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 Warren Hill, curator of the national bonsai collection in Washington D.C., believes that most commercial bonsai mixes are too coarse, causing the upper portion of the container soil to dry out too rapidly. He also emphasizes that the total dissolved salt value (TDS) is very important to bonsai, since too high a TDS will require more osmotic pressure than the roots can muster to pump water to the top of the plant. Most critical to the TDS total is sodium ions. A plant requires no sodium, therefore if sodium is present, it is only making the root’s job of taking up water more difficult. Thus when picking soil components, Mr. Hill advises to select those components that do not preferably hold sodium, and not to select those that already contain sodium. Recommended soil components include peat moss, preferably Canadian peat moss which is low in sodium and slow to decompose in low sodium concentration conditions. Mr. Hill recommends against clay components (as well as most organics) since these are anionic in nature (negatively charged) and will adsorb sodium out of the water, in turn
increasing the osmotic gradient and thus impeding water movement. Clay soil components which Mr. Hill avoids include turface, akadama and haydite - these would only be acceptable if you have very low sodium water and use organic fertilizers since inorganic fertilizers also typically contain high sodium levels. Other sodium sources to avoid include fir bark (fir tree trunks are often felled and floated to destinations in natural waters allowing them to accumulate sodium during their travel. Deionized water is helpful in avoiding more added sodium but water softeners should be avoided since they exchange calcium, magnesium and phosphorus in the water for sodium - making the problem worse. Gypsum (calcium sulfate) can be added to high sodium situations since the calcium ion displaces the sodium ions which are then washed away. Mr. Hill also states that a good mix needs to have at least thirty-five percent air space which can be provided by adding perlite, which he says is like adding air bubbles to the mix. The final mix is a combination of peat moss (or pine bark if a more alkaline soil is needed), sharp sand such as decomposed granite (chicken grit or blasting sand) and perlite. A recommended recipe is four parts sand to two parts peat or pine to one part perlite, with more sand added if a dryer, faster draining mix is needed. The following bonsai growers all first and foremost recommend sifting the fines out of your bonsai soil. Shane Cary, of Louisiana, uses a large variety of mixtures depending on the plant and the growing environment but among the mixes he has suggested an aggregate-bark mix, for example six parts aggregate (haydite, pumice, lava, or akadama) with four parts pine bark and three parts redwood / orchid bark. For trees requiring a “wetter” mix one could add more bark to the mix, or a use greater amount of akadama for the aggregate. As the tree ages and more time is required between repots, more redwood bark and less pine bark can be used which takes longer to break down. A mix of particle sizes also can be used with Shohin trees containing three parts 1/8” particles, and one part 1/16”. Medium tree mixes could contain three parts 1/4” particles, two parts 1/8” and one part 1/16”. A large tree could contain three parts 1/4” particles and one part 1/8”. Many successful growers in the South, including Gary Marchal of New Orleans, Louisiana, Vito Megna of Leander, Texas (near Austin), and Mike Hansen of Pflugerville, Texas (also near Austin), all use a varying ratio of haydite and pine bark. Approximately a fifty-fifty or sixty-forty haydite to pine bark ratio for deciduous trees, and seventy-thirty or eighty-twenty haydite to pine bark for coniferous trees, kanuma for azalea and agricultural pumice for collected juniper. Masaru Ishii of Gardena, California recommends an unvarying fifty-fifty mix of akadama to pumice. Some Canadian importers use one hundred percent decomposed granite for all trees received barerooted.  In the recent years I have personally been comparing the haydite/bark mixture to the akadama/pumice mixture. Obvious initial differences noted include the cost factor, with the akadama/pumice mixture costing about seven to ten times as much. I found the akadama/pumice mixture to be lighter (much nicer when carrying large trees), personally more aesthetically pleasing (although one could simply add a top dressing of akadama to an otherwise haydite/bark mix), and easier to discern when watering is required. More interesting, I made the observation that bonsai I planted in the akadama/pumice mixture provided more growth in a number of pine seedlings by an estimated thirty to fifty percent. Unfortunately I did not make measurements nor take before and after pictures. I did proceed with a very small, eighteen month project which I would like to share, which involved two year old Cryptomeria japonica yatsubusa cuttings in two inch containers, all approximately two inches tall at the outset. Five were randomly chosen and placed in a seventy percent haydite, thirty percent pine park mix, the other five were placed in a fifty percent akadama, fifty percent pumice mix, with all the particles being sifted and only those between the size of 1/8” and 1/16” being used. All received the same sun exposure, watering and fertilization regimen, and after eighteen months the akadama/pumice plants ranged from three and a half to seven inches in height, and the haydite/bark plants ranged from three to five inches in height. The mixture components were the only known variables among this group of plants.Boon Manakitivipart recommends a more complex mixture of one part akadama, one part pumice, and one part lava or haydite with a “handful” of activated charcoal and a “handful” of decomposed granite. He uses a large particle layer on the bottom of the pot for drainage, the actual mix in the bulk of the pot, and a smaller particle size top dressing not only for aesthetics but also to retain moisture in the upper part of the pot which dries out the fastest, so that roots will grow and thrive in the upper zone as well.
Ultimately, one must choose and create the mix that works in your zone, with your watering and fertilizing schedule, your species, your repotting habits and your sun exposure. All of the above artists use a considerably varied mixture with widely different water retention properties, different chemical qualities of the components, different physical properties and particle sizes and different uniformities of the mix. Yet, they are all very successful at growing beautiful bonsai. The one correct bonsai mix therefore cannot exist for all of us, so we should instead focus on why different mixes work for different individuals and use the answers to optimize our own soil choices. Much data is available on container plant production, but this is at best only partially analogous to bonsai cultivation. We must continue to perform regimented, well thought out and controlled bonsai cultivation trials and share our results so that the next generation of bonsai enthusiasts have a more extensive data base from which to extrapolate a mixture which will work for them.
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