When perennials bloom poorly or produce little or no seed, they can try again the next year -- but annuals do not get a second chance. And because annuals work hard to form flowers, gardeners want to help them put on the best show possible.
Like other plants, annuals need the big three nutrients -- nitrogen, phosphorous, and potassium -- along with smaller amounts of secondary nutrients such as magnesium and calcium, and minute quantities of trace elements, including boron and iron. Healthy soil, rich in organic matter, provides the basic nutrition most plants need, and usually contains adequate amounts of trace elements and secondary nutrients. In organically-rich soils you can probably grow many annuals without supplemental fertilizers. But at times adding compost is not enough. Some annuals want more nutrients, especially if they are grown in pots. Every time water drains through the hole at the bottom of the container, nutrients in the soil are carried along with it, and they should be replaced.
In addition to incorporating organic amendments into the garden soil each year, give periodic applications of fertilizers containing all three major nutrients to ensure that your annuals are well-nourished. If a soil test shows your soil is deficient in secondary or trace minerals, supply those as well (USDA and private soil-testing laboratories will instruct you on what to add to bring the nutrients up to acceptable levels).
For flowering annuals, use an all-purpose plant food, such as a 5-10-5 or 10-10-10 formula. Flowering plants have a special need of phosphorous and potassium to realize their blooming potential. Foliage plants will flourish with a formula higher in nitrogen (the first of the three numbers in a fertilizer formula).