evidence that composted leaves are a good source of calcium for your bonsai, in particular oak leaves. Should you wish to use composted leaves you should still sift it to remove the large particles as well as those which are too small or fine.

Charcoal

I’m not 100% certain whether charcoal falls into organic or inorganic but seeing as it is a plant product I feel I am pretty safe in inserting it here. This is not intended as a growing medium but it is considered an additive with several benefits. In water treatment carbon is used as a filter which can absorb, in its very fine porous structure, chemicals, minerals and by filtering, buffer the water to an extent. So clearly one of the benefits of using charcoal would be that it will act to buffer your pH.

Charcoal for bonsai trees

Image caption. Wet coco charcoal chips, ideal for use in a bonsai growing medium.

My teacher adds charcoal to many of his mixes, especially for the pines. My understanding is that he does so in part as it can also absorb pollution, a problem when you live in a big city and the tree stays in the same medium for many years where such pollution can accumulate with time. I am not sure but I anticipate that it will also absorb nutrients, although the rate at which it will release it is not known to me. I also do not know just how long it remains ‘active’ because when its matrix is filled it will no longer be able to absorb anything, therefore in water treatment it must be replaced annually or even more frequently. I don’t think it’s a fair comparison however because when used in a pot it is not going to be subjected to such high demands as a filter.

Unfortunately, I cannot give you an accurate measure of how much you need to add to a certain amount of growing medium, as most times we use our hands or a scoop as a measure and simply state; “Use a large handful for a medium container.” This is rather vague but then again, I don’t think you need to be too particular about the ratio. Do bear in mind that charcoal is rather water retentive though {look at charcoal in your braai after some rain and see just how wet it is} so adding too much will increase water retention of your mix.

As far as sources of charcoal go, I guess the most accessible source for you will be to have a wood braai, and smashing some of the charred wood up after it has cooled. If you do not want to go that route then you can buy charcoal from an aquatic supply store as it is used in filters. This charcoal is likely to be superior to what you would have it home as its normally made from coconut shells which is very porous and thus has a high surface area; what you want for maximum absorption.

In-organic Mediums

Stone

Stone features in some part in most widely used bonsai mixes. So I guess it is appropriate that I should begin this section by discussing it first.

Many books refer to decomposed granite, something which although is available in many parts of South Africa, is not available to my knowledge commercially. I assume the benefit to using decomposed granite is as it contains some minerals which it could release into the growing medium. However, I am not certain of this or how important it is, considering as you’re likely to get these trace minerals in any good fertilizer anyway. If you can get it then use it, if not don’t sweat it. However, for most of us, the stone which we can easily get hold of will be that which is used in sand filters.

It is crushed silica stone which has been graded to remove fine sand. I use this type of stone in my very popular General Mix. Stone is pretty much there to give structure to the growing medium, meaning that it helps to prevent the medium from becoming a solid, impenetrable mass. Too much stone on the other hand adds dramatically to the weight of the medium, which is not such a bad thing if you live in a high wind area but it certainly does make moving large trees around more of a back breaking activity.

It’s also important to note that stone has negligible water retention ability (in fact is generally only water droplets trapped on the surface of or between stone surfaces) and roots cannot penetrate the stone particles. This means that you should consider, when adding stone that the volume of stone you add is essentially taken from the available growing space in the container. If your tree is slightly over-potted (growing in a container which is slightly too large) this is not a concern, but when you are growing shohin or mame sized trees every cm3 is important as the pot is already so small.

Be careful to use too small particles, generally 2mm is as small as you want to go. You definitely do not want to use sand as this accumulates at the bottom of the pot and reduces oxygen exchange. If you use larger particles that's also perfectly fine, but just be aware that you will need to water more frequently and that whatever you are using as your organic (if you are combining organic materials with the stone) should be sifted to the same size. Its pointless to use 6mm stone and fine peat, the latter will simply sink to the bottom in time and this is not what you want.

LECA

Leca or light expanded clay aggregates were not originally developed for the agricultural sector but rather for the building or construction sector where it is used in aggregates used for building to make structures lighter, or for insulation to retain heat inside a building. However, it has now been used quite extensively in hydroponics and its variations, where plants are grown in a soil-less medium.

Essentially, LECA is produced by firing clay granules at certain temperatures. Many people confuse LECA aggregates with Akadama, but they are not the same and they behave completely differently. Aside from the fact that contrary to popular belief Akadama is not clay, LECA is. LECA also does not break down with time. LECA will retain its structure and so could potentially be sifted from old mixes and reused, although some disinfection might be advisable. It is available in several sizes and as a whole or crushed particle.

I personally think that as whole, large particles have no place in a bonsai pot, other than perhaps in the drainage level of particularly large or deep pots as a way of reducing weight and also to assist in drying in these lower regions of the container. The most useful grade, and what we use in our Professional Mix, is the mixed crushed and whole particle graded to 2 – 4mm in size. Although it is not much, the crushed particles do absorb more water than the whole particles as their inner, porous structure is exposed.

You can find LECA for sale as a single ingredient which you can mix to your own desired ratio with other mediums.

Pumice

Pumice is a naturally occurring volcanic product. Due to the way it is created in nature it has a soft texture and is rather porous.

Its ability to withhold and release fertilizer slowly is quite high and therefore when used in a growing medium less frequent fertilizing is required. Pumice is also water retentive but drains exceptionally well and therefore it would be difficult to over water and create conditions conducive to root rot.

Roots are not able to penetrate pumice, as is also the case with other media like Leca. However, unlike Leca where the surface of the particle is hard, pumice is soft and based on internet surveys, it becomes apparent that roots like pumice and readily ramify in mixes which contain a fair amount of pumice in it.

"If I had only one soil component to use for bonsai it would be 100% pumice. If you have akadama, add some." - Michael Hagedorn

Pumice can be combined with other elements such as akadama, crushed stone, composted bark or peat.

Due to the very good drainage qualities of pumice it is very often used to assist a sick tree to regain vigour and health. In such a case 100% pumice may be used.

Here are a few recipes from leading international bonsai practitioners:

Michael Hagedorn

80% pumice and 20% composted pine bark for strong roots and early tree development. Pumice provides the widest range of healthy growing conditions. You can over water it, under-water it, and it creates great fine root growth second only to pumice mixed with about 50% akadama.

Boon Manakitivipart

1 part pumice
1 part lava
1 part akadama
1 handful decomposed granite
1 handful horticultural charcoal

Bjorn Bjorholm

Akadama, lava rock, and pumice. Use a 1:1:1 ratio respectively for conifers and a 2:1:1 ratio respectively for deciduous and broadleaf evergreen species.

Peter Tea

2 parts akadama
1 part hyuga (Japanese pumice)
1 part coarse river sand (which is another way of saying decomposed granite)

Akadama

Akadama is a granular volcanic clay-like (although it is not clay) mineral which occurs naturally in Japan in a particular area only. It has been extensively used by Japanese gardeners and bonsai artists for many centuries as a growing medium for bonsai trees and other container-grown plants.

It is mined from the surface of the ground, after which it is graded and packaged. Unlike LECA it is not fired at high temperatures however when it is mined from greater depths it tends to be harder or it might be slightly baked. It has many unique qualities, one of which is that when it is wet it has a darker colour so indicates when watering is needed.

Akadama is unique as a growing medium and is sought after due to its ability for water retention and nutrients due to its moderate CEC value. However, at the same time it provides porosity and free drainage vital to the development of a healthy root system.

There are many outspoken individuals internationally against the use of akadama. They misunderstand the fact that after some time akadama does break down into smaller particles. Why this is a benefit rather than a negative attribute is because it mirrors the kind of growth you want. When you first repot you need a very fast draining mix with plenty of aeration for masses of roots to develop quickly. After a season or so you no longer want the kind of energetic growth which accompanies such root development, and you want growth to be more controlled to produce finer ramification.

Akadama can be combined with other elements such as crushed stone, composted bark, coarse fibred peat, or for best effect use pumice and lava rock or combinations thereof.

Here are a few recipes from leading international bonsai practitioners:

Boon Manakitivipart

1 part pumice
1 part lava
1 part akadama
1 handful decomposed granite
1 handful horticultural charcoal

Bjorn Bjorholm

Akadama, lava rock, and pumice. Use a 1:1:1 ratio respectively for conifers and a 2:1:1 ratio respectively for deciduous and broadleaf evergreen species.

Peter Tea

2 parts akadama
1 part hyuga (Japanese pumice)
1 part coarse river sand (which is another way of saying decomposed granite)

Summary

I should stress that the reader should apply some initiative and perhaps common sense when selecting which medium or mediums to use. Many times in books, magazines or web forums a certain medium is discussed. This medium then becomes vital in the minds of many to the growth of their bonsai trees, and they will go to great lengths in getting it. Let’s use for example decomposed granite. This is a medium which is often mentioned and although I know it is exists in South Africa also, to the best of my knowledge it is not available in the size we need nor can it be purchased commercially. So substitute. My wife does this all the time in her cooking and it seems to work for her! If you can’t find decomposed granite use any other crushed stone of reasonable size and colour (the latter consideration being purely aesthetic). If on the other hand you do want to sneak onto, or you have been granted access to some land where there are granite deposits and you collect some crushed granite that’s great, but I honestly would be very surprised if you will see any difference whatsoever. On the contrary, I am convinced that where granite is used extensively it is because it is freely available for purchase. The same comment applies to lava rock.

There are however mediums which do produce results in our bonsai’s development which are better than others and where no substitute is available, for example akadama and pumice. These two mediums really are the business when it comes to inorganic mediums.

There are no doubt other materials which I have not specifically mentioned in this article, but as I wrote earlier, I am only covering the most useful or best ones that are also commercially available.

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Terry Erasmus
Terry Erasmus

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