Well, that depends. When it comes to fertilizing tomatoes, a little bit can go a long way. You should usually not fertilize tomato plants the first few weeks after planting them, especially with fertilizer containing a high concentration of nitrogen. This may cause an abundance of green growth, with very little fruit production. It's fine to put down fertilizer in your garden about 2 weeks before transplanting tomato plants. After the tomato plants produce blossoms, it's ok to fertilize them every couple of weeks.
There are several different fertilizers that will work well for tomatoes. Most of them are readily available in stores that have large garden centers. Most all purpose fertilizers are appropriate. Pay attention to the 3 number code on the bag of fertilizer. These three numbers indicate the amount of nitrogen, phosphate and potassium respectively that are contained in that particular fertilizer. For instance, a 10-10-10 fertilizer contains 10% nitrogen, 10% phosphate and 10% potassium. A 5-10-10 bag would contain 5% percent nitrogen.
When fertilizing tomatoes, look for 5-10-5 or 5-10-10 fertilizer. This contains half as much nitrogen as phosphate. A higher phosphate and potassium number will encourage more fruit production. A lower nitrogen number will help the plant grow, without doing it at the expense of producing fruit.
There are several fertilizers formulated especially for tomatoes. Some of these fertilizers can be rather expensive. A standard, all purpose fertilizer can and will work just as well if applied correctly, according to the instructions listed on the package.
Most granular fertilizers should be applied at a rate of 1 1/2 pounds per 100 square feet. When fertilizing tomatoes with a granular product, make sure to avoid letting it touch the plants themselves. This could burn the plant and have adverse consequences. Instead, apply the granules in a circle around the tomato plants. After applying the fertilizer to your garden, water it in well.
If you are using a water soluble fertilizer (the kind that you dissolve in water and then spray on the garden), try to avoid spraying it on the plants themselves. Instead, concentrate your efforts at the base of the plant. A tomato plant receives nearly all of its nutrients through the root system. Additionally, spraying water on the top of the plant may encourage diseases to develop. If you prefer to use water soluble fertilizer, a hose end sprayer might be a good investment.
After fertilizing tomatoes, it's a good idea to apply a layer of mulch. Grass clippings, hay, etc... work well for this. Put the mulch down all around the tomato plants. This will prevent the evaporation of moisture. It also acts a control for weeds. At the end of the growing season, you can till this mulch into your garden and it will add nutrients to your soil over the winter.