Substrate or what is generally referred to as special bonsai soil is basically the media which contains and fosters the root system of a bonsai. Good drainage till the next repotting is the single most important factor in the survival and growth success of a bonsai, especially due to the smaller size and flatter shape of the bonsai pot. Several factors are involved in the successful formulation of substrates for bonsai, viz., geographic or climatic location, sunlight availability, species of plant varieties, watering practices, to name a few. Next to plant selection, the physical property of substrates in a bonsai pot will always influence the formulation of a man – made, tailor – made bonsai soil. Different soil compositions comprising of different substrates can allow a bonsai grower to meet the specific water requirements of the bonsai variety alongwith factors such as local climatic conditions, etc., as mentioned above. Generally, under good sunlight conditions bonsai need to be watered almost daily (especially in tropical climates) and at the same time be able to respond quickly enough to the demands of moisture and oxygen (required by roots). So the need for a good substrate formula assumes greater importance. The particle size decides the presence of spores (micro, midi and macro) in the substrate. Basically, substrates can be divided into three categories: a) those with good tensile strength (having particles with hardness which doesn’t break down easily and thus good drainage capacity but with very low or complete absence of nutritive worth), b) substrates which provide good nutrition (but usually lacking tensile strength due to which the particles break down easily into smaller particles over a shorter or longer period of time) and equally good CEC (Cation Exchange Capacity); and c) simple, locally available garden earth (either high or moderately good nutritive value, presence of local minerals, etc., but available in a naturally powdery form)

Substrates with good tensile strength:-

1) Stone crush or scoria, brick crush, sand etc., are ingredients with good tensile strength and can be used in high proportion to improve drainage but will be unable to retain water for more time. The use of coarse particles, especially sand, is used for improving drainage only. The use of too fine particles will not serve the purpose; sieved or graded aquarium sand is perhaps better than powdery sand. 2) Synthetic soil amendments with medium to high tensile strength: these have been introduced comparatively lately. Artificial or synthetic additives such as LECA (lightweight expanded clay aggregate) in its crushed form having sharp surfaces, calcined clay pellets, brick crush, pumice crush, are usually graded or can be graded, are usually sterile and their addition to the soil in a higher proportion will reduce the chances of soil compaction.

substrates with good nutritive worth, good water retentiveness and good CEC:

Humus, i.e., a content with decayed plants, insects and remains of small animals; peat moss or coco peat; crushed tree bark; leaf mold; well rotted manure are all organic ingredients. It should be understood that using too much organic matter will eventually fill the air spaces created by larger particles (breaking down and compacting). Organic content in soil will also decompose faster and will tend to compact, will clog and hamper drainage sooner, even if it is sieved initially. Repotting will have to be more frequent in such a case. This category of substrates will also hold moisture better and for longer periods, a boon in tropically hotter climates. Also, they will release and provide absorbed nutrition/fertilizers for a longer period (CEC).

locally available garden earth:

this substrate, if used exclusively in a bonsai pot may be OK in the short run but due to its basic clayeyness and tendency to solidify if not soaked properly, it may be harmful if the soil doesn’t get soaked thoroughly during consecutive waterings due to which roots may get trapped and dehydrate. It may also sometimes remain too soggy for longer spells an undesirable trait for roots.

Based on the three above mentioned categories of substrates, there can be three types of substrate combinations:

1) Purely inorganic, tensile substrates (usually used in temperate climates)
2) Purely organic, more water retentive substrates. In temperate climates it may be used rarely. but may be essential to retain a degree of moisture in the potting soils in semi – tropical climates.
3) combination of both inorganic high / medium tensile strength substrates and organic, water – retentive and nutritive substrates.
In most tropical climates or for tropical species in temperate climates (grown indoors or outdoors) a combination substrate may work better than either a purely inorganic or purely organic substrate. The proportions of both may vary from specie to specie of plant varieties, sunlight conditions (high, medium, or low), water requirements, etc., but generally the more the organic content, the less frequent will be the watering cycle. Since synthetic soil will not retain water, watering practices will have to be modified accordingly, especially for those plant varieties requiring more water; frequency of watering has to be increased especially in summer months. Also, in case the proportion of synthetic additives is high in a bonsai soil, the frequency of fertigation needs to be increased, roughly once every week during the growing season and on a fortnightly or monthly basis in the winter season. Soil bacteria may not be present to break down the nutrients initially so chemical fertilizers and later on as the soil acquires air borne bacteria, even organic liquid fertilizers can be used. Adding some slow release fertilizers during potting/repotting will help.

OTHER ADDITIVES:-

Perlite, vermiculite, diatomaceous earth, etc., are also soil additives which have specific functionalities but with lesser tensile strength. But it is advisable to limit them to 10% of the total soil volume because of their lower tensile strength and tendency of pulverization.

IMPORTANCE OF SIEVING OUT FINES:-

What matters for good drainage is the particle size. The coarser the particles (which create larger spores), the better would be the drainage and the lesser the chances of soil compaction hampering good drainage. However, in case of most tropical species, drainage and moisture retention go hand in hand. The solution is to screen and sieve out the fine particles and use only larger particle size to reduce clogging in the short run. Or alternatively to repot the bonsai more frequently in case the proportion of unsieved natural soil and other organic matters is high (as these are certain to break down and disintegrate sooner or later). Humidity or dryness both affect not only the plants themselves but also the bonsai soil. Care needs to be taken while making a drastic change in the soil media. Roots of all plant varieties may not adapt to drastic soil changes. The remedy is to cut wedge shaped areas or pies in the soil and gradually introduce new growing media which is entirely different than the previous one. The root ends in the old soil ball needs to be exposed to the new growing media to encourage them to grow. (Again Mychorrhiza). Main thing to remember is that making a bonsai soil is not rocket science or complex matter. All we need to do is to take care that the roots get water (vapour) and air (oxygen) and good drainage and the plant gets sunlight and nutrition through the use of reasonably bigger particles of soil and by including more of ingredients with good tensile strength. This will ensure good drainage for at least a reasonable time and thus ensure that the plant remains healthy and growing well. Initially each bonsai grower needs to experiment with different combinations of brick crush, aquarium sand, stone crush or scoria, new material like LECA, organic matter (manure or compost, leaf mold, cocopeat, humus, tree bark, etc) and arrive at a substrate formula which will suit their plants, climate, personal watering habits, etc. There is also no need to include every substrate in the bonsai soil. A mix of just three or four will do. A balance of large and small spores with the help of larger and smaller particles of soil seems to work better for most varieties of plants.