Richard’s short guide to the different types of soil that are used to pot up the bonsai trees here at Kew
Having covered the basics of repotting last year, this time round I’ll give an overview of the various different types of soil that I use for the bonsai.
Akadama, or Akadamatsuchi (in Japanese; “Red ball earth”) is the largest constituent of most of the composts that I use at Kew. It is a naturally occurring hard soil that is surface mined in Japan then sifted and bagged. It comes in different qualities or grades, with the harder grades (which come from deeper below the surface) generally costing more. Akadama will usually hold its granular structure for several years before the actions of frost and the growth of a tree's roots break its structure down. Akadama is a good all-round growing medium, as it holds plenty of moisture without getting too waterlogged, and also acts as a good “nutrient buffer” (taking up nutrients and minerals and releasing them slowly).
A 14 litre sack of “Double Red Line” hard grade Akadama bonsai compost
Before I use akadama I pass it through a sieve that has interchangeable screens with varying mesh sizes (a picture of a bonsai sieve can be seen at my previous blog “The Tools of the Trade”). This leaves me with three different grades of akadama: large grains, which are used for larger deciduous trees and most evergreens; medium grains, used for all the other deciduous trees and a few of the smaller conifers; and small grains which are used for dressing the surface of pots and potting up seedlings and cuttings. There is also a small quantity of dust which is discarded.
Akadama bonsai compost, sieved out into three different grades.
Top left is large grain, top right is medium and bottom is the fine grain
Keto, or Keto tsuchi (tsuchi in Japanese simply means soil or earth) is a sticky, usually black or dark brown soil formed from rotting plant material in boggy areas. Its sticky consistency is somewhere between peat and clay which makes it ideal for slab and rock plantings. It can be kneaded and sculpted like clay but does not dry out so readily. If it becomes dry, it will re-absorb water more readily than clay would. I usually try to mix in about 15-20% Akadama and 15-20% pumice before using keto for slab plantings. As far as I know, there is no viable substitute for keto available.
Using keto to build up the first stages of a “retaining wall” for a slab planted bonsai
Using keto to construct a slab-planted bonsai. The loose, large grained soil under the tree's rootball has been enclosed and held in place by a thin shell of keto soil. More about this later!
Pumice is a naturally occurring volcanic rock. Its foamy structure can be up to 90% air so it will often initially float on water. I mainly use it in soil mixes for pines and junipers where I will use between 30% to 50% pumice with the rest of the mix being made up of large grain Akadama.
Pumice, with a pound coin for scale
Kanuma is another naturally occurring volcanic rock, which only seems to be available from Japan. Though similar to pumice it is a lot lighter and softer and can usually be ground to dust between two fingers. Kanuma is most usually recommended for the growing of azaleas and rhododendrons due to its reputation for having a higher acidity than Akadama.
Top left is kyodama, top right is kanuma and bottom is a moler clay
based cat litter. Pound coin in the centre for scale
Kyodama is similar to pumice. Recently I have been adding about 25% to my deciduous soil mixes. I believe it is UK-produced so will have a smaller carbon footprint than imported soils. I also think that I am right in saying that it is a waste product from the glass industry so even more kudos to those of us trying to save the planet! Adding these harder, slightly abrasive materials to soil mixes is reputed to improve the fine root structure of trees.
A 9kg bag of kyodama
Some brands of cat litter are made from a type of “diatomaceous earth” (soil containing fossils of algae or “diatoms”) called Moler clay that is mined in Denmark. Though I have not carried out extensive tests, I have grown some Zelkova bonsai in cat litter for a number of years without the trees showing any ill effects. As you may appreciate, moler clay-based cat litter is significantly cheaper (and more readily available) than akadama soil, though the granules tend to be rather small. Moler clay is used as soil improver for sporting grounds, golf courses, parks, etc under the trade name “Terramol”. There is also another product available named “Biosorb” which appears to have similar characteristics. A form of moler clay is often provided in “spill kits” for mopping up spilled fuel or chemicals.
- Richard -
About Richard Kernick
Richard Kernick is the Bonsai specialist at Kew Gardens. He has worked at Kew since 2004, caring for and improving the bonsai collection while also working part time for the Alpine unit, helping to maintain their collection of woodland plants.
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