The proper soil composition is important for the health of your bonsai. It should be free-draining to allow the roots to breathe, but should still hold enough water and nutrients to feed the tree. The ideal composition depends on the requirements of the tree species, as well as your local climate. Local bonsai clubs are an invaluable resource in determining the ideal soil for your area.
Goals
Regardless of your particular soil composition, it is important to use a sieve set in the preparation of your soil mixture. This achieves two main goals.
The first is to remove fine particles from the soil. When wet, fine particles will form a thick paste and deprive the roots of air. This leads quickly to root rot.
The second goal of sieving is to separate the soil into three particle sizes. Hundreds of years of experience have shown that layering bonsai soil according to particle size improves root health and growth.
Three passes
Sieve sets, such as the one we sell, come with at least three interchangeable screens.
Start by passing the soil through the coarsest screen to separate the largest particles. Depending on your sieve, this will remove particles between 5—10 mm. (Anything larger than 10 mm should be discarded.)
Next, pass the remaining soil through the intermediate screen to separate grains approximately 2.5—5 mm.
A final sieve with the finest screen will keep only particles from 1—2.5 mm. Anything finer should be discarded. This is the dust that will strangle your bonsai roots.
For small bonsai or especially shallow pots, you may not need the largest particles. For intermediate and larger bonsai, arranging the particle sizes in layers will improve soil aeration and the dispersion of water inside the pot.
Repotting
Prepare your bonsai pot by placing drainage netting over the drainage holes, and affixing the anchor wires. Place a layer of the coarse soil (largest particles) in your bonsai pot such that it fills 1/3 of the pot, forming a slight mound where the tree will be placed. Next, place your bonsai tree over this layer, gently nestling it into the soil mixture.
Attach the anchor wires over the base of the tree, being sure to pad the areas of direct contact with rubber or foam.
Continue by adding the medium grain soil to about the 3/4 mark of the pot. Use a chopstick to work the soil into the roots to remove any air pockets.
Finally, fill the pot with the finer soil particles. Moss, if used, can be placed as a final touch. Water thoroughly with Superthrive and keep the tree sheltered until it regains vigour.
