Annuals grown in beds
Before you plant annuals, it is a good idea to amend the soil with compost or composted manure. You may also wish to work a natural slow-release fertilizer into the soil.
Once the plants are well established, you can use natural fertilizers, like seaweed, fish emulsion, hen manure, etc., according to the manufacturer's instructions. You can add these fertilizers several times over the season to encourage growth and blooming or to address specific problems (to pep up a weak plant, to correct a deficiency, etc.).
Annuals grown in containers
Container-grown plants are usually heavier feeders, because only a limited supply of nutrients is available to them and nutrients tend to leach out of the soil.
When preparing flower pots and window boxes, it is best to use potting soil specially made for container-grown plants containing compost. If it's not already in the potting soil mix, you can add natural slow-release fertilizer at planting time. Usually, less demanding plants will have their needs fulfilled for the summer. Extra fertilizing might be needed for more demanding plants during the season. Natural fertilizers (liquid seaweed, fish emulsion, hen manure, etc.) can be used a few times according to the manufacturer's instructions.
Warning! Not all annuals need to be fertilized. In fact, some annuals, including calendulas, cosmos, nasturtiums and portulacas, flower better when grown in poor soil.