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C O L I N   L E W I S   B o n s a i   A r t
B O N S A I   S O I L S   P A R T   1

O R G A N I C   C O M P O N E N T S

First, lets agree on what are the requirements of a bonsai soil recipe. Bonsai soil must:
  • Support the tree in the pot
  • Allow space for roots to grow
  • Provide space for oxygen
  • Drain well
  • Retain moisture
  • Retain nutrients
The degree of importance of each of these functions can vary according to climate and species, but any bonsai soil must satisfy them all.  Now let's get down and dirty....
ORGANIC COMPONENTS As far as I'm aware there is no physical advantage in using any organic matter in your bonsai soil, but there are some disadvantages. After a time any organic matter will break down into fine particles and, if mixed with other more stable ingredients, it will wash down to the base of the pot to form a pan that inhibits drainage. Once bone dry, most organic matter actually repels water and is difficult to re-wet.

Plants of all sorts can be grown in a totally inorganic medium - even gravel, or even pure water - but these require major and minor nutrient supplements as well as the introduction of all manner of microbes to compensate for the unnatural habitat the roots have to deal with.  However, a tree (or any plant) will thrive better and be stronger and more independent if it is growing in a soil that provides a natural environment for the roots as well as all the micro-organisms that make a soil 'alive'. To achieve this a certain amount of organic matter is necessary, but the truth is that a too much organic matter is unnecessary and often unproductive. Too much of the wrong sort can be disastrous!

Happily, most trees have enough old, more natural soil in their core to repopulate the pot with microorganisms as the roots grow and create their own organic rhizosphere (discarded root cuticle, dead root hairs etc). Adding more organic matter may not be always necessary.  This is something you will have to judge, but if you do add organic matter, here are some thoughts to consider.
Bark has a fairly high cationic exchange capacity (the ability to adsorb nutrients for future release) but it is generally very acid and it retains much of the nitrogen to fuel its decomposition, so it's usefulness as a soil component is dubious to begin with. Be honest, if you use bark have you ever, even on one occasion, found roots actually growing into the particles when you come to repot?  And if you think your trees are doing well in a mix of bark and whatever, what are you comparing them with - your past experiences or other peoples' trees that also grow in bark? There's bark and there's bark, and most of it is not very good at all!

Orchid bark: The small, seedling grade (usually fir bark) looks at first sight to be ideal.  But since it's intended for orchid growers, it is sterile - lifeless. The sterile particles of orchid bark don't even begin to decompose for many years. When they do, they will leach nitrogen from the soil and impair the trees vigor.  This is irrelevant, of course, because you will have repotted way before that happens and realized that perhaps you should rethink.

Mulch: If something is commonly used to spread on the flower border to suppress weeds, why on earth would you want to use it in your bonsai growing medium? It suppresses weeds not only by smothering the seedlings, but by creating an extremely inhospitable environment for roots - it is acid, totally nutrient-free, and it also leaches valuable nitrogen from the underlying soil as it slowly decomposes. Nasty!

What's sold as composted bark is usually no more than wood chips dyed to look like composted bark! After a year on the flower border, this is what you're left with. Not very good for bonsai!


Composted bark: Truly decomposed bark, sifted to remove dust and lumps, and used as an ingredient in bonsai soils at no more than twenty percent - okay. However, I have yet to find anything that is marketed as composted bark in the USA that is one hundred percent genuinely decomposed bark.  Dyed lumber waste mixed with a little semi-decomposed bark is about the best you can get, and it's not good enough - not for my trees, anyway.

Composting your own bark: This is the only way you'll get the real thing, but it takes time.  First, the bark must be crushed or chipped. Being nutrient-free, bark requires an external source of nitrogen in order to fuel the micro-organisms that cause decay. It's necessary to layer the bark with weed-free grass clippings, green vegetable waste, potato peel, fresh rinsed seaweed, and/or any other moist, high-nitrogen organic matter. You can even throw in some handfuls (not literally) of horse or cow manure to liven things up. Cover the heap with a few inches of garden soil and wait a couple of years.
On the other hand, after two years happily rotting away with some grass clippings, sea weed and kitchen waste, this mix of oak, pine and maple bark yields about 30% useable particles after sifting. This is the real deal. good bark
PEAT Sphagnum moss peat - or peat moss as it's curiously called here, is the best organic growing medium  for containerized plants known to mankind.  Unfortunately it is  non-renewable resource (once used up it can never be replaced) and these days it's hard to find a product that is coarse enough for our use. Some years ago you could sift out around sixty to seventy percent fines and leave thirty or so percent coarse particles, but not so now. All I have managed to find is very fine and perfect for clogging the drainage in your soil.  If you can find coarser peat in your area, it's worth a try, but sift it well first, and never let it become dry or it will repel water and become difficult to wet. Although peat doesn't contain a whole lot of microbial life, but it is hospitable to it, and it does have the capacity to adsorb nutrients for future release.
COIR Coconut fibers compressed into a block that expands when wetted - absolutely useless!  Coconut fibers are fine to begin with and once dried and compressed they become dust. As with so-called bark mulch, this attempt to capitalize on commercial waste leads to nothing apart from an inferior commodity.
LEAF MOLD Some folks are wary of using ingredients that come from the forest floor, citing the possible presence of pathogens and bugs that can be detrimental to bonsai. True, there is a slight risk of importing the odd nasty from time to time, but if you gather from areas where the plants are healthy and you're vigilant during the few months after repotting, there's no reason not to use leaf mold. Of course it will need to be sifted to remove fines and oversize particles.  Oak, beech, chestnut, buckeye and the like are the most useful since their leaves take longer to break down and provide more particles of useful size.  Pine needles are also good, especially for pines since the associated mycorrhizal fungi spread into the layers of decomposing needles. Easy to find almost anywhere, and the price couldn't be better!

Just about the most readily available and FREE source of organic matter.
Leaf mold is full of goodies like microbes and nutrients, and roots love it!
leaf mold
HOME MADE COMPOST Why not? Home made is usually better than any commercial product because you can control precisely what goes into it - or, to be exact, what does not go into it. No weed seeds, no onion or citrus fruit peel, no persistent roots (dandelion, crab grass, etc) and no un-shredded leaves (they will form a dose layer and will inhibit decomposition. You can use anything green, shredded leaves, seaweed, semi-woody stems, straw, and many other forms of garden waster.  A little soil added with turf roots helps introduce microbes necessary for decomposition, as does the addition of some high nitrogen fertilizer.  Urea is also a good compost accelerator, but I'll leave it to your imagination as to how you apply that!
MANURE Not on your life!  A small amount of well rotted manure added to the mix may help provide temporary nutrients and microbes, but it's very easy to overdo it so great care must be taken. And I repeat: it must be well-rotted, otherwise it can very easily burn roots and even kill your bonsai.  Best avoid it altogether.

Sphagnum moss is the best rooting medium on the planet - if it is fresh.  Unfortunately, it seems that although much of the northern USA is knee deep in sphagnum bogs, the only products available commercially are kiln dried and virtually lifeless bags imported from Canada, Chile and even New Zealand. It's mind-boggling that nobody has grasped the opportunity to capitalize on this easily harvested, highly renewable natural resource.  Dried sphagnum crumbles to dust in next to no time, whereas fresh moss retains its structural integrity for the duration of its use in a pot. If you can get hold of fresh sphagnum, all you need to do is run it through an old meat grinder or chop it into quarter-inch particles and add it to your mix.

I'm lucky, I only have to walk thirty feet from my back door and I can gather handfuls of fresh sphagnum - and I do so regularly. 

It's worth noting that sphagnum moss can be gathered in spring and summer, placed in a closed, clear plastic bag and left outside in the shade where it will live and even slowly grow for up to two years.

In conclusion, it appears that any organic matter available commercially is of little use, but home-made or wild gathered matter can provide ideal organic ingredients for bonsai soils.

Next time we'll take a look at the mineral ingredients for bonsai soils. Meanwhile, grab your bucket and spade and go hunting for suitable organic components ready for next spring.