Akadama is not available in this country due to VERY strict agriculture regulations! What can be used instead?
www.bonsainz.com/btarticles/soil.html
www.bonsainz.com/
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It is no good reading about Akadama from Japan, or Turface or TerraGreen (USA), as being an ideal growing medium: these are not available in NZ.
With the recent visit from Nobu, it is also apparent that definitions of compounds can vary - after telling us pumice was a good compound to use for our pines, he was surprised at the ease with which we could crush pumice - he thought of pumice as being a much more stable compound than that we have here in NZ, similar to a softer scoria.
Particle size is important: it must be efficient for its purpose, with a balance between good drainage and sufficient moisture retention; not so free as to allow the water to just pour through. Restriction of flow will allow the absorbent soil particles to soak up some of the water, and will help the water to disperse evenly through the soil in the pot.
Mame should probably have smaller overall particle sizes (eg 1-3 mm), larger trees should have larger particle sizes, having a larger pot/mass of soil, and will retain sufficient moisture with the larger particle size (2-6 mm is generally recommended) due to the combination of surface tension and the volume of soil acting as a barrier to water flow.
Soil is more than just drainage, however- it is also there to give stability to the tree (anchoring), and too-large particles will not pack sufficiently to give this stability
Porosity - Some rocks are slightly porous, and help trap moisture and nutrients (in solution). Scoria (in NZ, but referred to as ‘crushed lava’ overseas), pumice and zeolite (sold in stores as cat litter, due to its water-retentive qualities) are the most common examples An all inorganic soil will (given the right texture and particle size), support life. There would be no natural nutrient value and watering would be almost continual in hot summer conditions. Completely organic soils are also possible, but as they decompose and particle sizes reduce, the free-draining quality will also reduce.
Inorganic + Organic = Useful blend
Composted pine bark is quite good as the organic component of soil mix; it will retain moisture, and has a relatively slow decomposition rate, but enough to supply nutrients. A lot of potting mixes in NZ are based on this. Daltons do a good potting mix, with no moisture retention granules, with slow release fertilizer - the Warehouse 'Results' brand is in fact Daltons' make. Mixing this with gravel is sufficient for a lot of people, personally I sift out the fines [anything less than 1mm] to leave only free-draining soil, and then mix with gravel, pumice, zeolite, to give me my 'working' soil.
