Since a bonsai remains in a container for life, soil becomes a very important matter; the potted tree cannot extend its roots and find moisture or food like a tree grown in the ground. The roots do continue to grow, but they eventually become potbound and can no longer absorb moisture or the nutrients that are needed for healthy growth. As a result, bonsai must be repotted whenever they become root-bound. Evergreens, since they are slow-growing in general, can be left in their containers for three to five years. Most deciduous trees need repotting each year or two. Willows and other rapid growers may have to be repotted twice a year. When the tree comes out of its dormant period in the spring and just before new growth begins is the best time for repotting. The tree is taken out of its container and, before replanting in new soil, one third of the old soil and roots is eliminated. In this fashion, the trees are kept growing and healthy.

The basic ingredients for bonsai soil are subsoil, sand, gravel, garden loam, leaf-mould, compost and peat moss. They are dried in the sun, sieved into various sizes and stored until use.

It should be noted, however, that many successful bonsai have been grown in ordinary garden soil.

Subsoil is clay pellets, which are obtained by digging some 3 ft. below the soil surface. When first dug, the material is usually crumbly and not too sticky; it dries into hard lumps, and is a basic ingredient of potting soil. Coarse sand or gravel is necessary for good drainage and root formation. Local gravel pits or construction companies are the usual sources. Leaf-mould, compost or peat moss give acidity and nutrients. They are used in the proportions that each species requires. Garden loam is used with flowering and fruiting deciduous trees.

The bottom soil is made up of the larger lumps of clay, some leaf-mould - each particle .25 in. to .5in. wide - and some coarse sand. The main potting soil is made up of the same ingredients, with each particle about .5 in. in size. For topsoil the particles of the basic mixture should be screened to about the size of a grain of table salt.