Frequently watering containers results in the leaching of nutrients. These nutrients need to be replaced on a regular basis. A regular fertilizer program is needed to keep plants growing well and attractive all season.
There are several ways to supply nutrients. One is through the use of slow-release or timed release fertilizer. These products consist of water soluble fertilizer that is encased in a semi-permeable resin coating. When they come in contact with water, small amounts of nutrients are released to the soil for use by the plant. So, each time you water, the containers are getting "automatically" fertilized. These products are usually mixed with the potting media or placed on the soil surface at a rate specified on the label and based on the size of the container. Many of the products for use with annuals will supply nutrients for 3-4 months depending on the amount of moisture and temperature.
Another way to fertilize containers is to use liquid fertilizers. These products are mixed with water according to label instructions and then applied during normal watering. It is suggested to apply fertilizer to the soil media when it is moist rather than when it is completely dry. This helps to avoid potential damage to the plant. Reapplying every two weeks or so will provide adequate nutrition for most containers. You can adjust the frequency of application based on overall appearance and growth. You can also fertilize with every watering using only a half strength fertilizer solution. Occasionally use just plain water to leach the pot.
The choice of fertilizer analysis will depend on the kinds of plants you are growing. High nitrogen sources would be good for plants grown for their foliage while flowering and vegetable crops would prefer lower nitrogen and higher phosphorous types.