For raised bed vegetable gardening fertilization needs are the same as for traditional gardening. As you do with your garden soil, it is best to start raised bed gardening with a soil test if you are not using a soil mix. Before planting in the spring you should apply a general fertilizer. Spread it over the soil before planting and rake it in. Apply additional fertilizer in midseason and to areas where you are planting a second crop during the season. Do not over-fertilize the soil as this will lead to poor crops. Side-dress the crops during the growing season based on needs of each crops.
Also watch for signs of nutrient deficiency throughout the growing season. Nitrogen deficiency shows up as lower yellow leaves. Phosphorus deficiency will exhibit a purple discoloration and is often seen on the lower leaves of cabbage and broccoli. Potassium deficiency is more difficult to notice because it sometimes starts as a brown stippling of the leaves along the outer edges.
For improving the soil in your raised beds you will need a lot of organic matter every year because the organic matter decomposes and disappears, so choose a source that is readily available and inexpensive. Sawdust, ground bark, leaves, manure or chipped pruning materials are good options. For best result make sure they are well composted. In the summer, use compost to provide nutrients. In the fall, cover the beds with a layer of leaves or other organic material. As you continue to add organic material, the soil will improve each year.