Bonsai Soil – The Key to Successful & Healthy Bonsai Growing


All plants, be they ordinary potted plants or bonsai, need a medium to grow. The growing medium has to primarily provide firm anchorage for the roots and also be able to provide moisture and nourishment, at the same time allowing the roots to grow and breathe and to provide good drainage over a long period of time.The success of any growing medium or soil depends on the following factors –

a)     Good draining capacity: The draining capacity of the bonsai soil is gauged by its capacity to allow excess water to escape as fast as possible from force of gravity so that, given an equable climate (not too hot nor freezing cold), the soil goes from a state of wetness or sogginess immediately after watering to a state of mildly moist between a given watering cycle, presumably of twenty four hours. Good drainage in turn depends directly on the particle size and its tensile strength. The bigger the particle size, the better the spaces available for aeration, lesser surface tension & better gravitational drainage; the smaller the particle size, the lesser the spaces available for aeration, the higher is the surface tension & consequently poorer gravitational drainage. In the event of poor drainage the roots cannot absorb enough oxygen and consequently due to anaerobic respiration they produce ethanol which causes root rot and cause poor growth of the plant and eventual death in the event of continuance of the water logging.

b)      Tensile strength and size of soil particles: The particles of the soil need to be of a tensile strength which will resist crumbling under pressure, friction or changes in temperature. The absence of granular ingredients of good tensile strength would result in gradual pulverisation & compaction of the soil particles and lessening of its draining capacity due to reduction in particle size over a period of time. This state is referred to as structural breakdown of the soil. Material such as river sand, stone crush, brick crush, calcined clay, etc., have good tensile strength and therefore good drainage capacity.

c)      Cation Exchange Capacity (CEC): CEC refers to the capacity of the growing medium to retain nutrition and fertilizers and make them available to the plant. Water and nutrition absorbing capacity is present in some components such as clay, humus, diatomaceous earth, pumice, etc. However, clay, unless calcined or roasted, tends to crumble rather easily and particles will eventually find their way to the bottom of the pot causing a build-up of sedimentation which will hamper drainage, mostly sooner than later. Humus, on the other hand, as it is composed to organic animal or fauna waste, will not crumble so much and due to natural adhesion will also not easily sedimantate at the bottom of the pot. For the reasons stated above, the use of clay is not advisable in the long run. As to sand, since it is generally inert, it will practically have nil CEC. Diatomite, pumice, crushed brick, calcined clay, etc, due to their semi-absorbent capacity have a good CEC and will be able to substitute clay as they are less prone to compaction.

d)      Ph factor: Refers to the acidity and alkalinity of soil and its ingredients. For most plants a soil with a neutral ph works well for optimal growth of the plants. Where specific species need a different ph level, additives will be required to be used periodically. All soils will gradually become progressively more acidic due to the use of chemicals. To reduce the acidity and to increase alkalinity,  lime (calcium) is added to the soil as required.

e)      Inertia or Chemical reactivity: The reaction of the ingredients of soil to changes in temperature extremes may not be significant in the tropics but in temperate climates where freezing temperatures are prevalent it may affect the drainage to the extent that in freezing temperatures the soil would solidify through contraction and hamper drainage and is also likely to damage fine feeder roots. Of course, in such climatic conditions, it is highly probable that the frequency of watering may be very low and hence the chances of freezing in the soil might be avoided. The chemical reactions of the various ingredients of soil viz., sand, clay, humus, diatomite, pumice, crushed bricks, etc.,to fertilisers/reagents, etc., is practically neutral and hence it is not harmful per se. However, due to the semi-absorbent nature of these ingredients (except sand which is inert), these ingredients may  have a build-up of acidity over a period of time and soils having a large proportion of these ingredients will have to be treated for acidity.

For most plants which are grown in ordinary garden pots (made of clay or plastic or composite materials) the growing medium or soil consists of a simple formula of equal proportions by volume of unsieved garden earth and compost or manure. Due to its vertical shape a garden pot holds a substantial amount of the growing medium, so this formula or slight variations thereof work well for a long period of time. Even if the structure of the particles break down, due to the height of the pot the sedimentation at the bottom of the pot is not a problem for drainage over a long period of time if broken shards of earthen pots are placed at the bottom of the pot.

However, the environment of a bonsai is limited to the soil holding capacity of a generally flat pot which is significantly smaller than garden pots, so the soil needs to be composed of lerge particles of ingredients that have an open structure of soil which allow good aeration for good root growth & good drainage and should be heavy enough to support the weight of the bonsai and should also be able to absorb and provide nutrition for optimum good health of the bonsai. Horizontal drainage in bonsai pots is also much slower than in garden pots.  If ingredients with fine particles of soil are used, water-logging or prolonged sogginess of the soil due to poor soil aeration and poor drainage will result in root –rot due to the production of ethanol through anaerobic bacteria which germinates quickly in compacted soils. In such small pots, corrective action such as emergency repotting is usually late, i.e., at a time when root damage is in its extreme stage and almost irreversible.

The importance of soil in bonsai therefore cannot be overemphasised. It is the singlemost crucial factor on which hinges the success or failure to grow healthy bonsai.

Composition of bonsai soil:

In order to find the perfect formula for bonsai soil, bonsai growers have experimented with all kinds of local material including pulverised pine bark, saw-dust, coco-peat, city-waste, wood-chips, plastic chips, earthen pot crush, etc., using these as additives and in some cases directly as soil – substitutes.  Based on experience, climatic conditions and watering practices, etc., each bonsai grower has formed his own recipe of  the soil mix and due to the success attending individual circumstances, claims to have found the perfect formula of bonsai soil. However, given the diverse climatic conditions, plant species, watering practices, etc., it is difficult to rely on a single or fail-proof formula for a rigid proportion of the ingredients or a fixed recipe thereof. Proportion of the soil components / ingredients will necessarily depend rather heavily on these factors and will  accordingly vary between individual bonsai growers and also region to region based on availability of local materials.

The components of the growing medium or soil for bonsai are therefore necessarily a selective mixture of different ingredients such as sand, clay, humus, diatomite, pumice, crushed bricks, etc., which provide drainage, anchorage and also water and nourishment to the plants. 

A good bonsai soil should include a large proportion of granular sand or something similar in nature & structure and well-rotted manure, humus and common garden earth in such a proportion that the soil will not only be able to provide moisture & nourishment to the plant and anchorage to the roots, but more importantly, it will also retain a very high draining capacity for a longer time or at least till the next repotting. In a well-drained soil, osmosis will take place (flow of water from the soil to the roots) if the soil particles are not too small and spaces between them have adequate medium-sized spores or spaces for water which allow good air movement. If roots are in good contact with the soil, surface tension and capillary action will work hand–in-hand to ensure good water absorption by the roots resulting in the transition of a soaking state of the soil immediately after watering to an almost dryish state upto & before the next watering. Reverse osmosis (flow of water from the roots to the soil)  may take place in well-drained soil only if it is allowed to become bone dry for a prolonged period.

The drainage capacity of the soil mix can be increased by the addition of sand or similar material having good tensile strength to suit plant species which need extremely good drainage; humus, compost, coco-peat, etc., can be added to the soil for plants which need that extra store of humidity & nourishment. A granular structure the size of small grams or sweet peas work will in a moderate size pot. For bigger pot sizes of over 2 ft. length and depth of over 5 inches, a bigger particle size is better. Generally speaking, the greater the tensile strength and higher the quantitative & qualitative granularity of the ingredients the lesser will be the chances of a structural break-down of the bonsai soil and consequent sedimentation at the bottom of the pot. A well-drained soil which will retain its granular structure for a longer time is of utmost importance in bonsai and is the primary target in the making of the bonsai soil. The presence of a high percentage of granular particles leads to better growth and general health of the bonsai. Bacterial activity will also be optimal in such a soil; symbiotic and beneficial bacteria, earthworms, mychorriza, etc., will be fostered and anaerobic bacteria will be discouraged if drainage is good and there is no water-logging. (Anaerobic bacteria will almost always mean toxicity build-up through the production of ethanol in the plants)

Bonsai soil for the Tropical climates

In the tropics in summer especially, hot weather can dehydrate the plants very fast due to the larger surface area of bonsai pots; on the other hand, in the monsoons, rains are frequent and heavy in most places and there is a strong possibility of water logging in the pots if drainage is not good. The bonsai soil therefore needs to be very well drained with particles of good tensile strength but at the same time it should be able to retain enough moisture during the summer months which is also the major growing season when plants use up an amazingly high amount of water. In Mumbai, India, the soil for bonsai is normally composed of a good proportion of even sized brick crush which is cheaper, locally available and is calcined to a large extent due to kiln firing. River sand is also used whenever available. Other ingredients that are used are cow manure (prone to weeds), vermi-compost (comparatively weed-free), garden earth and leaf mould (Humus). Sometimes coco-peat and vermiculite (expanded mica crush) for moisture retention, rice husk which provides some heat in the soil (in small quantities) and/or crushed coal for making the soil sweeter and other soil additives are used especially for species such as serissa (serissa foetida), wax malpighia (carmona microphylla), indian hawthorn (malpighia coccigera), sandpaper (strebulus asper), etc., which need a little bit of extra moisture in the soil.  Lime is added to soils for species like junipers which love alkalinity especially during the winter season to improve needle colour.

Generally speaking a good bonsai soil for the tropics will have around 50 percent river sand and/or brick crush, 20 percent manure, 20 percent humus and 10 percent common garden earth. The higher percentage of brick crush/ sand ensures good tensile granularity, good drainage in the rains but the soil does not compromise on moisture content due to the addition of manure and humus, which, after their nutritive store is exhausted, retard the breaking down of soil to a great extent. Best results are obtained when all the ingredients are sieved, discarding all powdery stuff and only the granular soil is used. Sieving of all the ingredients prevents break down of the soil to a great extent.

The practice of importing soils such as Akadama and Kanuma from Japan is not prevalent in most of the tropical countries where the species of plants suitable for bonsai are so different from the temperate climates in their requirements that it is not known whether these ingredients will be suitable. As the locally available material is better suited & have already been tested on local flora, are plentiful and are considerably cheaper than imported materials, the need for importing these material is non-existent. However, calcined clay (which is semi-absorbent and of good tensile strength), similar to ‘Turface’ which is available in US & Europe is now being manufactured and is very easily available in different grades and is a comparatively cheap material gaining popularity gradually in India.

To sum up, bonsai soil needs to be very well drained for the successful growing of bonsai. Its composition may need to be altered to suit local climatic conditions, plant species (i.e., whether the plant needs more moisture or less, better drainage or normal). Conifers need better drainage and less frequent repotting therefore the proportion of river sand/brick crush is increased. Flowering and fruiting trees need humus rich soil for which leaf mould and coco-peat are added. Some other species need higher moisture; for them vermiculite is added to the soil. In drier areas, a little more compost and   vermiculite are added for added moisture and nutrition; on the other hand, in areas of heavy rain-fall, compositions with a higher percentage of river sand or brick crush or both is advisable. For mame bonsai grown in small pots, vermiculite, coco-peat and leaf mould of about 50 percent is added to brick crush which is preferred over river sand due to the semi-absorbent properties of the former. The particle size also can be smaller than that for bigger bonsai pots.

In all sizes of bonsai, however, the key factor is only good drainage; all else is but supportive window dressing.

Drainage, drainage and drainage of the Soil! That is the key to successful bonsai growing.