Article 09 – ‘Bonsai soils’

Bonsai soils
This article does not discuss every soil ingredient nor does claim which is best. This is a question that only the enthusiast can answer after experimenting over time with his/her own trees and care routine. However in general terms, the soil in which a tree was originally grown is probably the best providing it is growing well and healthy.

Soil contains a multitude of living organisms that consume, digest, and cycle nutrients. These living organisms include archaea, bacteria, actinomycetes, fungi, algae, protozoa, and a wide range of insects for example. Mites, nematodes, earthworms and ants all of which are important to the vitality of a soil composition, which in turn is vitally important to any and all growing mediums.

Of course, different horticulturists make various soil combinations for their trees, although much depends on species and from where they originated for example. Conifers Pinus spp. and cedar Cedrus spp. prefer the soil composition to be slightly acidic. As do some broad-leaved species Rhododendrons Rhododendron spp., magnolia Magnolia spp. and camellias Camellia japonica.

Deciduous species including Beech Fagus spp., Birch Betula spp., Oak Quercus spp. and Elm Ulmus spp. are predominantly nutrient-demanding, the autumn leaf fall provides an abundant rich humus, that decays quite quickly becoming the nutrients for a tree’s new growth in the spring. This composition of broad-leaf humus is not as acidic as the decayed needles from coniferous species such as Pines.

A bonsai is confined to a relatively small quantity of soil on which its health depends and this growing medium has to be conducive to its needs. Therefore, it must be able to retain water yet have good drainage and have the ability to allow for air circulation. Without this basic criteria the health and vigour of the tree will suffer.

Because the majority of coniferous species dislike wet soils, the soil composition should be less water re-tentative. Alternatively deciduous species especially those that flower and fruit require more water and are normally planted in soil compositions that have relatively high water retaining properties. Therefore, when preparing soil consideration must be given to the species it is intended for.

Organic and Inorganic Soils – can be described in the following manner:
organic – is plant matter which has decayed for example, leaf litter, peat, bark or compost.
inorganic – is material found in most soils sand, silt, clay and volcanic lava contain very little or no organic matter.

Inorganic soils can be difficult to obtain and can be more expensive than organic soils readily available at most horticulture outlets including, nurseries and supermarkets. But the question of what soil composition is best still needs to be answered. As previously stated the soil from where the tree was originally obtained (If from the wild) is probably best suited providing it is growing well and has a healthy disposition.

But, not all trees come from the wild, some are nursery cultivated, in this instance it pays to research the needs of a particular specimen or ask questions of the grower/vendor. Here are a few problems associated with different soil compositions:

Peat – has disadvantages in that it can be too water retentive meaning continually wet during rainy seasons. Alternatively peat in warm temperatures is prone to drying out and can be difficult to water properly leaving dry areas within the root ball.

Sand – although has good aeration has very little water retention properties due to high drainage rate, unable to store plant nutrients, is unstable in that its particles can be blown away allowing the tree to become unstable.

Silt – problems with aeration, water retention properties, drainage rate slow to medium to high, ability to store plant nutrients medium to high, can become unstable due to erosion.

Clay – water holding capacity is high with a low drainage rate, has poor aeration qualities, has good abilities to store plant nutrients is stable, but is very slow to warm in the spring.

One of the biggest problems when using any type of soil mixture is that it has the potential to break down and be compacted in the tree’s container. This often leads to problems with aeration and drainage that can result in root suffocation or rot diminishing the tree’s health. Arguably the only suitable organic component in a soil mix is bark as this holds it’s structure for a longer time before breaking down and does not reduce air circulation or drainage.

Inorganic soil mixes do have an advantage over organic, in that they are able to retain their open structure much longer without breaking down into a paste like substance. In addition, such materials have better water retention properties where any excess is drained from the container immediately and solves the problem of over-watering.

One such inorganic soil composition favoured by those dealing with conifers is Akadama a Japanese baked clay. It comes in a variety of grades purposely produced for bonsai and is imported into Europe and other Western countries. But the downside of using Akadama is that it can be difficult to obtain and usually only available from bonsai nurseries. Akadama is the favoured soil for many bonsai enthusiasts, but can be expensive for example, a 2 litre bag will cost approximately 9 -10 Euros. And if postage is required the price will almost be double. Furthermore, an Akadama soil mix has to be changed every 1 to 2 years.

Other clay soils available are Seramis, Turface, Oil-Dri – these are fired clays and are readily available in many western nursery outlets and are considerably cheaper than Akadama. These clays are fired for longer periods, thus the break down point is much longer than Akadama. Generally speaking all clays can be mixed with bark and grit for a faster drainage, but still have good water retention. Furthermore, nearly all deciduous trees are able to adapt from an organic mix to an inorganic one relatively quickly. But there is one important factor to remember which is; that conifers must retain some of their original soil as it contains mycorrhizae fungi necessary for health and development, which was mentioned at the beginning of this article.

In reality, there is no single soil mix best for cultivating bonsai, because of many factors including, the species, climate tolerance and seasonal changes. To day there is a wealth of information available on bonsai soil compositions written by both experts and novices alike, which can be confusing. But arguably the best way to get and indication of soils types and what is suitable for your needs is this approach.

Firstly, invest in a small soil testing kit, (approximately 10 €) which can be obtained from most all nurseries and some supermarkets. Secondly, take a walk in a forest find a healthy tree with good growth. Take a small sample (Close to the roots if possible) and seal it in a plastic bag remembering to label it. If more samples are needed from different species, you must change the implement or tool to avoid cross contamination. Now test the soil sample (s) according to the instructions on the box or packet and the results will give you a strong indication of the soil parameters of that particular tree. This method of approach is a good learning curve helping to build up your knowledge in determining what is a good bonsai soil mixture.

In the next post we will look at pots and containers. Until then BW, N.


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