A lot of nonsense has been written about composts. There are books around that state a compost for bonsai should be made from clays and sands from specific areas in Japan. This is misleading, what any compost should be is, free draining, while being capable of holding enough water for the tree, indeed some suppliers stock 'Bonsai soil' which is no better than any other potting compost.
Composts do not need to contain any fertilizer, as you should be feeding the trees on a regular basis.
Before going any futher you need to be aware of the soil preferences of the tree you are going to use the compost on. Apart from the nutrients and trace elements in the soil, there is one other chemical aspect you need to know, is the soil Acid or Alkali (Lime). Most trees are fairly indifferent to this, however Ericaeceous plants such as Azeleas and Camellias must have an acid soil, which is usually sold at garden centers for potting Heathers. Please don't assume that the peat (or substitute) is acidic, it's likely that lime has been added to it.
There are a number of well known formulations for composts and while the formulas are tried and tested, the materials used in them often contain a lot of dust, which will be washed down to the bottom of the pot and form a solid mass, killing the roots and tree. Coniferous trees will benefit from grittier soil than Deciduous trees. A good general compost would be 50/25/25 Peat/Sand/Grit with 40/30/30 Peat/Sand/Grit for conifers. The sand should be Horticultural, or sieved sand as others contain a lot of fine dust.
I'd define grit as any stone that would pass through a sieve with a 4mm mesh but not through a 2mm mesh. These larger stones keep the soil open and allow air to get to the roots. The effect is reduced somewhat at the soil settles but it helps to promote root growth after repotting.
It also increases the drainage. The last thing most trees need is to be sitting in a wet mass of soil (although some trees will benifit from these conditions (Willows, Swamp cypress)), it's a gateway for fungal infections. Grit and Sand, it should be remembered will reduce the volume of water a given amount of soil can hold.
Here in the UK, Horticultural Grit is available in any decent garden center, It's generally crushed, sieved granite.
If however you have access to someone who stocks either 'Akadama' or 'Kanuma' clays then you are well advised to use them. Akadama,is great for general use, Kanuma tends to be sold for acid loving trees such as Azeleas, but is no more acidic than akadama, it's just that kanuma is the 'local' soil for the main azelea growing area of Japan. Azeleas thrive in akadama. Both are granular clays and provide a wonderful open soil, giving amazing root growth. Before use you should use a fine sieve, such as a flour sieve to remove any dust.
If you have purchased a bonsai from a non specialist supplier there is a likelihood that the tree was mass produced in China, the country where bonsai (pen-jing) originated. Look closely at the compost it is almost certainly clay and you will not see grit or anything else that will open the soil and allow air to the roots. This clay will soon compact down as the tree is watered and choke the roots. The tree needs to be repotted at the earliest opportunity.
If you have not already done so, you may come across books on bonsai that recommend 'layering' the soil, Putting a coarse open soil at the bottom of the pot, building up to a fine soil at the top. This method provides excellent drainage but tends to keep most of the water at the top of the pot denying roots at the bottom enough water, while giving you an false impression that the tree is adequately watered. Current thinking is that one soil size throughout the pot is best, giving an equal distribition of water.
© Allen. C. Roffey 21:53 11/06/2005