First, a little ultra-simple plant biology.
Plants (including bonsai), maintain themselves and grow by taking in and processing nutrients, just as animals do.
The obvious difference is that plants in containers, can't search beyond the container to find them, the nutrients have to come to them.
One major portion of their needs is met when they absorb sunlight and take oxygen from the air.
The rest must come from the soil.
Those who practice the fine art of bonsai have known for years the very special qualities that a professional bonsail soil imparts to soil mixes.
A good bonsai soil profile is approximately 25% water, 25% air and 50% solid particulate matter.
Its unique properties help provide free passage of air and water to root systems while significantly reducing soil compaction.
However, when soils are compacted, an imbalance occurs.
Solid particles are pressed together, and water and air are squeezed out.
With less pore space, soils become too dense for the movement of air, water and nutrients. Also, the favorable environment for beneficial microbial activity necessary in a healthy growing medium, is lost.
With a commercial "potting soil" it can be difficult to tell exactly how much of each component or nutrients it may contain. However, for absolute beginners a high quality potting soil can be a good place to start.
Sooner or later though, the avid Bonsai enthusiast is going to want to mix his or her own soil.
And, since bonsai are confined to a small pot most of the year, year in and year out, that soil will need to be supplemented and occasionally replaced.
As most bonsai are a normal, and not a dwarf species, the tree must be pruned to be kept small.
Though growing toward the light, as most plants do, it must be wired and shaped to create the desired appearance.
But nowhere do competing elements need to be so precisely balanced as in the preparation of the soil.
Make sure yours has the following attributes.
Bonsai soil must be able to retain water well, since excessive drying is the easiest, and most common, way to kill a bonsai tree.
Many so-called "mall-sai" bonsai, bought at a store in the local mall, are nearly dead by the time they're purchased since they don't receive the proper amount of water, light or care.
Water is essential to life in itself, but it also acts as a vehicle to deliver nutrients to the roots and throughout the entire plant.
Humus, the organic components that remain after decomposition of plant material by soil organisms, along with clay are the two major factors that help retain water and nutrients.
But the soil can't be allowed to retain water too well. It must provide good drainage. When too much moisture remains in the pot, whether through excess watering or compacted soil, root rot is almost inevitable.
Proper drainage is achieved in part, by infusing the soil with small pieces of gravel or an "aggregate".
That helps create small spaces in the bonsai soil through which water can readily travel. Water then drains through the soil mixture, into the base and out of the drainage hole found in all bonsai pots.
The custom bonsai soil from Bonsai Boy is produced by expanding and vitrifying carefully selected shale in a rotary kiln at temperatures in excess of 2000 degrees F.
This process makes the soil sterile and environmentally inert.
The result is a natural non-toxic, highly absorbtive ceramic granule with a generally neutral PH.
It is dimensionally stable and will not degrade like other amending products.
In addition to allowing water to pass through, and not pool around the base of the plant to rot the roots...
a good draining soil allows for the easy passage of vital gases both in and out of the mixture.
Carbon dioxide, oxygen and nitrogen need to pass in and out of the plant and soil in order for photosynthesis and other essential biochemical processes to proceed properly.
A quality bonsai soil will have all those health supporting physical characteristics.
Beyond the need for good water retention and good drainage...
soils have to supply all the nutrients trees can't get from the air or produce internally using sunlight for energy.
Also, how much water retention is needed and specifically what kind and amount of nutrients will vary somewhat from species to species.
Flowering and fruiting species , such as, citrus, cherry, fig and others, require much more water than average.
Not only do they have to feed trunk, branch and leaf systems, but fruits and flowers take in more water and aspirate moisture much more quickly.
Soil is a mixture of inorganic and organic material.
Inorganic elements and compounds, such as clay, granite, ash and others help regulate drainage and supply nutrients.
Ash or ground volcanic rock, helps not only regulate water but supplies some of the needed nitrogen as well.
Organic components are made up of decomposed plant and animal matter, which provide nitrogen, phosphates and a host of other vital nutrients.
Mixing these two basic ingredients together in the correct ratio creates the soil appropriate for a given species and climate. By adding relatively more grit, for example, easier drainage is increased.
In the absence of more specific guidelines, a 50:50 mixture of grit and peat is a good starting point.
Grit, usually crushed granite or flint, provides good drainage while peat, typically moss peat, provides a spongy earth, making for good aeration and supplying needed nutrients.
Leaf mold or composted bark is sometimes a suitable substitute for peat moss.
Proportions will vary depending on species.
Pine and Juniper, as noted, should have more inorganic material to provide less water retention. Often the proportions change to as much as 75:25.
Provided the base of the pot contains a layer of gravel to keep the screened hole from being plugged, a perfect ratio isn't critical.
Akadama, a white Japanese clay, is the most commonly used fine-quality inorganic material used by expert bonsai artists in Japan. But it can be difficult to obtain in the US and UK.
Seramis is often used as a substitute. This more standard, orange-colored, clay is a good alternative. It has the added advantage that its color changes slightly as the moisture content varies, giving a good visual indicator of drying.
When preparing soil, keep in mind that all the elements of proper bonsai care are interrelated.
Proper soil mixtures vary with watering regiment and are dependent on local climate, air pollution content and several other factors. Consider your individual circumstances carefully.
If you intend to put forth the time, expense and effort to grow bonsai - which require more care than ordinary plants - soil is the last place you want to skimp on money or preparation.
Good Luck and Happy Gardening!
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