To understand exactly how your bonsai should be fertilized, you must be able to understand the basic make-up of fertilizers. Nitrogen is an essential ingredient for leaf and stem growth. Too much nitrogen however will make the tree produce long, stringy growth. Phosphorus encourages healthy root growth and helps the growth of new buds, whilst also protecting against disease and unfavorable winter conditions. As a general rule, feed your bonsai from early spring to late summer.
Most commercial fertilizers contain all three main nutrients plus some trace elements.