Bonsai are normal trees grown in a container. Their needs are the same as any other wild or garden tree: they need air, water, light and nutrients, and like many other potted plants the need to be pruned to mantain their viatlity, and they need to be repotted so they get a fresh supply of soil.
Watering your Bonsai tree
The biggest killer of Bonsai is improper watering. Getting the watering right for your Bonsai species is essentially to the health and survival of your little tree.
Lack of water can kill a bonsai quickly, you could in half a day's lack of water lose all leaves on a tree. Over watering - on the other hand will almost surely kill the bonsai too - as bonsai roots can suffer from root rot and eventually effect the health of the Bonsai.
In general Bonsai need to be watered only once every two or three days. The soil of your Bonsai pot must be damp, not dry or wet. The best time to water is early in the morning. Keep a watering schedule and water at the same time every other day, but only water of the bonsai needs it.
During summer you may need to water more frequently - if so water early in the morning and early in the evening. A good practice is also to soak water your Bonsai pots once or twice a week. This allow to a better circulation of air and nutrients to the roots as when the pot is soaked in a bowl of tepid water it forces out all the stale air. When the pot is taken out off the water bowl, water drains out from the bottom of the bowl and fresh air, oxygen and nutrients flow down to the roots.
In winter your watering cycle needs to be less frequent. Both the plant and soil lose less water through transpiration and evaporation and hence the soil tends to hold water for longer. Ensure that the soil is just damp not wet. A wet pot can lead to the plant standing in water for prolonged periods of time; thereby making it is more susceptible to root rot.
The safest way of ensuring your pot get the right amount of water is to look at the state the soil is in; if it is soaking wet or too damp do not water, if it is dry or near dry then water. Water less or more depending on dryness of the soil in the pot.
Feeding your Bonsai
All Bonsai need to be feed to grow and flourish. Bonsai that are not feed are more susceptible to weather changes, disease and pest. Their general well being suffers and the plants lack vigour and vitality and often end in straggly branch and leaf growth.
Plants get their nourishments from the air, water and the soil that it grows in. From the Air it gets Oxygen, Carbon, and some water and from the soil it gets Nitrogen, Potassium and Phosphorous called primary nutrients (NPK) and a host of other trace element, called micro nutrients, like magnesium, iron, zinc and copper. Any plant feed we use will have to replenish these nutrients to keep the growing medium fertile.
You can get these elements/nutrients from organic and inorganic fertilisers. Organic fertilisers, like humus, leaf compost, manure, bark release these chemicals slowly to the plant as the organic matter decomposes. Inorganic fertilisers release these nutrients "faster" to the plant. Both types of fertiliser have their benefits and draw back, but that is another discussion.
Inorganic fertilisers come in liquid and or solid forms and have a strength depicted by its NPK value. For example a fertiliser which has an NPK value of 10:10:10 means that it has all the primary nutrients in equally quantities in the preparation. A fertiliser that has a 20:10:10 NPK value has a Nitrogen level that is twice as much as that of the other two elements. This particular fertiliser is commonly known as a "high nitrogen" fertiliser.
Whatever fertilisers you use ensure that you always use the "dosage" recommended by the manufacturer. Too much of fertiliser will upset the balance of chemicals in the growing medium and can kill the plant.
The best general purpose fertiliser to use, in my opinion, is a fertiliser that has equal NPK value like, for example a 10:10:10 or 20:20:20 NPK value fertiliser. All good fertilisers will also have a balance of the trace elements nutrients too.
You can use a good quality house plant fertiliser or fertiliser specifically designed for Bonsai. Whatever fertiliser you choose ensure only apply the recommended quantities.