This may be the single most important topic pertaining to growing bonsai. If one cannot keep a tree alive, one cannot develop it into a bonsai; misunderstanding of bonsai soil construction and watering requirements is a common reason for failure.

     As a general rule, water a bonsai thoroughly, and only when needed.  To tell if it needs watering, scratch about a half inch below the surface; if it is light-colored and dry, then water the tree.  Do not simply water on a schedule!  Bonsai water requirements will vary greatly with type of tree, location, season, winds, and just about everything else.  Bonsai planted in pots with poorly draining soil are very easy to overwater.

     The root system of a tree growing in nature, travelling wide and deep who-knows-where, sustains it thru assorted adversity and weather conditions. Oxygen is necessary for the root cells to live and grow, and is available in various pores and pockets in the soil. Moisture likewise, near the surface in wet times and deep in the ground during drought.

     In contrast, the limited root system of a bonsai in its tiny pot is exposed to extremes and in a fragile home; special care must be taken in a small pot. Oxygen and water are vital. Oxygen would of course be available if the roots were hung in the air, but little moisture or protection; a heavy clay soil will hold moisture a long time, but allows for little space for oxygen. Therefore, a compromise is needed: a granular soil, with sufficient quantity of material to protect the roots and hold moisture, but still have pockets to hold air.

     A coarse soil will require more frequent watering that a heavy clay soil. However, this serves two important purposes --- it not only keeps the root system supplied with moisture, but replenishes the soil spaces with oxygen and keeps them from becoming stagnant. Fresh air is drawn down into the soil, replenishing oxygen and flushing out byproducts of metabolism and decomposition. Whether the bonsai is watered thoroughly from above or soaked in a basin, as the water rapidly drains from the bottom of the pot, picture the soil "inhaling" a breath of fresh air.

     Certain techniques can lighten the burden of frequent watering, such as wicking systems, humid enclosures, large pot, or heavier soil --- but none of these accomplish the beneficial freshening of the soil.

     When watering a bonsai, the entire pot should be thoroughly soaked. Have no fear --- it is virtually impossible to overwater properly-prepared coarse bonsai soil in a pot with large drainage holes. Excess water will rapidly drain thru the soil and out of the pot, leaving only moist particles and air spaces. It is very important that every corner of the pot be watered; the entire root system must be kept alive, to ensure the health of the tree. For example, if only the surface is wettened, then only those roots close to the surface will thrive; then a slight drought or late watering will cause these near-surface roots to die, jeopardizing the entire tree since there are no other roots functioning.

     There are many approaches to soil preparation and watering, which work when properly applied, but in my opinion this is the safest and easiest.

     More: Soil Components and Sieving