Bonsai are probably injured more often by improper watering than by any other single factor. No general schedule can be used for watering all plants. In the summer once or twice daily may be required. Size and type of plant, pot, soil mix, light, temperature, humidity and other conditions influence the speed with which the soil mass dries out. Soil kept either too wet or too dry causes plant roots to die, which leads to poor growth. Learn to gauge the moisture content of the soil by its color and feel. As the soil surface dries it becomes lighter.
Under continued drying, the soil begins to crack and pull away from the sides of the pot. When severe drying occurs, some damage already will have been done to the roots. Soil kept too moist becomes sticky and slimy, thus inviting root rots and other disease problems.
Whitish-colored deposits can form and are composed of salts built up from hard water and fertilizer. This is usually an indication of insufficient drainage, and not enough flow of water and air through the pot and soil. To avoid salt buildup, water your plants each time until liquid runs out of the bottom of the pot. This is also one reason that many people burn their plants with fertilizer - they leave behind salts that burn roots. Many homeowners soften their water by using a filtration system to exchange the calcium and magnesium in the water for sodium, creating a softer water that allows soap to function better. Plants use sodium only in very small amounts. Over time, sodium becomes toxic to plants.
Plants may be watered from either the top or the bottom of the pot. If one prefers watering from the top, use a watering can with a small spout. Each time, wet the entire soil mass, not just the top inch. Add water until it comes through the drainage hole in the bottom of the pot. Discard water that remains beneath the pot one hour after watering. Periodically plants should be watered by soaking. This is done by immersing the plant into a bucket of water so several inches of water are above the soil line. Then allow it to drain. Potted plants should always have good drainage. Occasionally roots may clog the drainage hole.