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Soil for Bonsai

by Lionel Theron

Many experienced bonsai growers have formulae for their soil mixes which work well for them under their particular climatic conditions, areas and circumstances.

If the mix works well it is unnecessary to change. Some growers use additives which they swear by and if proper controlled comparative experiments and studies have been conducted these additions may prove useful. Plants are extraordinarily adaptable and the soil they grow in is but one of the variables. Other variables include position, feeding, watering etc.

There are more and more manufactured products coming onto the market such as akadamia, absorbent ceramic balls and gel-like lumps all of which are being tried and may be useful and do provide profits to the manufacturers.

THE MEDIUM

Soil is the medium in which plants grow.

It is the layer that covers vast areas of this planet. It consists of:

  • Mineral particles and chemicals
  • Decayed and decaying material
  • Gases and water
  • Living organisms

MINERAL PARTICLE

The mineral content of soils is present in various sizes from the very smallest molecules which may be available to plants as nutrients, right up to gravel. Stones and rocks will not be taken into account as part of soil suitable for bonsai cultivation. In order to accurately define this constituent of soil the various sizes will be tabled as follows:

  1. Gravel 2 mm to 5mm
  2. Sand 0,5 mm to 2,0 mm
  3. Silt 0,02 mm to 0,5 mm
  4. Clay : 0,00 mm to 0,02 mm

If particles in a soil mix are too fine, they pack tightly and displace all air, making it difficult for plants to survive. Some of the elements required by plants are taken up in the form of gases through the roots or through the leaves. For most purposes, therefore, clay is unsuitable. The only exceptions are when a thin layer of clay may be used to cover the roots of a newly planted rock clinging style tree, or where clay is used to confine the soil of a planting on a piece of slate or flat rock.

Due to the nature of clay it is difficult when dry, to wet it, and once wet, it packs tightly and spells a slow deterioration to the tree. Soils that pack tightly are not friable.

Gravel and sand are usually sterile mediums and water passes through them at a fast rate leaching out any nutrients they may contain. Gravel allows big air gaps between particles, these gaps are very useful to the healthy growth of plants. Some sand and gravel may contain elements which dissolve slowly and alter, for example the pH of the mix. Usually river sand is stable, whereas sea sand would contain salts which could be harmful to plants. Dolomitic soil would alkalise the growing medium. Mined, crushed gravels may release large quantities of elements which may be detrimental to the tree. Some crushed gravel releases calcium or potassium in large quantities some of which may be washed out.

DECAYED OR DECAYING ORGANIC MATERIAL

Decayed or decaying organic material is of biological (plant or animal) origin which is present in the soil and plays an important role in the soil's structure. It improves the friability and moisture retentiveness of the soil. This organic material is a great source of nutrients for the micro-organisms present in the soil and for the tree itself, both as macro and micro elements. In addition, it provides such elements as vitamins and hormones which are necessary to support life.

Loam is a mixture of silt, decaying vegetative and animal matter, trace elements and nutrients. Loam is, however, an inaccurate description as it gives no clear indication of each constituent. Loam assists with moisture retention. Because of the lack of scientific description of 'loam' it is not used in the constituents of the growing medium described here.

LIVING ORGANISMS

Sterile soils are unable to support life. Most soils contain bacteria, moulds, fungi, insects, mycelia and a host of other small biological organisms. All of these small organisms play a part in the balance of nature. Soil itself is a whole underground Eco-system. A simple analogy of the dependence of one organism on another is that of the termite. In his tummy live some creatures called protozoa, their function is to break down the cellulose enabling the termite to digest the wood which he eats. Without protozoa the termite is doomed, and without termites this particular species of protozoa would have no food or protection. The mycorrhizal association in pines, ericas and several other species is well known. Ammonifying bacteria break down organic compounds into nitrates to become available to plants. Fortunately bacteria and spores of various types are prevalent in the air and water and usually re-establish themselves, even in sterile soils.

There are of course not only beneficial organisms, but also harmful ones, notably viruses (about which we know relatively little). It is wishful thinking to believe that, if soil is pasteurized, the bad organisms obligingly curl up and die leaving the good ones to thrive. Sometimes if so-called good organisms are too prevalent they become harmful, for example, an organism that normally consumes decaying material may have to resort to eating living material if there is not enough decaying material available. It is generally accepted that in healthy soils the balance of organisms is nearly correct, thus benefiting most plants.

GASES AND MOISTURE

For most trees an ideal structure would be about 50% solid, 25% liquid and 25% gas. Obviously this changes constantly, for example when watered, the liquid displaces the gases, then when it drains gases replace the liquid. Plants take up some elements in the form of liquid and some in the form of gas.

THE IDEAL POTTING MEDIUM

What is ideal for one may not be ideal for another. Fortunately plants are rather adaptable and compromise readily. It follows from the foregoing that the soil is used for bonsai culture should contain some gravel, some organic matter, and some liquid retaining material. A good basic mix suitable for nearly all bonsai would be;

  1. 25% to 35% sand. Particle size 2mm to 4,5 mm.
  2. 65% to 75% coarse compost which may contain a small amount of silt

SOME SPECIAL SOIL REQUIREMENTS

Most plants would drown if the moisture content of the soils is continually too great, however bog and aquatic plants have the ability to survive in water. Some trees, too have this ability, and then there are trees that need more water and others that need less. Willows and bald cypress will stand in water whilst wild olives prefer to have less.

Some trees, such as azaleas or maples prefer acid soils with a pH below 7, whilst others, such as olives and junipers, thrive in alkaline conditions with pH above 7. If it is known that a tree likes acid soil, peat may be added to the soil mix. Compost or leaf-mould should not be replaced by peat as peat is a sterile medium and contains no micro organisms, in fact it is not even supportive to micro organisms. Peat is good for water retention. Other additions to soils may be substances such as bone or hoof and hom meal, which is a slow acting phosphate. Care needs to be exercised when adding rich nitrogenous material such as animal manure or urea as they may bum roots. Care should also be exercised when adding undecayed wood products such as bark or wood shavings as they use nitrogen in the process of breaking down and may deplete the soil to the detriment of the bonsai.

The addition of charcoal may be helpful in the absorption of harmful gases and provide a sweet soil. Some trees are sodium sensitive, that is , they do not tolerate high salt levels. These trees may benefit from the addition of small amounts of alum (calcium sulphate).

Compost is vegetative and animal matter which has decayed in the presence of fresh air. It is often manufactured by gardeners using unwanted garden refuse with maybe a bit of manure added.

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Random Bonsai Tip

If a tree lacks a branch in a specific place you could in arch or approach graft a branch in the required area or thread graft through the trunk using a long shoot of the same plant.