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Guidelines For Creating Bonsai Soil.
by Randy Clark, Charlotte, NC
by Randy Clark, Charlotte, NC
The preparation of a workable potting mixture in which to grow bonsai is certainly not the most exciting or interesting aspect of the art, but it is just as certainly one of the most critical. Because the growing space in a bonsai container is limited, it is important that soil placed into it should perform perfectly. The health and well-being of the tree are dependant on it.
The right recipe for bonsai soil is like the right recipe for spaghetti sauce. Everyone has a slightly different idea of what should go into it, but the basic ingredients generally remain the same. Bonsai people will argue for hours about which ingredients will work the best. The actually fact is that most thoughtfully prepared mixtures perform fine so long as they provide for excellent aeration and drainage. The objective of this article is not to describe an exact mixture for making bonsai soil, but rather to discuss the principals and elements necessary for an effective potting mix so that readers can construct a workable medium tailored to their own individual needs and growing conditions.
Bonsaists spend a great deal of time sifting various soil components through a series of sieves. The objective is to make all components approximately the same size. In the process not only are larger unusable chunks eliminated, but also fine dust, which would plug up air holes between soil particles and inhibit proper drainage. Screening and mixing soil is mostly common sense and not rocket science. This is an important point to remember, because it is possible to get so involved in the creation of the world’s most perfect soil mixture that one can lose site of the principal objective... perfect drainage and aeration.
The actual components and the amount of each component used in any soil mixture can vary from region to region and garden to garden. Exactly what should be included in the final preparation is an individual matter. Components included in the mixture will be determined by several factors. First, what types of materials are readily and economically available in the grower’s immediate area. Second, what are the demands made by the local growing conditions, i.e., do you live in a desert or a rain forest? Third, How large is the container into which the tree is to be planted, i.e., shallow pots will dry out more quickly than deeper pots. Finally, what is the moisture and pH preference (acid or base) of the particular variety of bonsai being planted into the soil mix.
Any usable soil mixture must always meet two basic requirements if it is to have any hope of success. First, the mixture must drain water quickly. This is generally referred to as "perfect drainage." Second, it should be essentially pH neutral... that is, neither wildly acidic or basic. A pH value somewhere in the 6.5 to 7.5 range seems best. There are all kinds of pH testing kits available on the market. It is a good idea to get one and use it to test soils. Local agriculture extension agents also offer soil testing services for a modest fee.
"That Gravel You Plant Your Trees In"
The appearance of a correctly prepared bonsai potting mix is so radically different from the heavy black dirt the public usually purchases for their general potting needs that newcomers often describe bonsai potting mixes as "that gravel you plant your trees in." They are not far wrong in this assessment, but what they fail to realize is that there are some very specific reasons for preparing the soils in the manner we do and some very specific advantages to be gained by doing so.
Why do bonsaists insist on a very loose well draining soil mixture? The answer is simple. When god created little green apple trees... and all other plants for that matter... his plan was to grow them in the ground. It was man who devised the idea of putting them in containers. Horticulturally speaking, our bonsai are being asked to grow roots in what is essentially an unnatural environment... the small confines of a bonsai pot. Even though the container may suit our artistic vision for a bonsai masterpiece, it is an alien environment insofar as the tree’s root system is concerned.
When a loose, well draining soil mixture is used, it creates an environment into which the tree can easily grow new roots. Vigorous root growth translates into vigorous top growth and overall plant health. Because the growing space in a pot is limited, bonsai practitioners attempt to gradually trim away heavier roots, thereby making more space for the growth of fine feeder roots which are better able to nourish the plant. It is the development of these fine, hair like, feeder roots that a good bonsai mixture is designed to encourage.
This concept is fairly simple to understand. As proof, consider the kind of potting mixture which plant propagators use for the rooting of cuttings. Normally it is coarses sand or perlite. Both of these substances have a uniform particle size, drain water exceptionally well and have no fine dust which would inhibit air movement through the soil (aeration).
These are exactly the same characteristics on which a workable bonsai mix is based. The actual ingredients from which you assemble your soil mixture are certainly important, but regardless of the components,... if the final mixture does not have good drainage and aeration, it is wrong.