When you plant a tree in a pot, it's like confining an animal to a small cage. The soil is the plant's only means of acquiring everything it needs for survival: the better the soil, the better the health and performance of the tree.

Most plants can survive for a while with their roots growing in almost anything - from pure gravel to nothing but water, from clay to rotting wood. But to really thrive for year after year, they need a good balanced growing medium.

Most commercial bonsai soils contain 'fillers', such as hadite, large particles of grit or excessive amounts of turface - all of which take up space that could otherwise be colonized by the roots. They also often contain poor organic material such as dried and sterilized bark chips or dusty peat-based potting composts. The bark chips remain inert and impenetrable by roots for years, and when they finally begin to decompose they leach nitrogen from the soil, robbing the tree of a valuable nutrient. The peat-based composts rapidly break down into dust, which compacts, clogs the micro-pockets in the soil and causes poor drainage and finally root decay.

WHAT MAKES A GOOD BONSAI SOIL?

The most fundamental thing that roots need in order to maximize the health and performance of the plant is space to grow - after all, that's why we repot periodically. As I have said, most commercial bonsai soil ingredients are impenetrable, so roots can only colonize the spaces between the particles. The one common ingredient that is theoretically penetrable is bark, but this is so acid and so inhospitable that roots avoid it. Think about the last time you repotted a tree grown in soil that contains bark - did you see roots growing into and through the bark? No, right?

To provide roots with the best and maximum growing space Akadama is the perfect solution. Akadama is a Japanese clay-like granular mineral that permits roots to grow through the particles. If you have ever grown maples in pure Akadama (the best soil for maples) you will know exactly what I mean. When you lift the tree from the pot you find a solid block of healthy white roots - not a tangled web of brown roots wrapped tightly between the soil and the pot, leaving the soil itself virtually rootless.

Akadama, however, is expensive and if anything it is sometimes too water retentive for many species and for many growing conditions, so other ingredients can be added to create an optimum soil consistency.

Calcined clay, or Turface is an excellent additive, but it should be used only in relatively small amounts. It absorbs vast amounts of water and provides a 'buffer' which adsorbs nutrients for release later as required by the roots. If you use too much Turface its ability to hold on to nutrient salts may cause a toxic a build up of minerals and lead to "root burn". It's interesting to note that the manufacturers recommend using Turface as a soil enhancer, but never more than ten to fifteen percent (as opposed to the sixty to seventy percent some people use!).

Crushed lava is another excellent additive. As well as absorbing large amounts of water, the pore spaces in each granule are large enough to allow roots to penetrate. Lava also helps maintain an open structure and it contains many important minerals that are released slowly over a period of several years. Both Turface and lava can be sifted out of used soil and reused time after time.

Grit aids drainage and helps to keep soil open. The average enthusiast doesn't have the time to monitor each plant daily for drainage, so erring on the side of caution is a good idea. Most people use grit that is far too large. The grit I use is finer than in most other commercial bonsai soils. Finer grit means more spaces between the particles and a greater surface area to retain a film of moisture, which in turn leads to more fine root growth.

WHAT ELSE AFFECTS BONSAI SOIL?

Consider your bonsai, its container and the soil as a single, self-contained unit, and compare that to a tree growing in open ground. A bonsai's roots are confined in a small container, they are growing in a virtually inert and lifeless medium where they rely on you for all their needs - and their needs don't stop with the basic NPK of synthetic fertilizers. The soil in open ground contains many other minerals and countless millions of microorganisms that perform a myriad of functions necessary to keep the soil healthy and vibrant. They help the plant build resistance to stress, they aid the roots in absorbing major and micro-nutrients, and they help to prevent compaction and stagnation of the soil.

Fertilizers:

Your plant may not care about the type of fertilizer you use, but the soil, and it's microorganisms do care - a lot! It's rather like asking a human to survive on burgers and chips - it can be done for a while, but eventually the effects of an imbalanced diet would cause deterioration and probably death. Nursery container-grown plants are generally short-lived - or, at least, they only spend a few years in a pot, so the long-term effects of inorganic culture are not so much of an issue. But your bonsai, we hope, will live for a very long time in a container, so it becomes more important to give them the best possible growing conditions. You can keep your soil "alive" by using organic fertilizers that break down and feed the microorganisms in the soil, as well as providing some microorganisms themselves.

Trace elements:

Trace elements or micro-nutrients that are essential to healthy plant growth. Although required only in tiny amounts, a lack of trace elements can seriously affect a plant's health and resistance to stress. The most commonly seen evidence of trace element deficiency is yellowing leaves. Green veins and yellow spaces between indicates an iron deficiency, whereas yellow veins and green spaces between indicates a magnesium deficiency. Other micronutrient deficiencies are less easy to diagnose, but the effects are usually reduced vigor, lack of a second growth surge in summer, withering shoot tips, winter die-back and reduced ability to fight off fungal attacks.

Micorrhizae:

Trees growing in marginal conditions (which bonsai are) would probably not survive without mycorrhiza and, ironically, it is in marginal conditions that mycorrhyzae thrive. If your bonsai are well watered (not allowed to become too dry between waterings) are consistently fed, and are not stressed by heat, cold, drought, over watering, wiring or pruning, they may well not require the help of mycorrhizae. But most bonsai suffer at least one (sometimes all) of these stress factors. Micorrhizae will greatly enhance the trees ability to gather and regulate uptake of water and nutrients, as well as enhance its resistance to stress.