Container plants need more applications of fertilizer than in-ground plants because watering tends to wash nutrients out of the container, and the plants do not have access to nutrients from surrounding soil as garden plants do.
At planting time, add a slow-release balanced fertilizer to the potting mixture, or choose a mix with slow-release fertilizer included. If you are adding a slow-release fertilizer, choose one that is balanced in nitrogen, phosphorus and potassium, indicated by the numbers 10-10-10, 13-13-13 or 14-14-14.
Treat plants to a water-soluble fertilizer once a week midseason, when the plants start producing. When choosing one for container vegetables that are grown for their fruit (cucumbers, peppers, tomatoes, etc —as opposed to those grown for their leaves, such as lettuce), pay close attention to the nitrogen (N) content. Nitrogen is indicated by the first number in the trio of numbers that label fertilizers. Nitrogen support foliage growth. Choosing a fertilizer too high in nitrogen will produce lush, bushy plants with few flowers and fruit. Recommended formulas are 15-30-15 and 20-20-20.
Other tips –
Do not fertilize dry plants. Water them first, then water again with your water-soluble fertilizer, mixed according to package directions, an hour or two later.
Some gardeners make a nutrient solution for use with every watering. (For example, a water-soluble feed mixed at half the recommended strength.) If you go this route, be sure to irrigate once a week with plain water to flush some of the built-up fertilizer out of the container.
Learn the benefits of vegetable gardening without soil in Hydroponic Gardening.
Cut maintenance time with Incredible Vegetables from Self-Watering Containers.
Learn all about edible container gardens with McGee & Stuckey’s The Bountiful Container.
Make a prettier veg plot through Ornamental Vegetable Gardening.